Which do I trust? (speedometer or GPS)

Your GPS is far more accurate. The speedometer has many variables that can give erroneous readings. Tire size, pressure, alignment of the scale in the dash, and other thing that depend on how the car mfg. is measuring the speed to be converted on the gauge.

Virginia being so friendly to out of state speedsters, I wouldn't dare try any speed above the posted limit. I love my $$$ too much to validate the GPS. I'll settle somewhere between odometer and GPS.

Virginia being so "friendly" to out of state speedsters, I wouldn't dare try any speed above the posted limit. I love my $$$ too much to validate the GPS. I'll settle somewhere between odometer and GPS.

my car too, it seems pretty accurate, but when i get to about 70 its about 4 mph higher than my cars speedo

I think the dad installed some parent tracker gps thing. Anyway, the kid complained to his dad that he was not speeding. His dad said prove it and so he grabbed the gps data. It was the first case in the US to win. (first time he lost, they went to a higher court and the gps expert admitted an error and said the gps was accurate to 1mph)
How much was it wrong -
"Malone, a 17-year-old who was clocked doing 62 MPH in a 45 MPH zone on July 4 of this year, has more than just a GPS on his side."
http://arstechnica.com/news.ars/post/20071026-accused-speede...

http://arstechnica.com/news.ars/post/20080718-nabbed-for-spe...

"When the judge rules on the case in October, he could be let off the hook for his $190 speeding ticket. But the implications of the case go far beyond $190—it could change how speeding tickets are handled in the future. "This case has caught the attention of the nation, and it will set a precedent on how police departments use speed traps in the face of an increased GPS presence," Rocky Mountain Tracking president Brad Borst said in a statement. "The accuracy and reliability of GPS has helped bring this important issue into the limelight."

GPS is more accurate

On long road trips, I'm always the primary driver. And I typically put the cruise control on the speed limit for a while, then inch it upward about 4=5 MPH more. It freaks out my navigator.

But we've always passed the troopers sitting in the median of the interstates, and they never seem to blink. I don't tap the brakes to take it off cruise, and I'd have to agree with the concensus of this discussion: the speedo registers a few miles low.

Now the next long trip we make will be my first with the Nuvi 880, and I'm betting that it proves I am simply going the REAL limit.

It's good to know the GPSr upholds my driving habit.

VWs and Audis tend to report a faster than actual speed on the speedo. Not only does my GPS show that, but I have measured my speed using a tool called "VAG-COM". (VAG-COM is a diagnostic tool that runs on a PC and emulates that expensive diagnotic tools used by the dealer. It connects throught the OBDII port and reads directly from the various controllers in the vehicle.)

Generally, I have found that every VW or Audi I've owned display about 3 mph (5 kph) faster than the actual speed. In Germany where photo ticket cameras are quite prevalent, I'm sure it's a welcomed feature.

DOT allows speedometers (right out of the factory) to be up to 15% off. They're usually less than that but many tend to read a couple of percent higher than actual speed. Motorcycle speedometers (particularly those made in the orient) are particularly bad for this. Trust the GPS to be the more accurate of the two, at least when you're holding a steady speed.

Cheers

It is also worth noting that when your speedometer is inaccurate it will usually vary with your current speed. I know on my car, the speedometer is 3 mph faster than my actual speed when I am going 40 mph and 6 mph fast at 80 mph. These figures are with properly inflated new tires, so when the tires are old the number difference would be even greater.

My Dodge Caravan matches speed with my Nuvi 760 Exactly. This must be a popular topic, it came up on a firearms forum I frequent recently.

I bought a 2009 Jetta, and it never quite agrees with the GPS-according to the Nuvi I'm going a couple MPH faster than speedo indicates. The difference varies with speed though. I've verified the accuracy of the GPS against those unmanned radar things they put up to show how fast you are going. Unless both the GPS and radar gun are uncalibrated to the same standard, the GPS wins in my book. It's not worth $100 plus points on my license to find out the speedo was wrong.

Edit: should read ..than speedo indicates. I accidentally said GPS indicates.

The GPS appears to be more accurate. The speedometer on my pickup truck would say 70mph while my Garmin-650 would tell me 74mph. The flashing highway speed signs would always agree with the GPS reading. After a bit of contemplation, I remembered having installed oversized tires on the pickup. This would probably account for my difference in readings. Therefore, I tend to trust the GPS.

As a secondary comment, I seldom have the mph displayed on the GPS while traveling with others. Family members (especially the wife) tend to complain.

cheweez, you are right on with the tire sizes affecting your speed. I had inherited a full set of tires from my brother but they were slightly smaller than my regular tires. Anyways I have to go a little faster, by the speedometer, to attain the actual speed (e.g. 45 mph on speedometer is actually a real speed of 40 mph). I will often check the speed on my 660 and trust that reading over the speedometer's reading.

I am new to the GPS world but I believe the GPS is accurate. My cars speedo is about two MPH fast according to all of the little radar trailers beside the road and the GPS logs it the same as the radar.

Well, my motorcycle (like most motorcycle speedometers are off. Many as much as 15%. I know when my speedo says I am going 70 much of the traffic with the speed limit set at 60 are passing me.

I will put the Garmin 450 on my bike on Tuesday and ask a cop to check me out with his radar at two or three speeds.

Why bother, believe the GPS

Most recent Garmin units have the speed accuracy listed in the unit's specifications found near the back of the owner's manual.

The velocity is listed as .05 meters/sec .. This works out to 1/10 of a mile per hour.

Note that speed is calculated differently than positional fixes. This is why the speed accuracy is the same whether you're receiving a WAAS satellite or not.

It has to do with the circumference of your tire, your speed should always be a set % high or low.

So if you are going 70 miles per hour and your tires are 10% too small, your speedometer would read 77. But if you were only going 40 your speedometer would read 44. Thats why at "slower" speeds people are less likely to notice their speedometer being off than at the higher freeway speeds.

I have the 360 and it always shows the same speed as the speedometer.

My suggestion is to use the unit that shows the slower speed. Being 2 - 5 MPH out will not significantly affect your arrival time, but it could save you a ticket.

If you tried to argue the "accuracy" of your GPS in court against that of a radar gun, the first questions asked would be "When was the last time you calibrated the unit?", and "What standard procedure did you use?". These are the same questions that people can use to challenge radar tickets - and it is why the police calibrate their radar units so frequently.

Two Toyotas and a ford explorer, all three are within 1-2mph. Sometimes right on...Don't forget to take into consideration the slight amount of delay incurred for the gpsr to receive the constantly updating sat signals. Cruise control, road surface etc may reflect a slight variation between the speedometer and gpsr as well. A stretch of smooth level road will be more accurate than one with a rise or decline. The actual grade level can fool your eyes.

Anyway, try comparing the actual odometer readings. I've noticed a tenth or so off, and of course it varies with the total distance traveled. In other words, 1mile will have less of a discrepancy than 20miles. I'm sure it's probably the vehicles odometer that's in error. What are others seeing?

Hey~ How about not exceeding the speed limit

well since a gps does not only get coordinates but also elevation it will correctly display MPH on any incline. The system does not locate you on a 2d map, but can locate you 3 dementionally. It also dosnt calculate the distance you have moved on a flat plane relevant to the horizon but it maps you relative to the distance from the satellites. Weather you move up down left or right NSE or W it tracks that movement, and the time it takes to make that movement(which if correctly calculated translate to MPH) The system works in a 3 demintional plane, if it didnt then gps would be no better than radar.

but do you know that no one else has driven it that fast>? I can garantee you that a gps is ALWAYS more accurate that a speedo. a speedo is a gear driven device that has many diffrent variables that can change their reading including vehicle weight, tire pressure, road conditions (i can make my speedo say 60mph and not be moving at all!) gear ratios, tire size, and just like any other gauge... plain ERROR. I place my trust in Algebra, Trig, and TIME. These are all constants on this rock we are on, Gears and rubber are not.

only if the outside tire diameter is diffrent. Your rims have nothing to do with it.

the reason that your speedo is off more at higher speeds is because whatever the change in the drive gear line is, tire pressure, bigger tires, ect. and if you raise the ratio even .5:1 it will be an exponetial change. So at .5:1 gain in gear you would gain a half a revolution EACH revolution so at 10 you might be off .25mph then .5 at 20 1 at 40, 2 at 80 and so on. Exponential growth always comes into play when you mess with gear ratios.

OK, I've heard enough of this subject (come on, a year and a half long thread??? well, almost)

The actual truth of the matter is most definitely the GPS is much more accurate. period...

and here's why...

/RANT

The speedometer in your vehicle is based on a mechanical ratio between the output shaft of the vehicle's transmission, to a weighted counter weight inside the speedometer. there is no physical connection from the tires, wheels, or even the output shaft of the tranny to the actual speedometer in your dash. The transmission shaft has a helical gear cut into it's output shaft specifically for the speedometer cable to mate too. the end of the speedo cable is attached to a mating gear for the output shaft which can be changed (within a certain range of teeth per inch (or mm) to recalibrate the speedometer during testing after repairs or tire and wheel replacements. This calibration is normally done WITH A GPS UNIT!

Here's where it gets really tricky...

Inside the speedometer in your dash there are two wheels (not gears)... a big one, and a small one. both made of ferrous metals. the outer gear contains two magnets, and when it spins it generates a magnetic field into the smaller wheel which begins to turn in relation to the speed of the larger one. the smaller wheel is the one actually attached to your speedometer needle, giving you the readout.

Beyond that, there are other factors that can affect the accuracy of the speedometer. One main one is simply dust and debris accumulating between the wheels, a loss of magnetic attraction (yes, speedometers do wear out), a stretching of the speedometer cable due to a lack of transmission lubrication or age of the cable, improper calibration after a repair (may even be on a brand new, never registered vehicle), speedometer replacement or tampering, transmission linkage issues including worn out u joints, wrong size rear end ratio, and a HOST of other factors.

Most police jurisdictions here is the US will waive a slightly fast driver due to speedometer inaccuracies (normally 5MPH or less), not just to be nice, but simply because the case can be made for a mechanical malfunction as the root cause even on a new vehicle. (don't try this, as it's YOUR responsibility to know your vehicle and it's negligence on your part if you don't in a judge's eyes)

ALL vehicle manufacturer's have the responsibility to provide their end owners with an accurate speedometer within a certain tolerance limit due to the gear ratio between the two gears at the transmission output shaft.

Most newer vehicle have done away with this connection, and now measure vehicle speed through the brake rotor or drum mounted magnetic vehicle speed sensors for the ABS brakes in order to provide a more accurate reading (some trucks have this sensor in the rear differential). However, this too can be inaccurate due to brake shavings coating the wheel sensors (with metallic brakes), wheel sensor damage or misalignment, snow and ice build up, or defective parts.

Those that have this newer type of reading capabilities normally use the engine management system computer to calculate the ground speed of the vehicle, and can be "reprogrammed" by your local new car dealer much more accurately than ever before with proper equipment, regardless of wheel and tire size (actually, its only the outer diameter that really matters anyway.)

These vehicle also use a pulse modulation system to move the speedometer needle instead of the aforementioned mechanical system.

Another factor that GREATLY affects speedometer readings on both systems is road and wind RESISTANCE of the vehicle. (Ever notice how your mileage goes way down on windy days from a head wind? or a steep incline) How much power is required to actually move the vehicle, as these new systems use a calculation of vehicle tire diameter, engine RPM, transmission gear ratio, and wheel speed to determine your actual speed.

Besides, a GPS speed reading is a TIME based calculation of distance versus time to achieve your speed reading, and is a POINT to POINT reading. Whereas a vehicle speedometer is a GUESS (albeit a normally fairly accurate one) of distance traveled.
Kind of like the difference between using a tape measure to measure a distance accurately, or walking it off and guessing how many steps it took to cover the distance.

Please excuse the long post (especially for my first one!) but hey, I'm kind of tired of hearing about this, and wanted to clear a few things up.

Now, if you really want to discuss this more, I guess I can't really stop you, I just got tired of all the guessing and inaccurate information.

One more thing, excuse the spelling errors and typo's please. It's 6am and I haven't been to bed yet...

/END RANT

Oh yeah, one thing i forgot to attack...

the reason that your speed difference changes with speed is the simple fact that your tire diameter will grow or shrink due to centrifugal force reacting to the rubber in the tires. (the faster you go, the larger the diameter of the tire)
/END RANT 2

The gps is always more accurate. My Toyota Tundra is very close, but my wife's Toyota Camry is off my as much as 3 mph.

Long RANT excused - well done.

I would suggest the GPS is more accurate every time BUT beware of situations where there are rapid changes of speed or direction as the point to point average might be quite different to the actual instant readings at the points between those points... where are those maths types to explain.....

My nuvi 760 has ALWAYS been dead on the mark with the vehicle's reading. There has never been a detectable difference. I trust the Nuvi's accuracy.

I found out the hard way that my speedometer is correct. The whole time I was thinking that my recently purchased used car with slightly larger tires didn't have the speed calibrated. I was wrong. I also have a radar and he didn't hit me from the side of the road, but rather used his speedometer and video camera to record the speed.

I was pulled over by a state trooper in MD doing 83 in a 65. Fortunately, he let me off with a warning after I removed the keys from my ignition and put them on my dashboard. I thought I was only going 75 (according to my RoadMate 1412) but realistically, I imagine there may be delays in speed updates due to the time it takes for the signal to bounce back and forth from the satellite(s) back to my car.

It also begs the question how accurate is the reported "top speed" like if you bring your GPS onto a plane and use it with the pilot's permission?

Thanks for the insight into the working of modern speedometers.

My GPS and the road signs all tell me I'm going 5 miles an hour SLOWER than what my speedometer reads.. I've always wondered why.. But since my car normally is sold with 16" tires and I have 17" on the car that would explain why I'm always showing 5 miles more at all those roadside speed signs and on the GPS. Thanks!

OK, so now I'm really convinced that between the larger tire size and VW being the manufacturer, the speedo is fast.. good.. I can use the cushion.. every little bit helps

Seems to me Honda got caught with their pants down, The reason is that it causes the warranty mileage to expire quicker. And they should be sued class action style.

And anyone that believes Honda has your best interest in mind well then, you probably can justify buying a Honda because they assemble some cars here. Mean While the american auto industry is in the dumper laying off hundreds of thousands of people and closing dozens of plants. So don't think for a minute people tend to drive a little slower than they think they are, which gives better gas mileage figures because Honda has tweaked the Odometers and Speedometers to their benefit just to run your warranty mileage out faster.

SO trust your GPSr, you won't go wrong!

But presumably, your 17" tyres are a lower profile than the 16" ones, to keep the diameter the same. Or do you hear scraping noises and smell burning rubber?

I've checked out two VW/Audi speedos against GPS. They were both, as you found, 5mph fast, from about 35mph onwards.

Some of the VW/Audi systems have a settable 'overspeed' warning - that agrees with the speedo - not reality.

However, the Trip Computer agrees with the GPS (otherwise all its calculations would be wrong).

If you have Cruise Control; set a constant speed, zero the trip computer and observe Average Speed. Since Average and Current are the same when on cruise, you'll probably find that the figure agrees with the GPSr, not the speedo.

You'll find the speedometer in most any stock car will be calibrated a little optimistically. This includes American, Japanese, European, etc. I don't believe this is to get out of the warranty, but believe the manufacturers don't want suits because their speedos read too slow and the people that got tickets in the cars fighting them. I doubt any suit would win because all the manufacturers do it, not just Honda and the Japanese.

The big 3 American manufacturers didn't get in the hot water they're in by building superior cars, just as the Japanese cars didn't climb in the ratings overnight. These were slow processes that happened over time. The Americans were for quite awhile building disgraceful cars, and now the industry wants to be bailout and the CEO's fly to Washington in private jets to ask for money. That got a lot of peoples attention in the wrong way. Yes, the American cars now are on a whole a lot better than they were a few years ago, but it's going to take time to build people's trust again. There's a reason American car sales are in the pits that happened over time, greedy management and unions, poor build quality, etc. It's not like the auto industry didn't bring a lot of this on themselves. Then there's laws and other factors that the industry has no control over like the Japanese taxing the hell out of American cars over there and not playing fair. It's not like people flocked to foreign cars all of a sudden just for the heck of it. The American auto industry has a lot of doctoring to do on itself, and it's possibly a good thing the crisis has happened, maybe they'll come out of it wiser and stronger.

I believe we've been through this question before. We all pretty much agree that the gps is more accurate. One answer to this was that car mfgr's purposly set the readings 2-3 mph higher to avoid liability in speeding cases.

gusb

GPS! Speedos aren't very accurate.

So true. My poor bro had a nice Z-car which had the same problem because it just LOOKS FAST, even sitting at the red light. The only time they left him alone was when he borrowed our mom's "grannymobile" little blue Chevy stationwagon. And don't cha kno the grannymobile was the car stolen when it was parked in the French Qtr outside a restaurant? Go figure. Inconspicuous I suppose.

My vote is for tire size being the difference.

Cheers.

My vote is for GPS accuracy, but I experience the delay on the GPS response. I would not count on GPS readings when the speed is changing.

my garmin c320 show different speed with speedometer

gps show me 35 mi but speedometer show me 38 mi.

when i think about that , because speedmeter direct from my car, but gps is get date from gps station.

i think right or wrong ??

I agree. When travelling at a "constant" speed I think the GPS is far more accurate. But when changing speeds e.g. down from Interstate speeds to say 50MPH for road works, I trust the speedo.

For what it's worth, I'm a retired automotive technical training course writer/developer for a vehicle manufacturer.

Trust your gps.

lovoltage was absolutely accurate in the explanation of how vehicle speedos operate, and why none of them will ever read the same. Yes, the gps may have (insignificant) millisecond delays and are also subject to line of sight to satellite glitches, but the gps is BY FAR the most accurate speed indicator in 99.99% of all situations.

About optimistic readings and the manufacturer conspiracy.. the U.S. and other countries have federal regulations that require mfrs to provide indication accuracy within a few percent of actual speed but EXPRESSLY PROHIBIT any under-indication of actual speed. Makes sense to me, and given all of the manufacturing variables it's no surprise which side the error would be designed to.

Lastly, at the end of the day the only reading that matters is the one that law enforcement takes before writing you a ticket. If I were to go to the expense and hassle of pleading my case in court, I'd certainly rather have my gps track log than anything else.

I drive several cars and have found a bit of variation in the accuracy of speedometers vs. my Garmin 255W. I drive a 2000 Buick Century and find that at 60 the speedometer reads about 2mph high. I also from time to time drive a 2000 Dodge Caravan and find that at 60 it is very close to accurate. A Ford Freestar reads about 1 mph high and a 1996 Buick Regal reads the same 2 mph high that the Century does. None of these vehicles have original tires so that could be part of it. I have never driven a car where the GPS read a higher speed than the speedometer.

Is there any reason this thread is still alive ?

GPS is the one I would go by or which everone is faster

say the same MPH, about 2 or 3 MPHs faster than car speedo. I have driven by 3 or 4 different roadside speedos and they all agree.