Which do I trust? (speedometer or GPS)

If you live close to a stretch of highway with mile markers, it is easy to check your speedometer or GPS speed accuracy using only a stopwatch or even a watch with a second hand. The process is simple. On a calm day, find a flat, strait stretch of road where you can set your cruise control and not have to slow down or pass for 4 or five miles. Use a stopwatch to capture the time starting at the instant you pass a mile marker and stopping as you pass another marker 4 or 5 miles down the road. The formula is:

(distance X 3600) / (time in seconds)

So if you drive 5 miles in 4 minutes 28 seconds, that is a total of 268 seconds and your speed is

(5 miles X 3600) / 268 seconds = 67.16 MPH.

I have found you can get a pretty accurate speed reading in just 1 mile, but if you can hold a constant speed for 4 or 5 miles, you can get a very accurate speed calculation. Note that this formula will work equally as well in kilometers per hour assuming you can find kilometer road markers. For both accuracy and safety reasons, I recommend that you have a passenger watch the mile markers and operate the stop watch.

I have used this method several times and I find my Garmin 755T speedometer is right on. My 08 Elantra speedometer is consistently about 2 MPH fast at 60 MPH.

Most..(Not all) the user manuals supplied with Garmin GPS units list the specifications of the unit. The specs are usually found on the last page or two.

Garmin claims that the velocity accuracy of the Nuvi is better than .05 meters per second..... That works out to an accuracy of 1/10th of a mile per hour.

Note that this is when traveling at a constant speed with a good satellite fix. I don't know for sure, but I suspect all modern GPS units are the same or similar in speed accuracy.

You can easily check your speedo by counting the seconds between mile markers (on all the interstates and many other roads) . Take the sec. per mile then convert to miles per hour. The conversion factor is 3600/x as in 3600/60 = 60mph . The more level the road and the more miles you can clock the more accurate the results. Please be safe and use a partner for the measurements. The stop watch or GPS measurements are more accurate then most speedos or radar guns.

I agree that the GPS is accurate compared to the speedometer.

I would agree with the NUVI

I have compared (with a keen eye) the speed that my Nuvi displays with the speedometer of 3 different brands of cars (at speeds from 20mph to [WAY] beyond the maximum speed limit) and I have found no difference.

I have 2 vehicles and also a Nuvi 350 and a 780 and my findings are exactly the same. At 70 on the Nuvi's the speedometer readings on both cars is 74/75.

great lol now that we established that GPS speedometers are more accurate than the car speedometer.. so when a cop stops ya..and assuming he knows about GPS storing speeds u traveled.. he can look at it and give u a ticket based on your fastest speed lol.

love my nuvi !!

Way back when, I bought my first new car. The third time I got a ticket, I decided to fight it. I had the speedo checked. It was accurate between 0 and 35. Then the needle lagged behind. At 60mph, I was actually going 69, even though the odometer was accurate for miles traveled. The judge found me guilty for 2 miles over the speed limit. The ticket said 71 in a 60.
Since then I always checked the mile markers against my stopwatch at 60mph. My GF Toyota shows 62, while the watch & gps show 60. At the end of a long trip, the car shows a few more miles traveled than the gps.

I would think the GPS may be a little more accurate if it is getting a good signal

Not any longer. The systems now just use hall effect sensors for the abs system.

I have a Garmin 670 and I have it in kms and I can say that the gps is giving a slower speed than on my speedometer! The difference is 3-4kms.

About police radars, that is a crap shot, you don`t know if what they showed you was your real speed. I don`t know if we have any way to verify if the time of the fine was the same on the radar gun.
They can clock some one going 80 miles on a 70 mile zone and not reset the unit, and at they discretion stop you and fine you even if you where not speeding.
You know, proof is sometimes difficult to obtain because they have the radar gun and can be set with a press of a button.

Falcao

When I got my used Van I thought it was about 10 km over. When I put my GPS on I was right, it was 10km fast, so I go by my GPS.

ohwogo nuvi 750

I have three devices showing mph, my 780, the speedo and ScanGauge (plugs into the OBD port). The GPSr and speedo are always in agreement. The SG shows 1 to 2 mph less. A mechanic friend go with the GPSr and Speedo.

Same thing with me. Car reads slower than the GPS and right on with the speedometer in my truck.

Try going down a freeway and timing yourself between about 10 mile markers and then average the results.

I always set my cruise via the GPS. It has to be more accurate than my 100,000+ mile tranny. Set the speed at 72 - in a 65 and hopefully the police will leave me be!!! LOL

My Car Speedo and my GPS are pretty much spot onto each other.

They may float away by one mile per hour sometimes, but generally they agree on my speed.

That's exactly what I see on 3 different [brand] cars...regardless whether I'm driving slowly or really fast.

In general GPS is VERY accurate (especially when travelling in a straight line and with a clear view of the sky.

Car speedometers are required to be accurate or optimistic, but they may not read lower than the actual speed - Honda had a situation with the Gold Wing motorcycle a couple of years ago - cost them a bundle and necessitated a recall.

Plus they paid everyone who asked for their speeding tickets.

If your car speedo consistently reads lower than the GPS and if the vehicle is under warranty, get it repaired (and good luck with that). This, of course presumes that you haven't put huge oversized tires on the thing.

I personally have never had an issue where the GPS and my speedometer were different. They are always dead on. In both of my Hondas.

Maybe I am just lucky

I am one to believe the GPS to be right. I find that it is normally about 2 miles slower in speed than my speedometer is. I know the speedometer in one of my vehicles is wrong by 3 miles per hour. After getting my GPS I tested it against that vehicle and it was 3 miles per hour slower making me believe the GPS to be the correct one.

The GPS has always matched the speed detected on the portable speed alert displays that some local PD's place by the side of the road to alert travelers of the local speed - and their speed in relation to the limit.

Call me skeptical, but I think car manufacturers calibrate speedometers slower than the actual speed so the cars record more miles than actually traveled ... and the 3yr/36k mile warranty expires sooner. Just 2 MPH off, and you run out of warranty at 34k actual miles traveled although the odometer would record 36k.

It is actually the other way around ... speedometer reads faster than actual speed, making the odometer show more miles than what is actually driven.

But your point is well taken and I agree with you that the manufacturers do this by design for the reason you give. Also, it would eliminate any manufacturer liability issues for drivers going faster than they thought they were.

I trust my Gps it always seems to be the same speed as stated on the road side speed checker systems tat are placed on the road side by the police

As your tires wear it will change you ratio a small amount. A lot more for large truck tires.

Trust the GPS particularly if the signal is strong. I've tested against the speed indicators police put up at construction zones and GPS is always bang on

trust the gps

trust the gps

I always trust the GPS for my speed. Generally my speedometer is about 2-3 MPH off. If the GPS is reading 70 the speedo is showing 68 on average. Anyway the funny thing today is I had to change the the right rear tire today due to a blowout at 60MPH. Overall a smooth experiance. Anyway had to put the donut spare on. The whole time back to the house there was a consistant 7 MPH differance. GPS 70 and the speedo at 75-77MPH. Granted it is a smaller tire but I was still surprised at the speed differance for just one tire.

Myself just more proof that the GPS is going to be more accurate.

I would say speedometer - atleast that gives a less chance of getting a speeding ticket if you are closely over the limit

I recently bought a 755t and found that it consistently indicates that I'm driving ~4MPH faster than what my speedometer indicates in my truck - I then remembered that I had bought larger tires for my truck a few years ago, so I believed the GPS. My Yukon has the same size tires it orginally came with - the GPS & Yukon speedometer are usually consistent. I recently got a camera ticket in DC for going 11 MPH over in my truck (my speedometer would have indicated 7MPH over).

I'll play it safe and go by whichever is higher but I've found that I now rely on my GPS more than the speedometer. Also, since the GPS is mounted on the windshield, I don't have to take my eyes off the road to check speed.

In my wife's fine piece of german engineering VW Jetta, the Garmin says she is off by 4 mph at 65. My truck (Ford) reads in complete agreement with the Garmin. I haven't told her and so far no problem, She has a lead foot anyway

Steve

A few years ago, a high school kid got a speeding ticket. His parents had installed a Magellan GPS on his car and downloaded the tracks from time to time. They went with the boy to court and presented their evidence to prove he was not speeding that day nor several days before then either. The judge knew the police had used similar devices but he was not sure about it being used as a defense. He praised the parents for their concern. The judge took the easy way out and ruled deferred prosecution for six months. The ticket was later dismissed.

For the most part the courts will not accept the data from a track log as it can be manipulated once it is pulled from the device. I'm not certain if it can be manipulated while on the device but again, most - not all - refuse to accept the data.

Sounds like GPS wins. I have always heard that on most highways that police usually won't pull you over unless you are at least 10 mph over the speed limit.

Travelling 70 on a donut

This may have already been mentioned, I didn't read all 237 posts. I read an article about auto manufactures deliberately setting the speedometer to read about 2MPH fast so people will not tend to speed. From what I've seen maybe they should have set them 10MPH fast. My Impala is right on with the GPS, my wife's Vibe is about 2MPH fast.

Yep, not a good idea running that speed on a non-full size spare. It wasn't exactly a donut perse. It was indeed not a full tire. It brought the truck up to slightly lower then ride height. The rim was larger then normal. But only had a few miles to get home and the weather was turning. Again not advisable to do. Got a brand new tire the next day. Don't know if this had a bad sidewall or if I picked something up.

Most car manufacturers build in an error favoring higher speeds - I've seen anything between 2 - 5 mph. They don't want to underestimate for obvious reasons of criticism for getting folks speeding tickets.

Fred

The accuracy of the vehicle speedometer depends on many factors including tire size (as compared with original installed tires), tire pressure, temperature (which affects pressure) and other environmental factors. At the same time, depending on your location, the GPS accuracy varies from under 10 ft to over 50 ft (in my case with a 760).

Assuming your car is brand new and the tires properly inflated, the car's speedmeter should match your GPS speed. Anything else other than that, and I would use the GPS speed as a better indicator of how fat you are going.

Definitely the GPS. I used 2 different GPS units to confirm my built in speedo was wrong. My speedo actually reports my speed as being less than it is, so I am glad I checked.

But, isn't it true that the same mechanism that determines velocity also determines miles traveled for the odometer reading? So if true, then aren't you being possibly gipped on automobile warranties based on miles traveled?

Possibly - BUT, my car's speedo reads about 5% high and the odometer reads bang on . . . tested against measured miles. The GPS is for the most part bang on.

The optimistic speedometer is by design. I wrote about is about 100 posts ago . . . speedos can read high or be accurate. They cannot read low. Period.

In any case, as was pointed out, tire wear means that speed and distance readings are approximations.

And, car manufacturers will generally allow some leeway if the defect occurs within a couple of hundred miles of the mileage limits - then again, if you'r being a pig about things (looking for new shocks at 60,000 mile and after 5.01 years, that's quite another matter,

I found one of these radar signs along the road displaying your speed, the GPS was right on while the car speedometer was slow -3 mph at 55 mph.

My car and my gps are both the same. 885T

Before reading all the comments, I would have said to trust the car speedometer. After reading the comments I would trust the GPS. However, I have never noticed more than a 2 mph difference.
I follow the guideline that there is a 10mph "cushion" on the highway (speed limit over 50mph) and a 5mph in the city. When I follow that rule I don't get pulled over. However, the talk of photo enforced speed limits frighten me.

Well as far as the radar gun, only if it's been calibrated correctly. I read something where a driver challenged his speeding ticket in court. His lawyer had the COP shoot the wall, which the gun showed was moving at over 4 miles per hour.

Case Dismissed..!

Nuvi1300WTGPS

Nuvi1300WTGPS@Gmail.com

I won't get into the technical reasons but I don't think the digital radar guns used today require calibration. At least I can't think of any reason why they would.

Jack j

The gps is always the most accurate. I was disappointed to discover that my wife's auto (Toyota Camry) was off as much as 4mph.