Garmin Speed Display

I always go off the GPS. Usually car makers make the speedo show a little faster than actual speed. In all my tests with mile markers (60 mph:60 sec, 65 mph:55 sec, 70 mph:50 sec, and 75 mph:45 sec) the gps is always right on.

Daniel

Assuming the tires on your vehicle are the ones mounted from the factory and inflated to the proper air pressure it has been discussed in previous threads and the majority here believes the GPS is the most accurate.

Regardless of tire size, gearing, or other factors, the GPS will give you true speed. Period.

The GPS has to have a better accuracy otherwise how would the GPS be able display where you are?

I've always heard car speedos have a +/- 5% new out of the factory ...

Whenever I pass one of those unmanned radar displays, it generally matches my GPS reading.

Car manufacturers have for decades purposely made speedometers that show a slightly higher speed.

This makes you drive a little slower than you think you are going.

Another item is that if you have replaced your tires with a different size - that will make your speedometer wrong.

There used to be somewhat of a reason for a car having an inaccurate speedometer because they were driven by a gear in the transmission moving a cable to the speedometer and the speedometer being electro-mehanically driven by the spinning cable. There were multiple nylon gears but you were limited to a finite set of adjustability of the display. When the gearing and tires fit one of the nylon gears, the speedometer was accurate. Sometime they were off a little.

But these days the speedometer is drivn by electronics and the input is just a revolution sensor. So the connection is totally software and there is no reason the programming cannot be changed to make the speedometer accurate. But some manufactureres, BMW is one, will not let dealers "fix" the software. They want the speedometer to read faster than the car is going. My son calls this sort of thinking "nannying". I don't know if Toyota lets the dealer "fix" this by connecting a computer to the OBD II port and re-programming or not.

Jim

Very cool and interesting thread. I would have never known about this information. Thanks for posting this!

That reminds me that I bought a brand new 1968 Mercury Cougar XR-7 and got a speeding ticket on the way home from the dealer because the factory had put in a wrong gear. The judge accepted a letter from the dealer admitting that the gear was wrong and caused the excess speed at which I was clocked, and dropped the charge.

Has anyone else noticed the the mph display on the garmin is different between the map screen and the trip computer display. It seems to be about 1 mph different. Not a big difference but I would think they should be the same. And being as there is a difference, which one is right?

The one on the police's radar...

I put new tires on my wife's Avalon and noticed that before the GPS and display speed were the same. Now the GPS is 3 MPH faster than what I see on the speedometer. New tires are lower proflie and wider but same rim size. I would conclude the GPS is accurate and tires sizing can cause speed errors.

When I purchased my last car, I got a substantial difference between reported speed on the speedometer verses the gps. I tried two gps units, one of which is waas enabled, and got identical readings between the two. Thanks to a google search, I found out that model of car (BMW E39 series) consistently reports a higher than true mph. The reading I was getting was consistent with what other owners of the same model report. It is a factory setting that cannot be corrected by a dealer. I rely on the gps reading.

The speedometer indicated speed (at any given actual speed) will go up as the outer diameter of the tire decreases. Thus the speedometer indicated speed will even go up a bit as the tire tread wears off.

So I would have thought that your GPS should now be reading less than the speedometer, since the two were reading the same before you put on lower profile tires.

That is not quite true. I notice if I stop at a red light the GPS speed number they are still going down,while I have already come to a full stop. Yes it is close but not accurate.

"Position drift" while the GPS receiver is static can cause temporary small speed readings of 1 or 2 MPH. This is caused by position calculation error from small variations in signal strength and accuracy from the satellites. When moving, these small errors get averaged into the displayed speed and usually get cancelled out, having little or no effect on the displayed speed accuracy. I have tested the displayed speed on both of my nuvi's on many occasions using mile markers and a stop watch. The nuvi's are always spot on at highway speed (within the 1 MPH precision that the nuvi shows), while the speedometer readings on my 3 cars always read 2 to 3 MPH fast. I have also checked the nuvis aginst my OBD connected Ultragauge and it also matches. I find it interesting that the car's speedometer is off by about 2 MPH, but the OBD is reporting the correct speed.

and see a difference but only about 1 to 2 MPH with the Garmin giving the higher MPH. I also compared two Garmin units and they both gave the same speed. Not sure how much the diameter of the car tires make on the MPH on the odometer.

this issue has been discussed extensively. See node 6695:

http://www.poi-factory.com/node/6695

-jgracey

Here's a link where you can see the effect of different tire sizes. The circumference is the key parameter when you're figuring out the impact on speed. The OEM tire size should give you a baseline that you can compare the effect of different sizes to.

http://www.miata.net/garage/tirecalc.html

There's a bit of a caveat with that. Tire sizes are expressed as aspect ratios. So, a 205/70 will be 70% high as the tire is wide, i.e. 205 millimeters.

I can put on a 215/65 and it will be a marginal difference of -1.1% in diameter. Go wider yet, and the difference can actually go to the positive side. The wheel offset determines how wide you can go.

I noticed this a year ago when I purchased a Magellan Roadmate 1470, the Magellan speed was always 3 mph faster than my car speedometer, then I got a Garmin 2350LMT, the Garmin speed matched the car speedometer exactly, so one day I put both GPS's in the car and the GPS speeds where different.

I stand corrected. I was thinking "lower profile lower tire height" and ignored the statement that the new tire is wider.

I have noticed the same thing. In my RV I have a Scan Gauge II, The mph indicator is always a mile or two under what the speedometer indicates, my GPS is usually one mile above what the Scan Gauge indicates.

I always set my cruise control off the GPS, never the speedometer. I believe the GPS is the more accurate for the following:

I use a Garmin Edge 205 on my bicycle when riding and the speed is always calculated regardless of the size of my tires, the bike size, etc.

I can use the same speedometer in my car for speed and it is 100% accurate with my Nüvi 660 in the car.

Therefore - read paragraph #1.

By now you've come to the (correct) conclusion the GPS is more accurate than your vehicle speedometer. BUT, there's a way to fix that if it bothers you. There's a product called YellowBox that takes the reading from the sending unit (usually in the transmission) then corrects the number before sending the info to the speedometer.

I have one on my motorcycle (that was 10% off from the factory) and it's very simple to use. Just install, figure out the percentage your speed is off (comparing your GPS to the speedo) then set the dip switches on the YellowBox accordingly. The device is almost a must for those who've gone to much larger tires/rims on a truck.

Here's a link: http://yellr.com/

Cheers

Pontiac was spot on at 100kph
Toyota was 3 kph faster at 100kph

Run a red speed light camera and it will tell you how fast you were going, so long as you have the bucks to pay for it! Hmm, how much is 50 over going to cost?

I set my cruise control at 9 miles over the speed limit on the freeway and coast hwy.
Drive by the cops with the radar guns and never have been stoped.
I'll have to check my car speedo with the GPS.
Maybe I could be doing 3 more miles an hour
I think I'll just stick with my current 9 over, whatever it really is seems to work.

The GPS calculates your speed over a measured distance determined by GPS location over a rolling time scale. Therefore the speed displayed will lag a bit during acceleration and deceleration. But when driving at a relatively constant speed the GPS will be more accurate than the car's speedometer.

Same speed on both speedometer and Garmin 765t on my 2009 Chevy.

I drive a 2006 Ford E350 van and do "expedite deliveries". I have a garmin streetpilot c330. I know, I know, and yes it has grey whiskers and is very wrinkled. I have found it to be very adequate for what I do. I agree that I trust the gps speed over a short distance more so than the speedometer. The c330 has a feature that shows you your fastest speed, since you did a speed reset. Occasionally when I check fastest speed, It has read as high as 169 mph. Please go back and re-read my vehicle type. This is not a problem for me, I just get a kick out of seeing those speeds.

this has been my case as well whether in my dump truck or car the GPS always matches the radar read out, the truck speedo is always lower and the car is pretty much on the money.

This is the type of readout I'm referring too ...

http://www.trafficsafetywarehouse.com/images/RU2%208000.JPG

.

My GPS and Radar detector are in sync, the car is about 2 under. I would say both the Streepilot and 9500ix are the true reading.

...the auto makers are fined heavily if the speedometer reads lower than actual speed, so they build in a margin of error.

i believe gps is more acurate

I have used my GPS on my boat, on 2 motorcycles and many cars and I have found that almost all of the factory speedometers vary 1-4 mph. I always trust the GPS.

GPS is far more accurate than you car speedometer....

By whom? How and when is any discrepancy determined?

If you ever get a RADAR (LIDAR) ticket, be sure to check the calibration certificate. Many law enforcement agencies now a day don't have much budget to do it regularly.

If you ever catch an expired (invalid) calibration certificate, the traffic court will throw out that ticket without further question.

Personally, I got away 3 tickets in one city over a period of time.

I do my 10 over, and that's it. I feel no need to donate to the Gov coffers.

If you ever catch an expired (invalid) calibration certificate, the traffic court will throw out that ticket without further question.

Thanks for that info

I will keep this in mind, hopefully won't be pulled over anytime soon...

My Altima has a very accurate speedometer that I have checked with measured mileposts and a watch. My 255W is always within 1mph of the speedometer.

Agreed with the first quote. I doubt if Garmin gives any calibration certs with their products.

It's strange that my car speedometer always shows couple of km/h less than GPS.

I just wanted to thank all who responded to my question. This is a great web site, with so many knowledgeable members.