More Accurate - Speedo or GPS?

95Nelson · May 19, 2007

I recently bought a Garmin GPS and noticed that the speed the GPS shows I'm traveling in my 06 GT is about 2 mph lower than my speedo. In my other vehicle, the GPS shows about 1 mph lower. BTW, that difference is at highway speeds of 60-80 mph. My question - Which is more accurate?

'01 Steed · May 19, 2007

I'm gonna guess that each has a margin of error of atleast 2-3%, maybe more.

Easy way to check is to find a 10 mile stretch of highway and stopwatch that 10 mile run at 60 mph. Should be exactly 10 minutes. Any delta can be used to calculate your true speed.

Also, what is your tire situation?

95Nelson · May 19, 2007

I have the stock bullit rims and tires with about 15k miles - and only 1 really nice burnout/donut so far. I do have a custom tune and CAI, but I don't think he changed anything on speedo or gear ratio.

'01 Steed · May 19, 2007

Well, if anything, you're speedo reads faster as tires wear more.

Try the 10 mile dash thing and compare the GPS to the speedo. Post back. I'm intersted in the findings. If I had to pick the one I thought would be more accurate, I'd side with the GPS.

95Nelson · May 19, 2007

My assumption would be the GPS, too. It may be a while before I do the 10 mile check as I don't know when I'll hit the interstate again. When I've done that before I just used the distance on the odometer rather than poke along at 60 mph and time it. 60 mph is just too slow for me on the interstate. Of course I could try doing 90 and see if I average a mile every 45 seconds = 10 miles in 7:30 I'm sure the cops would understand it's test and let me go.

notoriouspig · May 19, 2007

cars are required to read at or lower that the speedo indicates... therefore most car manufacturers have there speedo read one or two mph below the inidicated speed... I am a police officer and were have to have are speedo's certified routinely... they on average read 2 higher that actual speed. Also having radar in my car i can constantly compare the speed indicated by the speedo and the radar... the radar is more acurate and will also read about 2 mph lower than indicated by the speedo... the higher the speed the more the error... but up to 80 the typical speedo reads 2 mph high..

Marine One · May 19, 2007

I went through the same drill with my GPS and my 07 when it was new. The speedo consistently read 2 MPH slower than the GPS. I used a stopwatch and confirmed that the speedo was registering 2MPH slow. I have the IUP with the trip computer built into the speedo. What I have found is that if I set the cruise control then select the average speed function and hit reset it will show the actual speed the car is traveling. I've verified this using the stopwatch on a measured course. Something worth trying if your car is equipped with this feature.

95Nelson · May 20, 2007

notoriouspig - thanks for the info. So if you pulled me over doing 90 mph in a 70 speed zone, would let me go knowing that I was only testing the accuracy of my speedo? You probably can't (or won't) comment on this, but it seems that most law enforcement personnel don't give tickets for anything less than 10 over, so I usually do 9 over (which is really only 7 over due to the speedo reading).

Marine One - thanks for the suggestion. I'll have to try the average speed check on my IUP as well.

351CJ · May 20, 2007

Every Ford I've owned (and there have been a lot of them) had speedos that read about 2 MPH higher than actual speed when the tires were new.

Aa number of years ago C&D tested a lot of vehicles for Speedo & Odo accuracy. While they found all the odo's to be very accurate, virtually every vehicle they tested has speedos that read high. Fords were actually about the most accurate at typically 2 MPH above actual speed. Some of the imports read over 5 MPH high at 65 MPH.

IIRC in the article although C&D could not get any official comments from the car companies, but off the record some of them told C&D that speedometers are intentional set to read high to keep customers from sueing them over tickets because the speedo was wrong.

notoriouspig · May 20, 2007

95nelson.....

maybe if u stopped and asked me to and there wasn't much traffic I would let u know.... actually i have done that for several people who have had broken speedos or have changed gearing....

Marine One · May 20, 2007

95Nelson said:
notoriouspig - thanks for the info. So if you pulled me over doing 90 mph in a 70 speed zone, would let me go knowing that I was only testing the accuracy of my speedo? You probably can't (or won't) comment on this, but it seems that most law enforcement personnel don't give tickets for anything less than 10 over, so I usually do 9 over (which is really only 7 over due to the speedo reading).

Marine One - thanks for the suggestion. I'll have to try the average speed check on my IUP as well.

Let me know what you find.

SlideWRX · May 21, 2007

The speedo should definitely read higher. I'd trust the GPS indicated speed; we've used GPS to calibrate the TSD trip computer in the rally car.

My mustang reads ~2 mph high at 70mph.

Tom

V. Barbarino · May 21, 2007

My lowrance gps system is about 2-3mph off when at highway speeds. The GPS is 2-3 mph higher than the speedo. I set my cruise control based on my GPS.

ps. I love love love love the digital speed indicator on my gps, wish I could get it in the middle of my speedometer.

95Nelson · May 24, 2007

Ok, so I tried the IUP - using the average speed reading and hitting the reset button. It worked and indicated the speedo was about 2 mph high. It's good to know as, like I stated before, I usually cruise at about 8-9 mph over the limit and haven't received any tickets for doing it - hold on a minute while I knock on a piece of wood

I, too, like the digital readout of the GPS for speed. I had that on a Buick Regal and liked it then. An anolog reading is harder to be accurate - am I going 62 or 63 mph... it's hard to tell. I don't think I'd like a digital tach but then again, I've never had one.

Thanks for all the info and advice. I hope this has helped others as well.

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