A little help please.  I have stock tires and as far as I know, the gear ratio hasn't been altered.  Yet my speedo consistently reads miles per hour slower than my GPS.  What could be the issue?

MATH!

 

Draw a chaulkline on the tire perpendicular to the ground. drive exactly one tire rotation. Measure that distance, then do the math to see how many of those rotations make a mile. You can also use a measured mile to time the car.

 

This is not unusual actually. Reading slower then GPS is better then reading faster (shortens the clock on the warranty). 

 

Since subaru is a small mfg they may use the same TCU and transmission on cars with slightly different tire sizes hence the difference. It is also possible your GPS is slightly off. 

I've always seen it the other way, speedometer reads slightly faster than a GPS.

 

I've also found that some speedometers dont read accurately compared to the speed signal input that the ECU gets.

For instance, my 96 Legacy L, before the lift kit when I was running stock sized 185/75/14 tires, the speedometer read dead on 70 mph, but ECU indicated vehice speed was 68, and GPS indicated speed was 66.

My 2004 Outback sedan is the same way. With stock size tires, it would read 3mph faster then both the ECU and GPS would read. I'm now 5mph fast since I swapped to a different tire size but it's consistently off. Haven't figured it out, maybe some extra resistance in the speedo head? I've got a lot of other projects to work on and it doesn't really bother me.

Yeah my 95 Legacy L's speedometer reads about 3 miles an hour or so slower than my gps also. Stock tires and everything. Just figured old car, old hardware, whatever. I noticed it a few weeks ago.

its pretty typical, most vehicle say they are going a few mile per hour faster than you actually are. its done on purpose, its supposed to make people safer, since they think they are speeding but they arent.

I was trying to see how much treadwear would impact it. Figuring 4mph slow at 55mph would be about 7.2% slow. With 225/60r16 tires it looks like it would take nearly an inch of treadwear to make it that slow. So not likely treadwear unless it's say super swampers that have been run down to slicks haha.

tires woulds be the main issues, but i recall Subaru and maybe some other brands forced to offer extended warranties because they had some cars' odos off by over 3%

If you consider that a 5% to 7% speedo error on the fast side adds 5000 to 7000 miles to your odometer at 100,000 miles, that's a lot. It also inflates your fuel mileage. If you want to sell your ride, at let's say 150,000 miles by the odometer, you actually only have 143,000 on it,not much diff, but it will cost you.

I understand the reasoning behind it, as far as safety is concerned, but it's annoying to go somewhere and you know its only 400 miles and the odometer say's 420 miles, it adds up U know. I put 215/65R/16 tires on my '03 Forester to help correct the speedo error,and it goes better on snow and at the beach, has more PSI of rubber contact to the ground and the speedo agrees with the GPS now. Seems like SOA should have done that, but they may like the higher EPA fuel mileage figures and seeing us more often at the dealership.  

If your worried about resale value at 150,000 miles, may I suggest that one never buys a new car.

 

Also  consider this, that tire sizes are not an exact measurement (physical measurement is) between mfg's. Who knows if your new car bought today will even survive to 150K (hey its a rough world out there). 

Edited October 10, 201510 yr by nipper