speedometer or gps, which do i beleive???

[peppster66]

Hi, this problem, has bugged me for some time now, when driving down the highway, the Gps will show that I'm traveling at 65 mph, but my speedometer will show that I'm only going 58mph, there is always a difference at all speeds, so my question is which one do I believe, Ive just been following my speedometer up until now, any thoughts on this would be greatly appreciated,,, thanks,,,,

[booster]

Originally Posted by peppster66

Hi, this problem, has bugged me for some time now, when driving down the highway, the Gps will show that I'm traveling at 65 mph, but my speedometer will show that I'm only going 58mph, there is always a difference at all speeds, so my question is which one do I believe, Ive just been following my speedometer up until now, any thoughts on this would be greatly appreciated,,, thanks,,,,

GPS for sure

[WJones]

I would think GPS also. There are many ways a speedometer could be off. Must less likely a GPS, especially in a straight line.
Do you have another vehicle? If so, it would be simple enough to try the GPS in the other vehicle and compare it to the speedometer.

[booster]

If you feel the need to verify, use the highway mile markers and a stopwatch. 60mph = 1 mile per minute.

A less direct, but very good is to use the GPS miles against the highway markers, as you can go longer distances that way for accuracy. If the odometer is correct, the GPS is very likely to be able to calculate the speed right.

[peppster66]

Thanks guys, I will try that test, I will let you know how it works, thanks,,,,,

[DCHitt]

Originally Posted by peppster66

Thanks guys, I will try that test, I will let you know how it works, thanks,,,,,

I'll bet you have big tires.

[booster]

Originally Posted by DCHitt

I'll bet you have big tires.

That would have to be a big tire size increase, I think. He has a 12% speed difference, so that would change the diameter of a 30" tire to 33.6", which is getting pretty large. A 295-75-16 is 33.5" diameter.

I think peppster has a Dodge so he would have started with 225-75-16 at 29.2" diameter or 245-75-16 at 30.5" diameter, so he would be at 32.7" or 34.1" respectively to have the 12% error. Add to that the fact that speedos usually read about 2mph slow, and it would be even more increase.

If the GPS turns out to be right, I would guess there is a problem with the speedometer or the sending unit to it.

[peppster66]

Yes, im using standard tires that the
roadtrek came with, I beleive they are 245/75/16's,

[DCHitt]

Originally Posted by booster

That would have to be a big tire size increase, I think. He has a 12% speed difference, so that would change the diameter of a 30" tire to 33.6", which is getting pretty large. A 295-75-16 is 33.5" diameter.

Math check.... I think that would be a 12% increase in circumference not diameter. Since he is using the standard tires the question is moot and my brain is too tired to figure out the real answer. C=2piR so I don't think it scales.

[booster]

Yep, pi times the diameter, no exponents so ratios work. Bigger tire equals less rpm, so his speedo reads low because it works off rpm. The diameters I listed were how big the oversized tire diameters would have to be to give him that much lower speedo to GPS speed.

[DCHitt]

Originally Posted by booster

Yep, pi times the diameter, no exponents so ratios work.

Thanks, I did say my brain was tired...