I bought a new truck. With 37" tires how many miles per hour off are most people seeing. I figured it would show on the speedo that you are driving slower than u actually are.
(Example- Driving 75 but your speedo only shows you going 70.)

Im pretty sure mine is backwards. When i'm driving 70 my speedo says 73.
If the speed limit is 55 I do 58.

Just thought it was supposed to be the other way around. sorry for the stupid question

Is there any way to reset the pcm ot gauge cluster? I did it on my 04 hemi just not sure if it is the same procedure? It is an 04 diesel, 6spd, 4x4.

 

What was your original tire size? If they were smaller than 37", the reading CAN'T be backwards. Do you have a GPS? It'll show your true speed and distance covered. You can compare that to your instrumentation.

Mine is 4.3% low. That is, an indicated 76 mph is actually 80. I just live with it, adding 4.3% to the mileage when I fill up.

 

Original was 265,70,17. I understand what u are saying and agree with u. However that's not what my truck is doing. That's why I posted. I just rechecked with a known good gps and when speedo says 50mph. My gps reads 47 mph

 

when mine says 60, i'm doing 70... when my speedo says 85 i'm going way too fast lol

 

It's a function of the percentage of height diference betwee what came stock and what's on it now. your original tire was about 31 inches tall. Going to a larger tire should make your speedo read slower than you are actually traveling. I dropped from a 33 inch stock tire to a 32 inch and my speedo reads about 3 % faster than I'm actually going. You went up 6 inches. How's the gas mileage up there?

 

I get about 16-17 city, 19 highway.

 

gotta love the cummins, more hp and torque and same if not better mpg

 

If you calculate fuel cost/mile, the difference ain't so great.

 

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

 

you bought the truck with the 37's on it already..... possible the previous owner had it calibrated, but just did it wrong