When I was reprogramming my truck to disable the MDS (with Superchips Flashpaq), the programmer makes you look at all the current vehicle settings. I noticed that the tire height was set to 31.5 in which is the stock 17's. I have 20's that are 33 in tall. I thought that maybe this was just what the programmer was set to from the factory, so I went ahead and changed it to 33 in. I drove the truck to work today and noticed that my rpm's at 70 mph are now 200 lower than before. I wonder if my truck was programmed for 17's from the factory. If I run my rpm's up to where they used to be at 70 mph my speedo now shows that I'm now going about 74 mph. What do you guys/gals think?

 

taller tires lower your rpm at the same speeds if the speedo wasnt reprogrammed

 

I bought the truck new a couple months ago, so I just figured the speedo should be accurate.

 

most speedos are never 100% accurate

changing your tire pressure can even offset it

4 mph is nothing major..... check it against a gps, i bet its even more off.... most speedometers are usually within 10 mph of real speed....

 

Speed past a cop and ask

 

yea, sounds like it was programmed wrong from the factory. heard about it plenty before, swapping wheels and all that.

 

My speedo reads exactly the same as my gps.