RPM's vs. Speed
 

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 :D

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.