duddin10

My dad has a 2007 5.9L truck. He bought it with a conversion package on it. The tires on it are 35x13.5x20. The speedometer has never been correct. He bought a triple dog with outlook monitor so he could adjust for tires. We put in the tire size height ( 33.50 inches) on the monitor and the speedometer was showing that he was going faster than he was actually going. So I updated to version 2.4.1 so we could try it on the programmer too. We put in and the speedometer and outlook both showed different readings. (5mph difference) The funny thing is neither was correct. We judged it by following me and my cruise set on 65. Ok, so we start trying different sizes and here is what we came up with. In all cases he would follow me and try to set same speed as myself in my truck.

In all tests my cruise was on 65. We also used my moms chrysler 300c as a test car too.

34" the monitor read 79 the speedometer read 70
32" the monitor read 75 the speedomter read 72
31.50" the monitor read 66 the speedometer read 69


So we now have monitor set on 31.25" and the triple dog set on 30.50".

With those values the monitor reads 66 and the speedometer reads 68..

My problem with that is that my stock tires and wheels are 30.50. How is it possible that by setting them to stock and close to stock size that they are closer than by entering the actual tire size? Can someone explain this?

Does anyone have the same size tires or something close to that? If so , what size do you have entered in? Sorry for the long post but we are totally confused.

cbrahs

on mine with 37 in tires, I take the speedo reading and divide by .83. If it says I am doing 65 I am actually doing 78mph. Have checked with several buddies. something is going on, maybe the dealer attempted to program for tire height the best they could and it is conflicting with your inputs?