Ive went to several different formulas and calculators and the number I get is 8.26 with 33's. My problem is when I'm going down the highway, faster than 55 mph it speeds up gradually till its about 5-6 mph off at 75. I only have 3 cal's left and plan on sticking with 33's. I have the Pro Comp A/T's from 2014. Although this may make my mpg seem better, lol I don't want my odometer climbing so fast!

 

Sounds like a bad speed control module.

 

I would get some string and wrap it around your tires to measure the actual circumference. I've seen "33s" that measure more like 32, and "34"s that are closer to "35s". Those number sizes can be pretty rough estimates depending on the tire manufacturer's tolerances.

 

Exactly. And most manufactures have RPM on their web site

 

What would the size of the tire have to do with the truck speeding up gradually on its own?
Guessing he is running with speed control on.
My truck did this also and I changed out the speed control module and now it holds the speed it is set too.

 

My truck isn't speeding up, it's the gauge. It's accurate up to 55 mph, then as u go faster, say 65, the gauge will read 71 or so.

 

It is the multiplier effect. The faster you go the further it is off

 

Changing the tire setup is not going to fix this.
In this case I would say its the speedometer head if it is just the needle that is off.
If it is both the speedometer needle and the mileage in the odometer that is off then I would say replace the PSOM.

Did you check the odometer to see if it was off with mileage markers or are you guessing it is off because the speedometer needle is off.

 

I just measured my tires with weight on it and it's actually 32". The conversion number is 851. Gonna try that out. I should have physically measured the tire instead of relying on pro comp's tire data. Looks like I'll have to push my reset button a couple hundred times to go back up to 851. I'll post my results

 

Well after taking a little highway drive after my CAL adjustment, way better results. I think it could be a tiny bit off, it seems to be pretty accurate now. My speedo used to fluctuate around 75 and go up and down, but I cleaned my VSS and it's connections and my needle is steady.