I've witnessed alot of posts on here from guys talking about their gas mileage (mpg), but I rarely see where they take into consideration their tire size on their mpg calculation.

Your odometer was calculated at the factory for your factory tires and if you change your tire size, your odometer will not be accurate.

Case in point; my '96 SLT came from the factory with 225/75/16 which gives me a circumference of 28.75 inches [((width*aspect ratio)/26.5)*2+wheel diameter]. However, when I changed my tire to a 265/75/16, the increase in width, led to an increase in sidewall height of 1.25 inches due to the constant aspect ratio of 75%, which increased my circumference to 31 inches or 8%. Therefore, I am covering 8% more miles than my odometer is showing.

So don't forget to add/subtract your tire difference to your mpg.

BTW...I'm getting 14.4 mpg (before adding the 8%) and 15.5 mpg (after adding the 8%), with a 13 year old 5.9l 360!

 

That's cool information. But all that math

 

Not to mention isn't our trucks calibrated for 28.5" tires? I know my speedo was off 3-4 miles an hour at 55mph before reprogramming to the 30" tire size that is stock on my truck.

 

yeah, but take into consideration that your new tires are heavier and wider

 

Its a dodge ram. aka stump puller, who worries about gas mileage

 

I could be wrong, but I've read in several places that the PCM will recalibrate for up to a 33" tire on these trucks. I have checked mine with a GPS and it is spot on with my oversize tires.

 

and also take into consideration that the truck may have came with 245/75/16(factory) like mine and but the door said 225/75/16.. belive me ive known the truck since it had about 20thousand miles.... and i got it with 42,000 with the org tires

just throwing that out there alot of these truck came with bigger tires than what the door said

 

what about rear gears? ima be moving up from the stock 3.55 open dif to a 4.11 det. locker setup and was wondering about that omw to work today.

 

The gears will help you move larger tires, obviously, but they are completely independent from your speed.

The trucks can't adjust for tire size. How is it supposed to "detect" the size of the tire? My SC 3815 said the truck was calibrated for 245/75R16 tires even though it came with 265/75R16 tires from the factory. If you have larger, oversized tires and your speedo is spot on, then it was calibrated wrong a while ago and you got lucky, or a previous owner who had the same size of tires on at some point had it calibrated for that size.

 

On a truck with ABS it would be easy for the PCM to recalibrate itself. It is measuring wheel rotation and speed all the time and it can compare distance traveled to see if there's any change and readjust. Like I said, I don't know if it can do this, but there are a number of sites where people have said it can. I know the history of my truck since it was new and the original owner says he never had it recalibrated.