I switched to 315s. My MPGs were about 2-3mpg off on the overhead from what I hand calculated. I got the speedo recalibrated and it is good now, but my overhead now is about 6 mpg high from what i recalibrated. What gives?

 

I would like to get my speedo recalibrated as well, how was yours done?

 

raceguy,

It's called adjusting the "pinion factor". The dealer can do it with a DRB tool that plugs into the computer access plug below the steering wheel, or you can do it with a Smarty.

Some dealers don't know how to do it or say it can't be done. Others will charge an hour to do it. Others will just do it for free. It only takes about three minutes if they are slow and waste a minute or two. Very easy with the right tool.

The dealer will select the size tire you have or enter a revolutions per mile number in one of the menus (depending on the year and tire size). The Smarty allows you to simply enter the diameter of the tires you're running. Get the diameter or the revolutions per mile number from The Tirerack.com or the tire manufacturer's website.

Fill in your signature so we can see what truck you have.



John

 

Yeah the adjusted the pinion factor. Charged me an hour for it! They used a figure of 537 revs per mile. Do the math, I dont think that is right. That equals out to a 36" tire. My 315s only measure 33". But why is my overhead 5-6 mpg off?

 

You need to adjust the revolutions per mile figure. Check on tirerack.com for the figures. The 315 BFG a/t's have 601 revs per mile.
I recalibrated my speedo at a dealership with that number and it is dead nuts on. My overhead computer has always read a higher miles per gallon even with the stock set-up. I hand calculate ~16 mph when the overhead reads 20 consistantly.

 

Ok thanks guys I will take it into the dealer and get it fixed. I really didn't want to spend $200 on a true speed, so this is great info to see.