I took the truck in this morning and told them to set the pinion at 601 as suggested here. I have a gps in the cab and when the truck says 60 I am going 69.9 on the gps(it is a solid unit that is very accurate) Can some one please help me get this right. It says on the door jam that it had 245/16's stock. Now I have 315/17's. I need to get it back in today so hopefully they will do it again for free since I provided them with the 601 rpm figure.

EDIT:Please note.... I have 3.55 gears

 

According to this tire size calculator, you'll need 587 revs/mile.
I'm using 599 for my 315/70-17's and it's about right.

HTH,
brandon.

 

Dealer punched in 601: I was 10mph over
Had them punch in 587: still about 9mph over.
I checked my GPS with my Jeep and it is dead on.

WHAT THE heck is going on.

315/70/17's

 

Maybe the truck is calibrated with the wrong tone ring on the ring gear? Have you ever cahnged the rear speed sensor on the rear diff? ???

brandon.

 

601 to 587 should have been a bigger difference than that.

Maybe he isn't clicking "save" when he is done putting in the revs per mile?


phox

 

No.... I haven't changed the rear speed sensor on the rear diff. The dealer I had do the calibration was out of town and they did it for $40. My local dealer wants 1.5 hrs labor !!? Jerks!!
I am so bummed over this. All I want is the milage to be close and for my shiftpoints to be accurate.

 

I don't know?

 

Agree that 587 changed to 601 should be more than just 1 mph difference!
I checked 2 different computer calculators - both come up 587 at 70 mph!
That should be right.
BFG says 601 revs at 45 mph. That # would reduce slightly as you go faster. Somewhere between those two...around the average.. 595 revs... has to be in the ball park.
Think I would trust the laws of physics over my GPS, assuming the dealer did his job correctly.
RJ

 

OH! Just had an idea based off of RJ's thoughts... Are you running the tires full of air? My BFG AT 315/70/17's sidewalls say the max PSI is 55psi and I run them at 55psi. Maybe that's the difference between 587 and 601. Are you carrying any weight in the back when testing the speedo? Does it change if you tow or not? Maybe the sidewall squat affects the speedo more than we thought. ???

brandon.

 

I had the dealer put in 600 for my 315/75/16 (265/75/16 stock) and it's within 1 mph.

 

Good point! If a tire can do 14 fewer revs per mile at 70 mph...than at 45 mph, than tire squat has to make a difference..No?

i'm still having trouble with the 14 fewer revs number...seems like an awlfully big difference. On the other hand...the bigger the tire (or at least, the lower the pressure)...the bigger the difference, I would think?
RJ

 

I run 45#'s all the way around. I'm going to the dealer one last time on Friday and watch over his shoulder and ask to see the input. This is very agrivating.

 

Turbo Deezle, do you have a fueling box hooked up that you're unplugging before you go into the dealer then plugging back in after you left? I put 315's on my truck and got the speedo recalibrated. I have an Edge EZ but I unplugged it before i went to the dealer so it wouldn't possibly interfere with the ECU during the reprogram. I went to the nearest gas station after having it recalibrated and plugged my EZ back in. Somehow, plugging the EZ back in caused my ECU to dump the new settings that the dealer just plugged in. I immediately took it right back in, told them about the chip being plugged in and they reprogrammed it for free with the chip plugged in. Speed is within 1 MPH now. I just can't unplugg the EZ ever.