MadocHandyman

When changing your tire size with a tuner, do you use the actual numbers on the tire to calculate or do you measure the tire with weight on to calculate?
I get almost an inch difference with both and not really sure what I should input into the tuner.

DieselDawg

most tuners let you adjust revs per mile. I just installed the 5Star TS Livewire and used a revs per mile starting point and then GPS to get it pretty spot on. Then just go out and go 65mph for 10 miles (big enuff sample for "slow timing finger syndrome") using mile markers. Should take 55 seconds per mile at 65mph.

Chris's FX4

On my SCT it wanted Revs/mile, so that's what I put in. The specs sheet on your tire manufacturer's website should have the revs/mile.

MadocHandyman

I've got a Diablosport InTune. Just the cheap model. It only allows me to input the circumference of the tire. If I use the tire specs, it's 30.5" diameter but if I measure it with the weight of the truck the diameter is only 29.5". I set it to the 29.5 and will use the gps to check it tomorrow.

DieselDawg

the dia of the tire is still 30.5"...the flat contact patch doesn't change that. Definitely double check with GPS and Mile markers to be sure.

vozaday

On my old superchips it asks that you measure the tire with the weight of the truck. As the wheel diameter is affected with the weight on it.

DieselDawg

that does not change the length of tread that will contact the ground nor the distance traveled.

MadocHandyman

Good point, thank you.

vozaday

I realize what you are saying, I'm just saying what my superchips said to measure. And trust me it made a difference when I verified with my gps. If I put in the actual tire height as per the tire specs it was off and when I measured from the ground it was spot on. That's my real world experience.

rockhead11

Ditto, or if you put 275/65/20 on and running 3.73 gears just change your gearing to 3.55.