rsutton1223

About a week after I put the lift on with 35's, I changed the tire size in ForScan as it was off about 6-7 mph depending on how fast I was going. After the change...the truck seems to drive a lot smoother and it doesn't seem to be affected by the tire size as much.

Am I feeling a placebo effect here or does that stat play into multiple parts of the tune/truck function? I have to imagine it affects the trans tune at least some. I know it does with the MPG calculation because that went up 1 mpg right after the change.

11screw50

Well, the mpg calculation should be higher than it was because the truck went from thinking it traveled whatever the distance for the number of tire revolutions with the stock size was to a more correct value. Basically the truck thought you traveled less distance before you corrected the tire size.

Eduskator

It would make sense to say that it does not affect anything else than the speedometer itself, but I'm not 100% sure. It basically adjusts the RPM (revs per miles) to match your new tire size.

And you are right, the speedo inaccuracy thing is exponential - the faster you go, the bigger the gap between the read & the actual speed is. At 50, it will read 55, at 70, it will read 80... something like that (vice versa, depending if you go with bigger or smaller tires).


Going with 33s (275/60r20) on my F150 this month (instead of the OEM 55 tires). It will correct the factory inaccuracy and my reading will be dead on accurate!

twoboosted

It does affect it
PCM thinks you are running 50 and you are really running 55.
Either shifts too early or too late.

MarkY

From your build thread, you have a 3.5 EB, correct? After changing your tire size in FOREScan, did it throw a fault (wrench)?

Isn't the shifting based on engine RPMs, not speed?

danlong

iTrader: (2)

Shifting is based on a multitude of factors such as speed, RPM, throttle position, etc... The speedo being off by several MPH will throw it off considerably.

Husker

iTrader: (4)

I don't understand why it would increase the gap the faster ya go???

danlong

iTrader: (2)

It's based on a percentage, so if you increase your tire size by 10%, your speedometer is going to read 10% slow. Therefore, when it reads 50 you're actually doing about 55, but at 70 you're doing about 77.

rsutton1223

No codes and it matches up perfectly with GPS now.

Husker

iTrader: (4)

Guess I'll have to prove that theory to myself. I'll set my speedometer back to stock, then do some test GPS runs at different speeds...I thought my speedometer was off by 3-4mph across the entire dial.