Questions for those that adjusted their tire size in ForScan
#1
Questions for those that adjusted their tire size in ForScan
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.
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.
#2
Senior Member
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.
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.
#3
Senior Member
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!
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!
Last edited by Eduskator; 09-07-2017 at 08:47 AM.
#4
It does affect it
PCM thinks you are running 50 and you are really running 55.
Either shifts too early or too late.
PCM thinks you are running 50 and you are really running 55.
Either shifts too early or too late.
#5
Senior Member
Isn't the shifting based on engine RPMs, not speed?
#7
Just The Basic Facts
iTrader: (4)
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 tire.
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