Does anyone know how to adjust tire size and gear ratio in forscan? I went for 331 to 373 gears and I’m putting 33” tires on it. I would rather do this with forscan than buying a tuner. Thanks!
2019 XLT 5.0 4X4

 

Drive it after you get the tires on it and compare the speedometer to a GPS. Between changing axle ratio's AND adding bigger tires I wouldn't be surprised if the 2 don't cancel each other out. You may find your speedometer doesn't need to be recalibrated.

If you're within 2-3 mph at 70 that is about as close as you'll get anyway. Your tires will get smaller as they wear down. If you get everything calibrated perfectly with new tires, you'll be reading 2-3 mph faster than you're actually driving when the tires are nearly used up. If you're reading 2-3 mph slower than you're actually driving while the tires are new, you'll be accurate later when the tires wear down.

 

isn’t there multiple reasons to update with correct inputs? Thought it would also affect the transmission/shift points (and not just odometer/speedometer)?

 

This was true for older trucks but on these new ones speed is measured from the wheels so I dont think it would change the speedo by doing gears.

 

yes. You want the truck to relearn the NV ratio(basically engine rpm vs tire rpm at a 1:1 transmission gear). The PCM will adjust the shift points accordingly

 

Yes, go into the BCM and change the settings. 3.73 should be a default option, but you may have to type in the tire size manually since I am not sure any 2019's came with a factory 33. I believe the tire size is input as the driven circumference, which means it accounts for the tire being compressed under the weight of the truck and isnt truly a 33" diameter. Best way to approximate it would be the measure from the hub to the ground, times 2, times pi. Hub to Ground * 2 * PI. That should put you in the ballpark.

For example, my 315/70R17 Raptor takeoffs are acutally in the computer as a 2672mm driven circumference. This works out to a 33.5" tire even though they are really a 34.5.

 

So how close is your speedo to GPS reading?

I have changed my tire size parameter and have no change to accuracy on my speedo or odometer. I am traveling about 6% faster (speedo error) and 6% further (odometer reading low compared to actual distance travelled).

I have a 2012 Scab 5.0 4x4 with 3.55s and 275/65/20 tires. The math says the difference from my OEM tires to these is about 6%. I have changed the tire size parameter, both in the AS-BUILT file and the regular fill. Neither made any change in this error. Am I missing a location to change the parameters (in a different module)?

 

There is a limit to how large of a tire the PCM will accept from the BCM on the 2011-2014 trucks. Above that size and the pcm will throw a code and ignore it. Scan for codes and see if you are getting one.

if that is the case you need to tune or device to adjust the pcm.

2015+ trucks I believe will allow up to a 315/70R17 or equivalent diameter.

 

2012s list that tire size as an option too. I tried that tire size with no change to odometer and speedo. I even did the PCM relearn what BCM parameters are in case that mattered. Still no change.

Is there another module that would calculate tire speed?

 

it might in the bcm but the pcm rejects it if its over a 32” tire i believe. You probably have a code if you scan

In the PCM tune, there is a setting to tell the PCM to read the tire size from the bcm or to use the size in pcm itself. From the factory, the pcm is set to read from the bcm but if the bcm reports too large of a size(because you changed it) the pcm doesnt like it and will ignore it