35 to 37 Speedometer Calibration

[RaptorDrew]

My '24 is at the dealership and their mod shop getting a series of goodies installed and I had asked that they calibrate my speedometer (since I put 37s on my 801a) while it was there. They just advised that they don't yet have a solution for the '24s. Does anyone know of a new tuning option on the market which provides for this?

(Yes, I have used the Search function and no, I could not locate an updated assessment.)

 

[hanj0208]

Go check out forscan thread. Plenty of information there. I went from 35 to 37 and still haven’t done it yet. It is about 1-2mph difference. No big deal.

 

[Hemmy]

Go check out forscan thread. Plenty of information there. I went from 35 to 37 and still haven’t done it yet. It is about 1-2mph difference. No big deal.

I also went from 35 to 37's. How are you determining your speedometer difference? I am using Waze and at higher speeds on the highway, I am 3 mph off (truck is reading slower than Waze). In town, I am 2 mph off. Is Waze accurate?
I have to do the Forscan, just waiting to do it. I have to take my truck in for some work and don't want changing anything prior to going to screw up my extended warranty.

 

[hanj0208]

I also went from 35 to 37's. How are you determining your speedometer difference? I am using Waze and at higher speeds on the highway, I am 3 mph off (truck is reading slower than Waze). In town, I am 2 mph off. Is Waze accurate?
I have to do the Forscan, just waiting to do it. I have to take my truck in for some work and don't want changing anything prior to going to screw up my extended warranty.

Same. Using gps speed by google map or Waze. Your speedometer is going to show slow than actual speed.

No warranty is going to be voided. I done few other forscan “mod” and they never questioned or voided my warranty when I took my truck for service multiple times.

 

[RaptorDrew]

Go check out forscan thread. Plenty of information there. I went from 35 to 37 and still haven’t done it yet. It is about 1-2mph difference. No big deal.

That's what the mod shop uses but apparently there's not currently a fix for the '24s and '25s.

 

[hanj0208]

That's what the mod shop uses but apparently there's not currently a fix for the '24s and '25s.

Yeah. I just noticed on forscan thread. I heard they are trying to add many VIN as possible. One of the member reached out to their team and was able to use forscan. I am sure it will be available to 24-25 shorty.

Once it becomes available try doing it yourself instead of paying someone. It isn't too difficult. If I can do it, I am sure everyone else can.

 

[MrLiou168]

Assuming the inverse is also true? I have a dedicated snow set of 35's that I use in the winter months here in the Rockies. So I need to calibrate from 37 to 35.

 

[taquitos]

Assuming the inverse is also true? I have a dedicated snow set of 35's that I use in the winter months here in the Rockies. So I need to calibrate from 37 to 35.

Yes you can go either way. You can set your tire circumference to anything you want actually. If you look up changing tire size with forscan you’ll find there are four values used to set tire size. It just so happens that with 35 and 37 tires only two of those are different.

 

[KustomAF]

I also went from 35 to 37's. How are you determining your speedometer difference? I am using Waze and at higher speeds on the highway, I am 3 mph off (truck is reading slower than Waze). In town, I am 2 mph off. Is Waze accurate?
I have to do the Forscan, just waiting to do it. I have to take my truck in for some work and don't want changing anything prior to going to screw up my extended warranty.

It’ll be off by a percentage. The ACTUAL diameter of your original tire/actual diameter of the new tire. The odometer will be off by the same percentage. About 3.6% if we’re talking stock 35 ko2’s to 37” ko2s

 

[MORaptor1]

There is a simple forscan code change already built into the bcm/ecu for these (21-23 for sure) trucks. There is no need to measure anything. Just look it up. You only swap 1 or 2 digits. The trucks were all built to be either 35 or 37, so the simple change is there.