[vtirocz]

Can the dealer update the tire size in the BCM, if using a production tire offered on the 6th gen? For instance, on my SS, if I want to switch to the SS 1LE wheel/tire and not impact my speedometer or traction/stability control.

Is it possible to do this without going to the dealer?

I'd appreciate any info on this topic.

[wrankin]

I researched this and it is possible to program in tire sizes. PRAY does this for some local dealerships.

I spoke to my local dealership because I would like the MMR M017 knockoff version of the 1LE wheels and 1LE tire sizes. They would only update the tire size if I purchased the wheels from them.

Bill

[TJay74]

The tire size is not in the BCM, the dealers have no way to update or change the tire size programming at all. They can only program the tire size that VIN had in it when it left the factory.

If PRAY is doing it I am going to guess he is using HP Tuners to do it for the dealers and it is costing them at least $200 or so for each car he does since he would have to license each VIN individually.

[KingLT1]

+1 to what TJ said. Tire size is adjusted in the ECM.

[vtirocz]

Originally Posted by KingLT1

+1 to what TJ said. Tire size is adjusted in the ECM.

That's interesting. Wouldn't a change to the ECM tune void the powertrain warranty (related to anything controlled by the ECM)? I'm surprised GM didn't make it easier, since wheel/tire swaps are common on this type of car.

[VinnAY]

Totally different software and hardware I'm sure but the Mustang guys I believe figured out some "as built data guide" or some such that one thing you could do was change tire sizes.

[wnta1ss]

The tire size is in the ECM as mentioned. I have not read a stock SS/1LE tune, but read a supposedly stock ZL1 file (same 305/30-20 Supercar 3 as the SS/1LE) and it had the same tire revs/mile as my stock SS tune (275/35-20 F1 runflats) had. So it looks like GM did not bother to adjust for the slightly shorter tire in that case.

[Alpha1BC]

I had SS 1LE sized wheels/tires on my SS, no re-cal needed. GPS speeds were still accurate to speedometer speeds.

[arpad_m]

Originally Posted by Alpha1BC

I had SS 1LE sized wheels/tires on my SS, no re-cal needed. GPS speeds were still accurate to speedometer speeds.

Are you sure? If yes, then I may not understand tire sizing, please let me know why this is the case.

[TJay74]

Here is the SS 1LE / ZL1 tire info:

Size 1 285/30R20 Size 2 305/30R20

Compare
Size 1 Size 2

Diameter
26.7" 27.2"

Sidewall
3.4" 3.6"

Circum.
83.9" 85.4"

Revs/Mile
755 742


Here is the SS:

Size 1 245/40R20 Size 2 275/35R20

Size 1 Size 2

Diameter
27.7" 27.6"

Sidewall
3.9" 3.8"

Circum.
87" 86.6"

Revs/Mile
728 732

[TJay74]

GM drives the speed off of the rear wheels, so there is only 10 revs per mile difference in the 2 main tire sizes GM uses for the Camaro. Even the tires on the I4 and V6 has the rev per mile listed at 730.

The ZL1-1LE is listed at 756 rev per mile, so I suspect it has a slightly different calculation used for its speedo calibration.

[wrankin]

Originally Posted by TJay74

GM drives the speed off of the rear wheels, so there is only 10 revs per mile difference in the 2 main tire sizes GM uses for the Camaro. Even the tires on the I4 and V6 has the rev per mile listed at 730.

The ZL1-1LE is listed at 756 rev per mile, so I suspect it has a slightly different calculation used for its speedo calibration.

Interesting info.

Does anyone know what the differential front to back tire sizes or ratio is that causes problems with the ECM or ABS (not sure what controls this) reducing power when drag racing? I've heard about a fuse pull work around to remedy this, but also heard that it also cuts out the power steering.

Bill

[Shawnqa800720]

Great thread guys!

I am far less concerned about speedometer impact than I would be about other vehicle control programming, and I am curious if anyone on this forum would know?

Things like the Snow and Ice traction control, I assume, judges wheel rotation and slippage, and adjusts to help the driver maintain control. If the diameter changes, then the minute calculations the computer is doing to adjust would be off, and could impact performance (or safety).

wnta1ss, you mentioned you read the file - was that just a speedo file, or does that govern all control systems related to tire diameter? If you don't know, do you know anyone on these forums who may be able to help?

I think this would be an important piece of information to know - very relieved if tire diameter is a single entry to govern all related control systems and Chevy didn't deem it important enough to be be specific to every model/variation. But would need to be cautious if tire diameter is linked to any other control systems (drive mode selection, ABS, etc.) and is varied by model within those other systems.

[Captrob]

This post came up just in time. I am ordering the MRR M017’s on Monday. The one thing I was still uncertain on was to try and keep the same heights 40/35 or go with 30’s all the way around.