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Discussion Starter · #1 ·
I recently bought a GDE tune for 3.91 gear ratio (even though mine has 3.55's) because it was too good to pass up.

I was thinking of possible workarounds for the different gear ratios, and decided to try tricking the BCM by changing the tire size. Currently, I have 275/60-20r tires on the truck which translates into a 33" diameter tire (275/60R20 Tires). Accounting for the higher gear ratio, I want the truck to still think it has 3.55 gears and 33" tires, so using a simple ratio formula: 3.55/3.91 = x/33" where x is the fake tire size. This yields x = 29.88" diameter. Intuitively, this makes sense, because if the rearend ratio is numerically higher, the tires would spin faster at the same speed. Since the rearend isn't actually changing, the tire size needs to (virtually) spin faster which means they need to be smaller to achieve the same speed.

Tire circumference(inches) = PI * D = 3.1415926 * 29.88" ~ 93.87"​
Tire circumference(mm) = 2,384 mm. This is the number I put in via Alpha OBD. The original diameter in the BCM was 2,577 mm​

For some reason, I could not update in the ECU using Alpha, but I took it on a test drive anyway since I was able to update the BCM. I ran it both in normal and tow mode for a total of about 20 miles. No problems other than the engine seeming a little sluggish in normal mode- could be I need to figure out how to update the ECM, or possibly this won't work for the transmission without a tune. Or it could just be that this engine is sluggish (it has been kind of a dog from the beginning compared to my other truck).

Anyway, I'm sure there is something wrong with doing this, but I wanted to put this out there to see what kind of problems this could cause. Feedback is welcome and appreciated!
 

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Discussion Starter · #4 ·
But is the speedo correct after the tire size change?

I've read of some who used an ECM with the wrong gear ratio with no ill effect. I think GDE claimed it was only an issue when using cruise control and tow/haul together.
The speed seems to be accurate, but I need to find a radar display in a neighborhood, or have my wife pace me on the freeway.
 

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Discussion Starter · #10 ·
The speed seems to be accurate, but I need to find a radar display in a neighborhood, or have my wife pace me on the freeway.
Update- I took a 3 hour drive this past weekend using tow/haul and CC. I found that the the indicated speed (on the odometer) was about 1-2 mph lower than the set speed on the CC. Annoying, but everything worked fine!
 
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