LBZ tire size change

[BigTex]

I know this should be a simple change, but I need to make adjustments for larger tires on an 06 LBZ. Looking over the available options in the tune file, the only thing I can see at this point is table H0136 (Tire revolutions per Kilometer) under the speedometer section.

Do I just need to calculate the % difference between stock tire size and current size, then adjust the H0136 accordingly? If so, will this correct speedometer, odometer, etc...?

[GMPX]

Yes, the fancy calculator is not implemented in the LBZ because it is missing a few parameters that we need.
I don't know the exact formula, but someone over at Diesel Place has done the % change and it seemed to work O.K

Cheers,
Ross

[BigTex]

Great. Thank you Ross. I figured it was something like that since the LBZ software is still undergoing changes. I just wanted to double check.

Have a great day and thanks for an excellent product.

Richard

[Turbo595t]

Hello,

I did a tire size change on a LBZ last week. The rev/km that gm has was 424, I calculated the stock tires to be 411 rev/km and the new 285 size tire to be 382, but when I put the 382 in the speedo was high reading 69mph when it was supposed to be 65mph, so I added the 13 rev/km from what I calculated and used 395 rev/km and it was fine.

Did you run into something like this?

Thanks,
Brian

[Trippin]

395 is what I had calculated/used for 285 75 16.

[Turbo595t]

395 is what I had calculated/used for 285 75 16.

I was in a hurry so I googled some tire calculators and first one I used came up with 382rev/km and so did the second. So I broke down and did it by hand and a calculator and I came up with 381.72rev/km........ so I don't know what's going on.

(285 * .75)+2 = total side wall height

16 * 25.4 = wheel dia mm

add those together then divide by 2 to get the radius.

Then 2pie(r) for the circumference

then divide into 1000000 millimeters for revs/km

I came up with 381.72rev/km.

I could have used pie(dia) for the circumference, but for some reason I have always used 2pie(r)

Brian

[BigTex]

Yes, Turbo595t, I had the exact same results and the same calculations. I had already calculated the stock 245.75.16 tires out to 411. When I saw 424, I thought that the speedo must have been high from the factory. I was moving to 305.70.16 tires, which calculated out to 382 revs. Afterwards, I decided to take the ratio of my calculated tire heights and apply it to the mysterious 424 number that was originally in the PCM. I multiplied 424 by .92944 (which is 382/411) to end up with 394, which put it just right.

[SSonnentag]

I know my speedometer read about 68 mph when my GPS said 65 mph, so the factory numbers do indicate more miles traveled than actual, resulting in a better "perceived" fuel mileage. :)

[Cobra#3747]

I figured out on the Revs per mile that going from a 245 to a 285 was about -8%

So I just -8% the rev table. GPS put it pretty much right on