LBZ tire size change

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...?

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

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

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

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

Quote:

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Originally Posted by Trippin

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

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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

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.

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. :)

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