Tires and Gears

[Growler]

20,000 miles have come and gone and I'm still waiting for the stock 235/75R16 Tombstones to wear out. I'm looking to replacing them with 255/85R16's so I can keep the stock aluminum wheels and gain a little ground clearance when they wear out.

 

By my calculations, if I do this change I'm going from a 29.9 in diameter tire to a 33.1 in diameter tire. By my calclualtions, this would put my speedo (and associated mileage, etc.) off by about 10%.

 

Now, the 8.1L engine that I have was offered with either a 0.373 or a 0.410 rear end. I got the 0.410 in anticipation of eventually getting larger diameter tires.

 

I think that, if I got the computer reprogrammed for 0.373 gears at the same time I got the 255/85R16's, I would still be within about 0.7% on speedo, mileage, shift points, etc. [((29.9x0.410)-(33.1x0.373))/(29.9x0.410)=0.007]

 

Does this make sence? And, if so, will the dealer reprogram the computer for 0.373's if I tell him I had the truck re-geared (with no further evidence)?

 

What do you think?