Speedo Calibration Question

[512silverado]

Hello everyone. I am an infrequest poster, but an avid reader of the board.

 

This weekend I got a 7" Pro Comp lift and 35" BFG A/T's for my 99 Silverado. My question is this: When using my Hypertech programmer to correct the speedo for the new tire size, do I program it for 35" tires, 34.6" (actual size according to BFG's website), or 33.75" (actual diameter as measured by me)? The tires are 315/75/16 (sometimes referred to as 35/12.50's).

 

Any assistance would be appreciated.

[512silverado]

Just looking for an opinion...not a definitive answer.

[Bish]

Just looking for an opinion...not a definitive answer.

 

<{POST_SNAPBACK}>

 

 

 

The correct method is to draw a line at the center of the contact patch. Put the line on the tire and driveway. Move the vehicle so the tire travels 1 revolution and draw a line on the driveway. Measure the distance between the 2 lines and divide by 3.1416 and this gives you the tire height. Since Hypertech only gives you 1/4 or 1/2 inch increments, pick the closest. Bear in mind it will change a little as the tire wears but I've never changed mine after setting it properly.

[512silverado]

Thanks. I'll get that done tonight.