New 2001 Sierra owner question about oversize tires and speedometer

[HoagieFish]

Hi

 

My son just got a new (for him) 2001 GMC Sierra 2500HD. It has 275/60R20 tires on it. Looks like the stock size would have been 245/75R16. We think the speedometer may never have been recalibrated to account for this. To address this would we need to have it done via a programmer, change a speedometer cable gear or would the gear ratio in the rear end need to be changed (stock is 4.10, from calculators looks like it should be 4.44 and pricey). Or am I mixing some apples and oranges here and gear ratio has nothing to do with that and, if so, what would the benefit to changing (or detriment if left as is) be to changing that. I'm also assuming that if the speedometer was never recalibrator, the odometer is likely off.

Thanks much for the education.

[RyanbabZ71]

purchase a programmer like a hypertech, plug in the new overall dia and move on.

[Nick17]

Yes it will have to be change via a program or you can use a GM scanner which you can't get personal access too but its an option

[RyanbabZ71]

Gm scanner / tech2 will not work with non stock tire sizes.

 

 

Ryan