what hydra said

 

Point may be moot now but when I got my new tires at Costco, they had a computer priogram that gave you speedometer calibrations for different sizes. You enter current tire size then the new size and it gives graphic of what the indicated and actual speed will be.

 

Well, I guess I'll just have to buy stock size equivalent tires since I can't shop at Costco ... 275-60 is an odd size . But I'm not due for new rubber yet. I know my speedo is off by about 5mph since I'm running 265-70 tires. Pity the speedo gear swap is out because when I get my 20s on, I'll be back to stock diameter. I can swap back & forth on my Jeep.

I'm gonna look further into the exiter thing. Might be possible ...