Been thinking about the calibration by the TRUSPEED....
If you're looking to upsize by many sizes then this gizmo may be well worth your money.
If your gonna go from 265/65/17 to 265/70/17, possibly not.
Here's why:
265/65/17 is basically a 30.56 inch tire
265/70/17 is basically a 31.6 inch tire
Speed is calculated on circumference (C) of tire, right?
C= 2*(pi)*radius or Diameter*(pi)
So,
C (circumference) of 265/65 is 30.56*3.14= 95.9584 inches
C of 265/70 is 31.6 *3.14= 99.224 inches
There is only a difference of 3.2656 inches. Therefore, each revolution of your new tire only makes you go 3.2656 inches further.
Percent difference is (3.2656/95.9584)*100=3.4%
As it has been mentioned in previous posts at each speed your going 3.4% faster than it is showing.
100mph = 103.4mph
70mph = 72.38mph
60mph = 62.04mph
30mph = 31.02mph
15mph = 15.51mph
For the price of $81-$184 how many of us really care about the change in speed or change in odometer? Now, if your changing gearing or getting some monster tires, I'd be first in line. Another item to keep in mind is the chart, in the pdf, Doodz gave a link to. For each given diameter or circumference there is a range. The shown range in the pdf was 30.5 to 31.5, then 31.5 to X. The first choice looks close to what your already working with. They claim a 1% close to true, maybe by getting the calibrator you can knock off 2.4% of the error.
Hope this adds to the discussion...and no, I'm not an engineer.
G3 Trout Runner
