i bought a evo today and i did the calculations in the book for tire size i have 275/60/20 and came up with 2638 edge and factory speedo are 4mph off of eachother and gps is about the same..is there a trick to figuring this out?

 

2638 is the circumference with no weight on the tire (squishing it, which reduces the effective radius or the circumference. Usually, knocking about 3% off the "unsquished value", gives a number that is very close. So, 2638 x 0.97 = 2559. Try that. What you want is for the Edge speed to agree with the GPS speed. If you get that right, your dash speedo will normally read 2-3 mph higher in the 70 mph range, but your odometer will be right.

If you need to increase the Edge speed, increase the Tire Size (TS) value. Reduce the Edge speed by reducing the TS value.

Ford designs the factory speedo to read high, I think to avoid lawsuits when you're caught speeding. There will always be errors in the dash reading, because it has to convert an electrical signal to an analog one involving a mechanical gauge.

- Jack

 

will try that..thank you for the fast response!!

 

that calculation got me exactly where i needed to be!! thanks man..

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