UKGT500 Posted April 18, 2014 Report Share Posted April 18, 2014 A few days ago I was shocked to find my speedometer MPH was a LONG way out compared to my GPS. Actually I don't use my GPS that often but when I did last year my speedo and GPS pretty much agreed spot on. Anyway the only reason I can now think why I am so far out is because of my Brenspeed tune which I loaded onto the car via the supplied SCT tuner. Now I know this is quite well documented regarding setting "wheel revolutions per mile" being the way to change speedo calibration and I have found a couple of calculators. Thing is every calculator I have found online points to 748 (or thereabouts) being the number to set on the SCT. Closest on an SCT is 750 but when I set that I am still out by the same amount which is: speedo shows 70mph, GPS: 80mph! Obviously this is enough to get a ticket or two... This afternoon I kept trying different settings and the closest (still 2mph at 50) is 720. I'm going to change it to 715 which may make it closer. Thing I can't understand is why there should be such a discrepancy. Of course you could say "forget about it if you've cracked the correct setting" but a calculator is a calculator: i.e. a mathematical way of working something out and the difference between 748 and 720 (or 715) is quite a lot... Have to confess I should check my tire pressures. My rear gear ratio is stock at 3.31;1. The tires are stock 285/40/18 still on its' first set of tires....! Malc Link to comment Share on other sites More sharing options...
54First Posted April 19, 2014 Report Share Posted April 19, 2014 Have you checked the gear ratio on the SCT? Steve Link to comment Share on other sites More sharing options...
msb64 Posted April 19, 2014 Report Share Posted April 19, 2014 Something is off, just not sure what that would be though. My oem rear tires are just under 84 inches in circumference (inflated, no load). Divide 63360 (inches in a mile) by that 84 and you get approx. 754.3 revolutions per mile. Not sure if the gear ratio is even a factor, with a given known tire circumference, a wheel sensor counting revolutions and the ecm already tracking time, that should be all they need to calculate mph. Link to comment Share on other sites More sharing options...
UKGT500 Posted April 19, 2014 Author Report Share Posted April 19, 2014 Yes I just checked to see the gear ratio and it says "stock value". The same with the wheel rpm: when I scroll through the options it doesn't tell me what number I have previously chosen - I've just been keeping a note, changing the value and reloading the tune but to get the speedo to agree with my Sat Nav I am now a long way from 748. Not sure if I've heard of the speedo itself even being the problem though I doubt that somehow. Malc Link to comment Share on other sites More sharing options...
2010KonaBlueGT Posted April 19, 2014 Report Share Posted April 19, 2014 Something is off, just not sure what that would be though. My oem rear tires are just under 84 inches in circumference (inflated, no load). Divide 63360 (inches in a mile) by that 84 and you get approx. 754.3 revolutions per mile. Not sure if the gear ratio is even a factor, with a given known tire circumference, a wheel sensor counting revolutions and the ecm already tracking time, that should be all they need to calculate mph. The Vehicle Speed Sensor (VSS) is on the output shaft of the trans so yes, the gear ratio factors into the equation (because the gear reduction is AFTER the VSS). Phill Link to comment Share on other sites More sharing options...
msb64 Posted April 20, 2014 Report Share Posted April 20, 2014 Thanks for the update Phill. It seems odd that they are still taking it off of the transmission output shaft, just like the old mechanical setups. Wonder why they chose to keep doing it that way? If they did take it off of a wheel instead, it would have taken the rear gear ratio right out of the equation. Besides calculating mph, would there be any reason why they might want to reference engine rpm against transmission/driveshaft rotations? Are the rear wheel sensors just for traction control and ABS braking then? Link to comment Share on other sites More sharing options...
6-Speed Posted April 20, 2014 Report Share Posted April 20, 2014 The FRPP Procal II instructions provides an equation for computing revolutions/mile according to tire size (See step 20A on page 4). 772 rev/mile for the stock tire size worked well for my 2007. http://www.fordracingparts.com/calib...structions.pdf Link to comment Share on other sites More sharing options...
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