Joes95GT said:
That would get close. I'm going to agree with the majority here and say that I don't believe there is a combination of drive/driven gear made for a car with a 3.90 gear.
Joe
That's not true. The majority is wrong then. The method that most here use is Hanlon's calculator. It is not accurate. Specifically, it does not calculate revolutions per mile for the given tire size accurately. People here believe it's data, and spread that disinformation like wildfire. I'm not trying to discredit anyone, just trying to clear up this stangnet misconception.
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The formula to determine speedo gears is:
rear gear ratio X drive gear teeth X tire's revolutions per mile / 1000 = driven gear
This is a well known formula and is referenced in the Ford Racing 2004 catalog on page 129.
These examples assume a tire with 810 revs/mile. This is a common size for 245/45-17's. For this example, this is not important - just that the tire size stays consistent between examples.
so with 3.90's:
3.90 X 6 X 810 /1000 = 18.954
compared to stock 3.08's:
3.08 X 8 X 810 /1000 = 19.9584
The closer that you get to a round number, the more accurate the speedo will be. The farther off, the less accurate. So for 100% perfect accuracy, you'd need a 18.954 toothed gear. Obviously, gears don't come in fractional sizes. You have to use the closest round numbered one.
The calculations show that both stock gears and the 3.90's will have nearly identical accuracy. If you assume that the speedo is calibrated to the stock 3.08/8/20 setup and takes in account for the slight inaccuracy, you have to assume that the 3.90/6/19 setup will also take that slight inaccuracy in account, and be as perfect as you can get.
This is based on math, not myth.
I discussed this in greater detail here-
http://forums.stangnet.com/showthread.php?t=389857
and
http://forums.stangnet.com/showthread.php?t=392055
Rick
. The Speedcal can be purchased from Dallas Mustang. They have install instructions on the website and the install seems incredibly simple.
