[QuantumMotorsports]

Best way to do it is to go out on the highway and put your speedo on 60 and time how long it takes to go a mile (use the highway markers). Take the number of seconds it took to go a mile and divide 3600 by that number of seconds (so if it took 63 seconds, you'd have (3600/63) = 57.14. That's how fast you were actually going. so then, take 60 divided by your actual speed and multiply by the number of teeth you actually have on your speedo gear, and that will give you the number of teeth you need. then you can go to ford and ask for a gear with that many teeth. So say your actual speed when your speedo says 60 is 52, and your speedo gear has 21 teeth. you take (60/52) X 21 = 22.15 so the closest you can get to being correct is a 22 tooth speedo gear. Unless you change the drive gear in the tranny like someone said. But, actually, you should be able to take your new rear end gear ratio, divide by your old rear end ratio, and multiply by the number of teeth your speedo gear has, so like (4.10/3.73) X 21 = 23.08 so 23 teeth. See what I mean? Oh, and there's no way that your tach is off because of a rear end gear change. As a matter of fact, when you don't change your speedo gear, but you change your rear end gear, if you just look at the gauges, you won't be able to tell there was a change, because if you used to cruise at 70 in 5th gear at 2500 rpm, it will still say 70mph in 5th gear at that RPM. Untill you change your speedo gear. If you have any more questions, just ask me.