[cliffyk]

FWIW--here's a thingy I put together a while back, it's a Rear Axle Ratio Guesstimator.

To use it just get going 40 to 50 mph in 4th gear (3rd for autos) and make note of the selected speed and rpm at that speed; then plug the numbers into the calculator. If you have non-stock rear tires that are of a different outside diameter than stock enter the tire size too.

Here's an example using my GT with 3.73 gears; 50 in 4th is just a bit over 2500 rpm:



It uses a fuzzy math algorithm, meaning that the input values for speed and rpm do not have to be dead on.

It's pretty accurate...

[blkfordsedan]

^^Cool! I like that a lot better than doing the long-hand formula I posted awhile back! How do you make those custom calculators? I have several functions I would like to make them for, like compression ratio and such.


Best way is to drive it at 60MPH in fourth gear and see what RPM the engine is running. (60MPH = 1 mile per minute, which make the calculating easier). You also need to know the rear tire size, so you can calculate the tire circumfrence or you can measure the diameter.

1 mile = 5280 feet = 63360 inches
tire circ (inches) = 3.14 X diameter (inches)

63360/tire circ. in inches = wheel Revolution per mile
(This is also the wheel revolutions per minute @ 60MPH, since 60MPH is equal to one mile per minute)

Wheel RPM X Gear Ratio = Engine RPM

So....

Engine RPM/Wheel RPM = Gear Ratio

Note: this only works if your speedo is accurate and your transmission is in a direct (1:1) gear.

[zipmaster07]

Originally Posted by BlackPony99 I heard ford racing gears tend to be noisey, but an alum driveline would help lol and if you still don't believe they are 4.10's, install them, turn your tire one full turn and count how many times your driveline rotates (hopefully 4 times + 10% of a turn)

I have 4:10 gears and the aluminum driveshaft and IMO the driveshaft doesn't reduce noise at all. Don't get me wrong I love the driveshaft, but I didn't buy it to reduce the noise.

Good luck with the gears, I have to say this was one of my most interesting projects. I did mine myself and I must say it was enlightening (not necessarily good, not necessarily bad). All-in-all though, I'd do them again.