ford gear ratio

[tynall44]

is there any way of telling what gear ratio u have the tag is gone and its a funny vin tag just a few numbers nothing like original,,,i know u can take it apart and count the teeth but hey who wants to do that ,i know the truck came out with a 240 six and a three speed manual trans if some one knows about what they put with that combo please let me know. thx tom.

[FORDification]

The VIN doesn't include that info...but your VIN tag on the door should give a rearend code for the rearend ratio, in the second line.

An easy (though not extremely accurate) method is lift the rearend and then simply make a mark on the driveshaft and then slowly turn a tire, counting the number of revolutions the driveshaft makes vs. the tire. If your driveshaft makes 3 complete revolutions to one of the tire, you have a 3.00:1 ratio. However, if you've got an open rearend (not a Trac-Lok) then using this method will be pretty inaccurate for reasons I won't go into here. In the case of an open rearend, you'll need to secure one tire (just drop it to the ground so that only one tire is up in the air) and rotate the tire twice and count the driveshaft revolutions.

Or, if you have a tachometer, you can use the following formula to figure your gear ratio:

[heep70]

Keith, If I want to know how fast (MPH) I am really going with oversized tires. I could use??????

MPH = (RPM X Tire Dia) / (Gear Ratio X 336)

OR

RPM = (MPH X Gear Ratio X 336) / Tire Dia

Right?

Now are these formulas here and the one you have above with a tranny gear ratio of 1:1? I know the formulas are not meant to be exact.

See what happens when I am board at work.

[FORDification]

Yes, those are the formulas I've been told to use, but I've never heard of any mention of a tranny's final drive ratio having an effect on them. But of course, as you mentioned, it's not an exact science, just a good ballpark figure....and I since I believe most common transmissions have a 1:1 final drive ratio, I'd imagine that's what was taken into account when coming up with the formula in the first place.

You'd need to know your oversized tires diameter, and this is what one website advises for figuring that, as opposed to simply running a tape measure around the outside.

A real accurate way to determine your tire diameter is to draw a line on the bottom of your tire, then draw a line on the ground which lines up with the line you just drew on your tire. Then move the car until the car makes one full rotation of the tire and mark the ground where the line on the tire is at the bottom again. Now measure the length between the two lines and divide by pie (3.14) and you'll have your tire diameter to use on the calculator (don't forget to use inches). This is the preffered method to use in determining your actual tire diameter.

I suppose this method might be a bit more accurate than the tape measure, since it will take into account any tire squish caused from the weight of the vehicle.

However, I should also insert a disclaimer here that I totally suck at math. In fact, if it wasn't for my sister being in the same math classes as myself in high school, and if I hadn't known the combination to her locker so I could copy her homework assignments each morning, I'd have probably flunked it completely.

[cobraF100]

All this Math is giving me a

2 + 2 is 5 ....right.....

[heep70]

OK, thanks. I do well in math. The answer is 3.2534 not 5


I was just trying to find out how fast I was going with 26" tires and 5th gear being a 0.78:1 gear ratio. I cam up with a 9.1% or 5 - 6MPH difference.

[Nefarious67]

OK, thanks. I do well in math. The answer is 3.2534 not 5


I was just trying to find out how fast I was going with 26" tires and 5th gear being a 0.78:1 gear ratio. I cam up with a 9.1% or 5 - 6MPH difference.

Several sites have calculators to figure out info such as: rear end ratio, speed vs rpm, ect. My favorite is: Smokemup.com You have to be a member now to use the tools but its free and doesnt take long. All the tools are under Automath, the one that would help you is Automath -> Gearing -> speed for rpm range.

[heep70]

I got pointed to this one just a minute ago.

http://www.intercotire.com/site62.php

[willowbilly3]

I alwaysjust figure it myself. Since the tire circumference is in inches, I just convert a mile to inches (5280x12)=63360. Take for instance a 31 inch tall tire is about 97 inches around. divide 97 into 63360 and you have the number of revolutions per mile your tire is making (653.19). From there is is simple to find whatever you need. If you had a 3.50 gear, you will be turning 2286 rpm at 60 mph. (3.50x653.19) If you were turning, say 2500 rpm at 60 mph your rear end ratio would be 3.83 ( 2500 divided by 653.19)

[NM5K]

i know the truck came out with a 240 six and a three speed manual trans if some one knows about what they put with that combo please let me know. thx tom.

The door tag will have the axle code.
Most likely , it's a 3.70. That was the
stock tranny gear for f-100. I'm assuming
this is a f-100? Stock gear for the f-250's
was the 4.10. I've got a 74 f-100 with a 240
and 3 speed. It's got the 3.70. Most of the
sixes stayed with the stock gear, as they
needed the torque... My 68 F-250 has the
4.10. With the difference in wheel/tire size,
the rpms for each truck is almost the same.
MK