Changing your Gear ratio

[Tank]

I'm pretty new here so sorry if this has been asked before. I just bought a '72 f100 with a 360 and 4 speed manuel tranny. It tops out at about 60mph and I was wondering if changing the gear ratio in the rear would help it run a little faster. I don't care to have a racer but it would be nice if I didn't feel like it was about to explode when I get on the free way.
Here's a pic of my new bump:


ps. where does the term "bump" come from?
Thanks

[fordman]

the front and rear ratios on a 4wd have to match. i forget why. but what is your rear ratio now?

[papabug71]

Yup, if you do the rear, you have to do the front as well. If you dont the transfer case will turn into a small bomb.

I would figure in a nice 4x4 like yours you would want the low gears. If you go very big it will kill your power.

[Tank]

Well that brings up my next question. How do I find out the current ratio? I know I sound like an idiot but I am a motorcycle guy who is expanding his knowledge into a long time dream of mine to own a classic truck.

[papabug71]

What is the code on your door tag?? We should check that first. Its the metel tag on your D. side door. It should say axle & have a 2 digit code under it. Mine is code 17 for 3:25 gear ratio. It could also be a letter followed by a #. My dentside rear end im swapping in is a A2 code, for 3:70 limited slip.

Im sure they have been changed if they are that low. I dont think the 9" had many really low gear choices back then. EDIT: I see you could get 4:11's in the 9" in 1972

Another less precise way is to jack up the rear end of your truck, put it in neutral & count how many times the driveshaft spins for every one revolution of the tire. Put a mark (with a sharpie) on the driveshaft for referance. Just be sure you do it safe. That will give you a ballpark estimate of what you have.

For instance: Say your driveshaft turns 4 1/2 times per every one revolution of your tire. That would come out to be about 4:56 gears.
Im willing to bet if your topping out at 60 & you have what looks to me like 33" tires, it probably has 4:56's in it.

Hope this helps.

PS, I would give my left nad to have your truck. It looks VERY buff.

[Alvin in AZ]

Yeah what they said. ;) ...but also look for axle tags... :)
http://www.panix.com/~alvinj/file12/Ftags.jpg
The two upper tags are from Ford9inch axles.
They are both 3.25 gear ratio but the second one with 3L25 on it, has limited slip.
Ford calls it "Traction Lok". :)

Alvin in AZ

[lee_ford]

ps. where does the term "bump" come from?
Thanks

The bump running down the side?

_1012452.jpg

BTW: , Looks like a good start to a labor of love.

[Alvin in AZ]

Cool one, Lee. :)
My '75 parts truck is a dent side...
http://www.panix.com/~alvinj/file12/F100front1.jpg

Alvin in AZ

[Tank]

What is the code on your door tag?? We should check that first. Its the metel tag on your D. side door. It should say axle & have a 2 digit code under it. Mine is code 17 for 3:25 gear ratio. It could also be a letter followed by a #. My dentside rear end im swapping in is a A2 code, for 3:70 limited slip.

Im sure they have been changed if they are that low. I dont think the 9" had many really low gear choices back then. EDIT: I see you could get 4:11's in the 9" in 1972

Another less precise way is to jack up the rear end of your truck, put it in neutral & count how many times the driveshaft spins for every one revolution of the tire. Put a mark (with a sharpie) on the driveshaft for referance. Just be sure you do it safe. That will give you a ballpark estimate of what you have.

For instance: Say your driveshaft turns 4 1/2 times per every one revolution of your tire. That would come out to be about 4:56 gears.
Im willing to bet if your topping out at 60 & you have what looks to me like 33" tires, it probably has 4:56's in it.

Hope this helps.

PS, I would give my left nad to have your truck. It looks VERY buff.

Cool! I'll check the tag when I get home from work tonight.

also thanks for the "bump" answer. Suddenly it seems like something that I should have figured out myself!

BTW: Convincing my wife that I couldn't live without this truck was almost as painful as giving away my left nad! So I completely understand what you mean.

Thanks All.

[Tank]

Cool one, Lee.
My '75 parts truck is a dent side...
http://www.panix.com/~alvinj/file12/F100front1.jpg

Alvin in AZ

Looks like your parts truck has served its purpose well.