well i'm sure theres a million threads just like this but i have a 69 f100 and going to be making a 3000 mile trip soon so i'd like to be able to do 70-80 mph and not 55-60. i have no idea what gears it has in the rear end, any ideas? where can i buy these new gears? can i do it myself? how much would a shop charge? does the tranny shift according to rpm or mph ? how will things be for me around town and acceleration?

 

Well, you need to do some research first. Does the rearend still have the TAG on it? It will say what the ratio is, as long as it hasn't been changed and old tag left on.
Do you have a tach, do you know what rpm you are turning at a certain speed. This will help determine what ratio you have now.

If you're truck was a run of the mill 360/390 automatic then most likely it has 3.25 gears out back unless it was optioned up to 4.10 for towing? There aren't much lower sets available, but they do have 2.75 in some older big luxo-barge cars. If you can find one of those and put it in you'll save a couple more hundred rpm.

You can buy new gearsets. Then you have to have them installed and if yours has alot of miles then a bearing kit will be needed. You could do it, but you need a few specialty tools to do it right and you HAVE to do it right or you'll tear the gears up in a few hundred miles. The best you can do is call around your area that does that kind of work and see what they would charge.

The tranny shifts by vacuum signal from motor. That's the little rubber line off the back of carb that runs to modulator on tranny. It senses the load you are putting on it and shifts accordingly. Gears out back will not really affect it much, though it will shift a bit later since you'll be taking more effort to get up to speed with lower gears. Nothing at all to be concerned with.

All in all, if you have 3.25 gears I wouldn't worry about it. Not worth the time/money to change. Speed limit was 70 mph back when these truck were made, so it will cruise at that speed all day.

 

yeah i can cruise at 70 but it just seems the rpms are high, i'll do some research and fill you in some more, thanks for the help. will high rpms hurt the motor?

 

such a far trip scares me and i would hate to get stranded in a place where there aint no shops or even a gas station for 100 miles but its such a great truck and great trucks are hard to find

 

On the VIN plate look where it says AXLE post the code here, it can be decoded.

 

The high rpms occur because there is no overdrive. If your engine and other driveline devices are in good condition you should be OK. Be prepared to make frequent gas stops.

 

go to the top of this page see the spot that says articles/specs theres a drop down list click on auto calculaters.look in there,thers a mph-rpm calculater. ill bet youd be in the 2:75-3:00 gear range depending on your tire diamiter.just remember that the smaller the motor the harder it will be to hold a higher speed at a lower rpm luss drinking even more fuel.i shure hope that makes sence.im not to good sometimes at wording things rite

 

^^^ i think i understand what you mean. i always thought less rpms means less fuel consumption, but then again the motor is trying that much harder to pull the truck

 

where is the vin plate ?

 

I wanted to gear my F100 higher too so i just put taller tires on it. the tires I chose cost about the same as aftermarket gears but when you add the labor to have a shop install those gears it was a better option. I did not go much over stock diameter, I agree with Wizard, you can make the engine work too hard at low RPM,s and suck down a lot of gas.

 

the vin plate should be on the drivers side door just above the the latch.if your door is origanal itll have all the codes so nummberdummy can tell ya everything your truck come with new like engine/trans,axle,paint,ect. go ahead and list every thing it has on it