Well i got my tach installed so i could finally see what my cruising rpms are. At 60mph i'm running 3k rpm and i have 3.25 gears. I have a 302 and a Toploader Tranny. Is this about average for the rpms? Or do you have any other suggestions on what gears would help me out more with some fuel economy with my daily driver, but not sacrifice to much power downlow.

Thanks again.

 

well, you must have a lower rear end then i do. I run 80 at 3000 rpm. I run 60 at 2300 rpm. If you want more fuel economy go with a higher gear, such as a 3.00 or maybe a 3.25. but you might already have one of those. i dont really know. I have a 2.80...

 

I have 3.25 gears

 

beat me to posting a similar question. I have a T-5 and 3.25 gears theoretically...anyway, i've heard its recommended to swap to a higher one such a 3.5 or 3.89. I dont really want to upgrade to something too high like 4.11 but i was wondering what you guys recommend. I've seen 3.7 and was wondering if thats good for someone who wants a good accleration but not like hyped on it

 

The Higher the gears (From 3.25--->4's) Gives you better low end correct? So if i want better mpg i should go more towards 2.77's huh.

 

Couple of questions for you Midnite...

You have a toploader, 4 speed?
Are you sure the gears are 3.25s? or is that what a PO told you they were?

I have 3.5s and a 3 speed FMX and I only turn around 2500 RPM at 60MPH in 3rd. You should be turning less RPM at 60 with the higher gear then I am turning.

But yes if you are looking to get a lower RPM at cruising speeds (50-60mph) you would want a a 3.0 or 2.73 or something along those lines.

 

Like mine, your car was probably delivered with a 3.00 open rear. When I rebuilt my rearend I installed a 3.70 and added a posi ofcourse. It's lots of fun around town, but way too much gear for highway cruising. My car has the FMX auto tranny too, which just magnifies the problem. I am returning to the 3.00 gear during my resto, already bought the parts from Currie Enterprises. I would think this gear would be a good compromise for you, going to a 2.80, you may really miss the low end pull you have now. Just my .02 cents. BTW if you don't have a posi rear, while your messing around with it, make the upgrade, you will not be disappointed.

 

tire diameter makes a large difference. 1 inch in diameter makes for nearly 10mph at a given rpm.

If you want good mileage stick to 3.25 and numericaly lower gears, I like 2.70 gears. With a 4 speed it will lug a stock 302. Those gears work better with an automatic, because the torque multiplication from the converter helps get the car moving.

Anyway its easy to figure out rpm vs gear ratio vs tire daimeter. you can go th thie site on the coral and input everything, http://www.corral.net/tech/gearcalc.html or you can do it with a calculator.

(gear ratio x 336) / (rpm x tire daimeter) = mph

(3.25 x 336 = 1092) / (3000 x 25 = 75000) = 68.68 mph

Math can be usefull with cars, and its not like algebra in hich school because you have a real number with meaning. it is algebra though, no biggie.

 

We counted the tire rotations to the drive shaft rotation and it is 1:3.25. The 9" Rear end has a tag that states its 3.25 gears and that it has traction lock. Although right now only 1 tire spins when i floor it so i assume the clutch packs are worn. I have a posi unit that i will get installed but i need to find new bearings for it and hope its in good condition (garage sale item)

The tranny is out of a 71 Mustang. This car is kinda put together with a donar car sadly. So are you saying keep the 3.25 gears in the rear end and get new transmission gears? Or should i just go down to 2.73's because i will eventually get a 351 in the car.

 

For the best of both worlds, get an AOD. 325's are not all that much of a gear over the stock 3.0 's......