Heres one for you. Decided I was going to change rear-end gears to squeak some more mileage out of my truck. Every since the truck was new (in 1974) I have assumed the gears were 4:11. At around 65mph I'm cranking out a little over 3100rpm so that figures about right. My door code says 4:10, the third member tag says 3:84 (which I have never heard of in stock Fords) and the original window sticker has 3:55 on it. I know the factory tow package probably changed what the original window sticker had but here is the twist....I just pulled them about 30 minutes ago and it's a 3:25!!! 39 teeth on ring, 12 teeth on pinion. My biggest question is, why are my rpm's so high if they have been 3:25's since the beginning? Is there another gear I'm not thinking about that I could change? I got 3:00 ratio third member to replace but that doesn't help a whole lot, only about 200 +/- rpm at any given speed. Now I'm thining about the 2.75 to 2.50 range to get where I need to be. Any ideas guys? Truck is the one in my signature...

 

What size tires do you have? My 77 with 3.00 gears, C6, and 23x11.50 15" tires turns about 2400@70 if memory serves me correctly.
I went from a 3.50 to a 3.00 and lost 2 MPG. The torque converter slips too much at lower speeds and waste fuel.
I want to go to a 3.70 gear now.
Jimmy

 

I'm running stock type tires of P235/75/R-15 (28.9in). It's a 2wd so I can't play with the tire size too much. Do you think torque converter slippage would be a problem dropping down to 2.75 or 2.50?

 

......Input from anybody else?

 

Well I don't know what my RPM's are at 70 for sure, but I know that the old girl will do 100 with no problem, 3.73 gears top out at 70-75MPH. The engine just won't twist any faster.

I have 2.75 Open gears on my 79 F150, 351M, C6. I gt about 15 MPG on the highway at 75 MPH. I run LT 235 / 75 / R-15's.

I really don't think you are going to gain anything if you already have 3.25's. That does surprise me though. Is the 460 factory? Has anything been changed that you know about?

 

I have a 2wd and fit 31's with no problem on the rear. Just make sure that the offset or backspacing on the rim allows it to tuck inside the fenderwell.

As for going too low on rpm's that's why they make the tow-master type converters. They stall at around 900-1000 rpm instead of 1750-1900 on stockers. That small a tire with the 460 is over-revving that big engine. As long as the torque converter will allow it you can cruise at 2200-2400 no problem. As far as the 3.25 gears they were pretty common to my knowledge. All of my fords of that vintage either had 3.50 or 3.25 I've only ever seen one that came with 4.11's.

You can always try the 3.00 gears. In a 1/2 ton it's not that hard to swap out carriers if it doesn't work how you want it too.

 

I've got a chart that I did a while back that helps me figure rear end ratios/tire sizing for a variety of applications. Here's what i came up with if it's any help.

The C6 has a 1:1 final drive, and with P235/75/15 tires (28.9"x9.3"), and a 4.11 gear, rpm at 65 mph is 3104. That makes me wonder if the speedometer and/or tach may be off or you've got slippage with the converter.

If tire size and mph remain the same:

3.70 gear = 2801 rpm @ 65 mph
3.50 gear = 2649 rpm @ 65 mph
3.25 gear = 2460 rpm @ 65 mph
3.00 gear = 2271 rpm @ 65 mph

The rpm numbers get factored up a bit depending upon the driveline efficiency and type of converter, but I've found that they are usually accurate enough to help select a gear or figure what the existing gearing is.
You said rpms were about 3100 with the 3.25 gear. Info here says rpms would be 2460. That's about a 25% difference in rpm readings - if the speedo read just a little slower and the tach read a little higher, that could account for the difference if the converter was not slipping. At that rpm it should not have any slippage. You might want to verify that the speedo/tach are accurate before going to the higher (less than 3.00) gears.

 

Thanks for all the input guys. The 460 in the sense the block is the original factory block but I rebuilt it several years ago. The only thing I didn't different when rebuilt is a RV cam and a .20 bore but everything else changed was all external like intake/carb/ignition etc. Transimission has never been rebuilt and it's strong as an ox with no slippage whatsoever. I used to run 30x9.50 all around but wound up not liking the looks of it. The original tires were G78-15 and the comparable newer tire is the P235/75-15 so that's what I went back to. Willd, that's sure a good idea about the tow-master type converters. That might be the way to go. Chevdog, thanks for the charts. I researched this stuff till blue in the face converting different ratios/rpm/speeds with all the available programs out there and your numbers verify what I have already written down. I know my speedo is OK, it's new. I guess I'll go ahead and exchange the 3.00 ratio for a 2.75 or 2.50 since mine wound up to be the 3.25 already, just to see what it will do. I'm always up for trying new things just to see the results .

I'll let you guys know what happens!!

 

higher gears don't allways mean better milage you could be putting the engine into a rpm range that will cause it to run with the power valve on your carb open (lugging the motor) have you tried tuninig the carb ie.powervalve or main metering jets how about dist.advance recurve there are shops with chassis dyno's that may be able to find the "sweet spot for your engine and its mods maybe you coul find a taller set of tires to try out

 

Well guys, thanks so much for all your input. I wound up putting in 2.75 gears and WOW!!! Cut my gas mileage in half!!! I went from 10-12 mpg to 16-18 mpg!! Can you believe that with a big block!! I'm just shocked. And....I have no loss of power at all even for pulling. Pulled a 18' stock trailer the other day and I didn't even know it was back there. Incredible. I researched tires the other day and the stock ones of G-78/15 were even a little bit shorter than my current P235/75/R-15. I just can't believe how much of a difference it made. I knew it would change some but not this much. I did find out the hard way though that you need to know if you have 28 or 31 axle splines to team up with the differentials. The junkyard originally gave me a 2.75/28 spline and I didn't know until I got it cleaned up, bolted up and tried to put my axles back in. Threw a couple of wrenches across the shop on that one . They had a hard time finding a 31 spline differential at the gear ratio I wanted but finally found one out of a 82 with a six banger. I guess this is definitely the 'sweet spot' for my old reliable. Thanks again.

 

That's great news, Tinman, Congrats!!

 

From your figures it appears you have INCREASED your fuel mileage by 40%-50% not : Cut it in half :.

I don't have any problems running my 2.75's with my 429 yanking a 5000# trailer.

 

But did you change the speedo gear to correct for the gear change? Your numbers probably aren't what you think, your odometer is just moving at a different rate.
Jimmy

 

Nope, speedo gear has been changed too and I have already tested the new gear with a radar.

Mil1ion, what figures are you talking about? I've already checked fuel milage through 4 tanks of gas and it has been raised. I used to get about 50-60 miles per 1/4 tank, now I'm gettin that with an 1/8 tank. You used to could 'watch' the fuel gauge move, now it's like I'm driving my car and don't worry about it!!