2.77 rear end

rbcyouthdude · Post by **rbcyouthdude** » January 9, 2007, 7:16 am

Anybody have this setup in your truck. I have the factory 3 speed with a 351M. When I get rolling I have to shift by 10 MPH. I have been offered a 2.77 (Have 3.73 now.) I know I will be losing some pulling power but I mainly (95% of the time) just drive empty. Wanting some milage out of my daily driver.
Is this to high?
Michael

jakdad · Post by **jakdad** » January 9, 2007, 9:44 am

Hi Michael,
If your wanting any milage from a 351m, the trans or rear end is not the place to start. Move farther forward as under the hood. Maybe a 302................

banjopicker66 · Post by **banjopicker66** » January 9, 2007, 10:35 am

jakdad, I must respectfully disagree with you about your suggestion for improving mileage.
The rear end is the easiest place to start with modifications to improve your mileage.
Changing the engine is not only a lot of work and effort, but the change will bring only very minor results - if any at all, and a 302 may actually decrease his mileage with the setup he has.

Now if he were to go to a 240 or 300, that might make a diffference. The 302 is a short stroke motor, and gets its best mileage and performance with higher RPMs.

banjopicker66 · Post by **banjopicker66** » January 9, 2007, 10:46 am

Now as to changing the rear end, No, the 2.77 is too tall for your 351. I put a 2.75 behind my 352 (similar power band) and the gears at takeoff were terrible. I had to feather the clutch even at a stop on level ground. I advise against this ratio.
A better ratio would be 3.25. 3.50 was the standard ratio for the 352 with a three speed to provide a balance of power and mileage. You could go to a 3.25 without any serious problems, I think. 3.00 would be pushing it, though.

There are a couple of considerations to keep in mind when changing out your rear end. The Ford 9" rear came with 2 different axle splines, 29 and 31. Your rear is probably a 29 spline axle setup. The punkin you put in must be able to accept the 29 spline axles, or you will end up going through a lot of modifications to make it work. So, you need to check the donor punkin before you try to put it in.

Secondly, the yoke on the front may not have the same U-joint size as your current punkin. This isn't a problem; take the chunk and the rear driveshaft to Napa, and have them to sell you a conversion joint, which has different sizes on the crossbars.
The mudders and 4-wheelers do this trick all the time with transplanted transmissions, rear ends and drivelines.

jakdad · Post by **jakdad** » January 9, 2007, 11:47 am

I agree with the 3.25 to 3.70 ratio for the rear end. I strongly disagree that you will ever get respectable milage from a 351M regardless of rear end ratio. The best milage can be achieved by being on a down slope, then cut the key off and coast.

winr · Post by **winr** » January 9, 2007, 12:09 pm

I had a 3.50 3rd member in my 65 short wb with 352 3 speed on the column.

I now have a 3.25 in it.

My rear tires are around 28" tall.

[EDIT: oops, I meant 2.75 gears]
[3.00 gears may be to tall as others already stated.]

Richard.

banjopicker66 · Post by **banjopicker66** » January 9, 2007, 2:49 pm

AHA! Now I see your point and agree completely.
I know very little about the Modified series if engines, but if I understand it right, they can be made to do well with a cam and timing gear change.
If he had to do more than that, such as buy aftermarket heads, then yes, an engine change might be easier and cheaper.
Thanks for the clairification!

jakdad · Post by **jakdad** » January 9, 2007, 7:01 pm

Right on! And a head change would be in order. The Aussie 2V heads wake up these old 351M-400s. They also get better milage with these mods but still wont match up to a 240-300 or a small block. I still love the sound of the in line sixes with dual exhaust......................

FORDBOYpete · Post by **FORDBOYpete** » January 9, 2007, 7:48 pm

There's No Replacement for Displacement when tall gears are the order of the day, IMHO. the 351M/ 400 has all the design of a Big Block high displacement engine, but it is a low displacement engine in truth. I say you're pushing the envelope of probability running below 3.25:1's with a 351M/ 400 engine. :roll:

AND sluggish lo end performance of 3.25:1 gears'll just about kill any gain in TE or Mid Range operations. A 351 is not a lot of displacement no matter what Engine it is making the power W or M.

I'd either go for a good running efficient small block or go whole "Hog" and get BIG CUBEs. Then you could pull 2.77s without worrying about torturing your lower end, which is the weak point at lower RPM with Modifieds.

But this is just my opinion here. :roll:

FBp

MadMaxetc · Post by **MadMaxetc** » January 9, 2007, 8:03 pm

I runn a 300 with a 3.00:1 rear. This is the tallest I would ever go. I have a built 300 (see my signiture) and I have to feather the clutch a bit.

3.25's would be the limmit. An engine swap is easy to do if you have a picker, the right pars, helpers and a little $ (500 for stock stuff should do it.)

But to ansser your Q. No. do not go with that gear.

64 f100 · Post by **64 f100** » January 9, 2007, 9:00 pm

I'm running 2.75 in my 66 shortbed, and ran it to Denver and back. Got lousy gas mileage, mainly because of speed I was running. Didn't make much difference how much weight I was hauling. Went empty, and came back towing one and had a cab in the back of mine. The 2.75 gears were hard on the clutch, but not tall enough, for good highway mielage at speed. The modified motor will never be a good gas miser, but it can be helped, with cam modifications. 3.25's is what most three speed's I've had were equiped with, so you should be able to get by with 3.00's, but don't look for much gain in mileage. If lucky, you might get 2 mpg depending. I have yet to install my overdirve tranny I got from Banjo, and he says it didn't do well with 3.25 gears, but he was running less torque. He had a stock 352, and I'm running a modified 390. I don't run out of power very often. Bigest problem I have is rattle and ping for not running premium. With the overdrive, and 3.50 gears, which is what Banjo said his setup was, the overdrive should be like having 2.63 gears. I'm seriously thinking of running 3.25's as an experiment. This would put it at something like 2.3 in hid at highway speed. Truthfully, a five speed might be a better choice for you. If you go with your present gears, then it would be about what I would end up running with a set of 3.25's. Better choice would be to change out the engine, if your serious about gas mileage. The heads on the 351m are the culprit, kinda overkill for a small displacement motor. If your going to keep that engine, you should check into retiming the cam.

Rich

banjopicker66 · Post by **banjopicker66** » January 9, 2007, 9:12 pm

I might add that now I am running a 2.75 - but there is a 460 and a C-6 in front of it now!
I get about 15 MPG at 70 MPH with an RV Cam, a 1470 Edelbrock 750 CFM carb, and a C-6 out of an F-350.

Rich - I tried a 3.25 behind that overdrive for a short time, but in OD, I had to get to 80 just to gt in the power band. I really think you'll like 3.50 with the 390 in overdrive - maybe the 3.25, but my money is on the 3.50.

rbcyouthdude · Post by **rbcyouthdude** » January 9, 2007, 9:30 pm

thanks for all the info, I thought tey were to tall. I will try to find 3.25's if I even mess with it at all. I get around 11 mpg, just wanted more.

Michael