Slick 60's

I have been over all the rear end advice i can find in previous discussions on this site and am confused by all the technical talk as i am new to all this. I have a lot of basic engine knowledge and am very good with body work but i have never worked drive train so bear with me. My truck is not intended to be a work horse, the most hauling it will do is my motorcycle in the bed. I want the truck to get the best gas mileage possible as i enjoy traveling to shows and such. It is has the stock 300 motor and stock 3 on the tree along with the origional rear end for the 66 f-100, whatever that may be. What would be the best combo of rear end and tranny to get the optimum mileage. I am open to completely replacing the tranny and keep in mind that i also picked up a 9 inch 2.75 ring gear and pinion pretty cheap. any thoughts would be helpful.

Bishop

I have found a good combo for a 4.9L aka 300 CID I-6 is a 3.50:1 rear & either a 5 Spd Manual OD or an AOD transmission behind it.

Although the 4.9L is a torquey engine I believe 2.75:1 is too low for it to perform well as it'll have too low a ratio (tall a gear) all the time to pull thru, everytime. What you may gain will be more than lost pulling away from stop signs, traffic lites, up Interstate ramps or hills in other areas. Not to mention the constant shifting & wear on things.

I run a 3.00:1~ 9" FoMoCo Rear behind a 460 in my 66. That makes 510 Lb/Ft of torque and it works well, but the 300 in my 88 F-150 just doesn't have that sort of raw power.

I have learned that being under powered is like being over geared and niether one is efficient/ effective in terms of optimal overall performance, and that's what you say you are striving for, right?

FBp

I agree with FBP on the rear ratio. I had a 352, with a three-speed, and I went from a 3.50 rear to a 2.75 - and it was terrible.
I had to feather the clutch to get the empty truck moving, and that was on level ground - forget about doing it on a hill!
Worse, the engine didn't reach the power band RPMs until doing 80 MPH - so mileage didn't improve at all.
With the 300 you have I recommend you go no lower than a 3.25. 3.00 might be OK, but my gut feeling is to stay at 3.25 or 3.50.
I have a 460 now also, but the extra power in front of a C-6 lets me run a 2.75 with no problems.

I'm running a 3.00 rear with a 300 six and T18 transmission.
It seems to be a good combo able to stay with the flow of traffic and it pulls a moderate load well.

Mileage wise it averages 13 mpg. Engine is stock with efi exhaust manifolds and duraspark 2 ignition.

Isn't a 3:25 the standard ratio with a 352 (1966)? Does a 3:50 let you drive all day at 70 MPH?

I run a Highly modified 300 / 3speed / 3.00:1 combo and it is about the max I would do. The 3.50:1 would be wat I would do if you keep the 3 speed. If you cange trannys to one with OD I would go down to 3.70:1. This will keep the RPM high enough in OD to still make power.

I am putting a T-5 behind mine in a few weeks (next weekend I swear! ) Look out for more info on that.

...OH and I get 16 mpg

Look down here
|
|
\/

I had a 3.50 behind my T-18 (3 speed with a granny gear) and I moved to a 3.25 along with larger (7.00x15) tires and now I have a little trouble getting started. First is still waaay to low and second requires me to feather the the clutch more than I like.

Course its with a 240, which has less grunt that a 300.

The issue of tire size matters a lot in these discussions as well, FYI. If you are running smallish tires, then 3.00 or 3.25 is fine behind a 3 speed.

I will say this -- with a 3.25 and then 7.00x15 tires (about as tall as a 235x75R15) my truck rolls really nicely at 65 at about 2200 rpm, and will go up to 8o but the wind noise, roaring of the engine and feeling I am riding a flying brick makes me want to slow down soon.

I had a FI 300 6cyl with a 5 speed and 3:50 gears and stock 235 75 15 tires and it ran great, I had to watch my speed it would want to do 80 on the highways and pulled most hills easily in fifth. Got 19 mpg on highway. I put 31 inch tires on it and it made it a dog, I switched back to old tires within a week. I had the same setup in another truck , except had 3:08 gears and it was a total dog, always had to floor it up hills and alot of shifting, gas mileage was terrible too.

Phil