So I'm about 2 weeks away from having my car ready for state inspection. It's a 1976 Gran Torino sedan, 351M engine, Motorcraft 2150 carb, FMX automatic, 9" rear axle with 2.75:1 gearing. I've been playing with some online speed/gear/rpm calculators, trying to figure out the most efficient speed to run the car at, and what different transmissions or gear ratios or tire sizes might help. I'm using a formula for finding the sweet spot of an engine's speed, based on a piston speed of 16.4 to 19.8 fps.

Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

For the 351M with its 3.5-inch stroke, this comes out to roughly 1700-2000rpm. With the stock transmission, axle, and the P205/75R14 tires on it now, that rpm range gets plugged into this calculator:

MOTOR - A Rear Axle Calculator

47.8 to 56.2mph would be the ideal cruising speed for efficiency. Knowing that the national speed limit was 55mph back then, this seems ideal for back then. But since typical travel speeds on the backroads of central Texas are 65-75mph with 60-70mph speed limits posted, I'd like to move that range up to 60-70 or so. To do that I need to make about a 25% difference in something.

A Ford AOD transmission should be the same case size as the FMX, should bolt up to the driveshaft, transmission mounts, and the 351M, assuming that this particular 351M bolted to this particular FMX has the Windsor bolt pattern on the bellhousing. I'm not sure if FMX transmissions were made with big block bellhousings or not, still researching that. I do know M-block engines were typically big block bellhousings, but that a few 400's got the small pattern and most 351's were small pattern but don't know if any were big pattern.

The problem with using the AOD is it throws the sweet spot into too fast. 0.67 final drive means a 33% change, putting my ideal cruising speed between 72 and 84. I won't see those speeds unless I go way out by El Paso, and I have no need to go there. So to use an AOD on this car I would also have to change something else. The ring and pinion could be changed to a livelier ratio. 3.50:1 gears with the AOD gets me a cruising speed of 56-66mph and 3.25:1 gears gets me 60-71mph, right where I want to be. That or I can run smaller tires, like little 13-inch lowrider 5.20X13's. And that's just not what I want.

My question is, did Ford make any other versions of the AOD, or any other overdrive transmissions, that have a 0.75 or 0.80 fourth gear, preventing me from having to change out the final drive gears? My axle, my tires, and a 0.80 overdrive, gets me the same 60-71mph cruising speed I want.

I also know changing tires and wheels will help, but I don't want to put on anything larger than what stock would have been. HR78-14 translates roughly to P215/75R14, only a small change from where I am now. P225/70R14 would also match up and be a bit wider but the same diameter. The change wouldn't be much anyway. Finding a transmission with a 0.80 gear would do it, though. Is it possible to build an AOD, like from one of the big builders, I'm sure some of them advertise on here and all, with a custom overdrive gear ratio?

 

I don't know the scientific part of it all, but I know a car the size od a torino would benift from from a gear size of at least a 3:54. I still would give you decent mpg and better punch off the line. The 2:75's maybe good for high speed cruising. A gear that high could even hurt mpg as the engine will labor on short hops betwwen stop lights. You could also run synthetic fluids in the engine, tranny and diff.

 

One year of the 400 had the small block pattern. The 351m was intro'd much later, and is a 400 block with a shorter stroke. Thus, it has the BBF/385 series bell, just like the 400. And you can get an adaptor for the AOD to fit that pattern, but like almost any transmission change, you might save gas, but you'll not save money.

What mpg do you get now? Figure out a 10 or even 20% improvement. Do the math.

You'd be better off rebuilding the 351 with some compression and the earlier timing set, if haven't done that already.

Happy Motoring!

 

I'd go to a larger tire size. I had 225/75R15s on my Galaxie, and could turn low 20s on long highway trips with a 351w, FMX, and 2.75 gears.
Those 14's are tiny, and you'd be hard pressed to find a proper tire for it. Your car weighs at least 4000lbs, most tires that size are about a 1500lb rating. Those tires don't have the capacity to handle the car at it's listed GVW, especially if you don't have a 50/50 weight split.
The larger tires have a better weight rating, and also being taller give you a higher effective gear ratio.

 

Now where is he going to find the deluxe Torino color-matched wheel covers in 15 inch? Them tires he's talking about are the factory size, and are rated high enough to hold the road-hugging weight, if little else....

 

351m with a smallblock pattern I've never seen 351c and 351w were popular but they actually made the 351m was that 76 only or what. You might check with gear venders seems like they were 21 or26% but I'm not sure.

 

351m started '75. 351c was gone, 351w was somewhere at the time.

Gear Vendors won't bolt to an FMX, made only for C6 afaik. $3000 solution + a different trans.

 

Second that, plus the bigger tires will look better, IMO. I tried a Holley spreadbore on my '75 Ranchero 351W to get better mileage, with dismal results.

Try to use all the other mileage aids, like synthetic lubes, clean air filter, proper tire inflation, etc. No matter what, if you drive 75 instead of 55, you're going to get lower mileage, even if you play with engine rpms.

 

Are you sure it's a 351m not a 351w? IIRC the 351w came out in '69 or '70 and almost alkl cars could be had with a 289, 302, 351w, 351m, 400, or 460. If it's a 289, 302, or 351w then you can run an AOD behind it and get better mileage.

 

The 9" was also made in a 2.50 ratio. Since that's 10% higher than your current unit, it would increase your ideal cruise speed by 10%.

I believe there was a 2.40 and possibly a 2.25 ratio made. They might be hard to find as many people wouldn't have saved them, feeling that higher numerical ratio units are more desireable.

 

You would want enough torque multiplication to move that weight of car and not burn the bans up on the tranny. A 2:50 gear ratio is way too high for that size of car. I would want at least a 3:54 in there. It seems a classic like a 76 would be more of a weekend cruiser and mpg wouldn't be so much of a factor. If I had that baby it would get the works, holley 4 barrel, cam, headers and shift kit. I would love to have a 79 ranchero GT with a 460 to play with.

 

I'm not promoting either increasing or decreasing Occupant's gear ratio, just providing information regarding one way of achieving his goal of lowered rpm at freeway speeds. Obviously acceleration will be decreased with a corresponding decrease in numerical rear axle ratio.

A low numerical rear axle ratio shouldn't cause burning of the bands. Slippage, i.e. torque multiplication will occur in the torque convertor. In a properly functioning transmission the band will apply and not slip. The transmission will behave basically the same whatever the rear axle ratio is, changes in acceleration will be the result of changes in overall mechanical gear ratio.

 

I'm with Pfogle on the 351....I had a '76 LTD with a 400, but I think cars with 351s, including the LTDs, were 351Ws, the 351M was only put in trucks. I could of course be wrong.

My LTD had a 2.75 or 3.0 rear end, definitely higher than a 3.54. It kind of sloshed out of stoplights, but it would cruise at 60-65 pretty much all day and pull about 17 MPG doing it, IF I could stay out of towns.

If you don't have a particular reason to keep the 14s, I'd try 15s, say 215/75 or 225/75-15, like Ford Six said. It's getting hard to find much selection of larger-sized 14-inch tires since mostly they're used on smaller cars any more.

 

The 351M was introduced in passenger cars (LTD/Torino & Mercury Marquis/Montego & Cougar) in 1975. Also used in 1977/79 Birds, LTD II's.

First used in F100/350's in 1977, as was the 400.

351C: 1970/74, cars only. 351M: 1975/82. 400: 1971/82.

351W introduced 1969, used in cars thru circa 1985. Econolines first used 351W's in 1975, Bronco & F150/350's first used 351W's in 1981.

351M/400 = 8 valve cover bolts / 351W = 6 valve cover bolts.

The lowest numerical 9" rear axle was the 2.47-1, only offered in 1977 & 1978. The next choice was the 2.75-1.

Considering the bulk of a Torino, there isn't much you can do to improve gas mileage.

Money spent (wasted, IMO) for a change in the axle ratio, or to install an AOD will buy 100's of gallons of gasoline.

Even if one was to install both, the improvement in gas mileage might be 1 to 3 HIGHWAY MPG tops.

The AOD is an expensive trans to repair and NEVER should be driven in heavy traffic in Overdrive.

100's of 1000's of AOD's were overhauled in the 1980's because in heavy stop and go traffic, the trans is constantly shifting up/down, in and out of O/D, which fries its guts...in a very short time.

btw: 1973/76 Torino's came with either 14 or 15 inch wheels.

 

Wow. That about covers it. Thanks, Bill.