well guys i have a 66 mustang with the stock 289 and c-4 tranny. unfortinatly the tranny is from what i belive to be a late 70s f series pick up.

i would imagine that the gearing is a little different. i also have doubts about my rear end. i spun the driveshave and found that i went around about 3.74 turns for 1 turn at the wheel. so i would suppose the gearing is something like3.75 or so. im not sure if this gear ratio was an option.

ok enough run around. this setup gets me about 9 miles to the gallon and my car runs 55mph at about 4000rpms. should i be getting this kind of fuel economy?

i would like to get a different setup as this car is my daily driver, and i was leaning toward a AOD tranny and a lower ratio locking rear end ( maybe around 3.14 or so.)

PLEASE EXCUSE MY CRUDE SYSTEM i am new at this game and not very keen when it comes to ford setup options.

any help would be great. anything at all.

 

I would go either AOD or lower gears, but no need for both. The AOD can be a pain to set up, but the 4th gear is worth while for a daily driver. The AOD will get you about the same as a 2.6:1 rear ratio with your current 3.80 gear ratio (if you have an 8"...3.73:1 if you have a 9").

9 mpg is kinda low, even for that kind of a gear ratio. I'm getting about 14 out of a built 302 with a 4 speed and 3.80 gears (3k at 60mph). I'm guessing you have some short tires on there, too. Your tires should be about 25.5" tall. You may want to think about a tune up and carb rebuild to make sure the engine is running its best. It is possible the power valve is opening, so you'll want to attach a vacuum gauge to the engine and drive around so you can see what kind of vacuum you have and make sure the power valve rating is the right rating for your engine.

 

well that is one problem i failed to mention, the only non stock componants on my car are a holley 500 carb. after doin the equation with my dad i realized that that is way too high of a cfm. and i also have a pertronix falme thrower igniton. i guess i can change the jets in my carb. is that correct? i also know i need new plug wires and with that i would put new plugs. that is the only thing i can think of as far as a tune up that would need looking into.

and i have stock look alike rims. but wider. 14X7 and my tire measures right over 25 inches. like 25 and 1/8 or so.

 

your carb size is fine.

You are getting 9 MPG because you have a c4 with a 3.75 rear.

I used to get 5 MPG with my 67 when it had a 351w, c4 and a 4.11.

Here are 3 possible ways you can drastically improve MPG (I'm sure there are more):

1. Don't drive over 40 for prolonged periods (not practical) - cost $0 (you could also lock out your secondaries, but 4k at 55 means you most likely need them to maintain that speed

2. Replace the 3.75 with a 2.79 or 3.25 - our coupe with a 289, toploader, and a 3.25 gets 22 cruising on the highway - cost $600-800 for a new thrid member with the ring and pinion setup, then 2 hours for you to pop it in on the driveway and refill the case.

3. Put in an AOD and keep the 3.75 - my fastback with a 408c stroker, 4.11 gets 16-17 cruising at 55 - that one time I drove that freakin' slow but I ussually get 12-13 MPG driving shall we say 'aggresively' - cost $600-800 for a good used stock AOD, more if you want a new one, or one with upgrades, $300 for the torque converter (unless you want to try and reuse the one that comes with the used AOD), $400 for x-member, linkages, yoke, tv-cable, holley throttle bracket, and $100 or so to shorten you driveshaft.

oh yeah and on edit, you will need a new flywheel and to change the imbalance on it to match your harmonic damper.

 

I assume you are talking about a 500cfm 2 bbl Holley. It may indeed have a blown power valve. Loose the Pertronix, I have never been impressed with them. The later Ford DurasparkII is an excellent high output ignition, try to get one from a 1977-early 80s truck, it lacks the computer interface. A 302 distributor will work fine in a 289. If you go with the AOD, the throttle control can give you a fit, but Lokar makes some nice linkage kits. My 66 GT350 had two 465 cfm Holley four barrels, 3.89:1 gear with F70-14 tires (probably 225/70 14 modern). It got 20-22 mpg on the highway at the then 70mph speed limit. Make sure your timing is set BTDC not ATDC, Ford scales run both ways. Make sure there is no vacuum applied when checking. Make sure the vacuum and centrifugal advance (if there) are both working. If you only have centrifugal advance try to achieve 36-38 degrees total advance with it. Oh, the Shelby had no vacuum advance, just a stock Autolite dual point distributor. Good luck getting it right.

 

500cfm is right....unless it's a 2-barrel, then it's too big. A 500cfm vacuum secondary carb will definitely have the secondaries open at 4k cruising and will kill your gas mileage (which is part of the reason I don't like vacuum secondaries). For high cruising RPM's, a double pumper is the definite way to go.