gear ratio/gas mileage

Took the Gal out yesterday and checked the mileage, 8 mpg. I didn't really expect much better but even with a 22 gallon fuel cell my driving range is pretty limited. With my Ford buddy and myself it weighs about 4600 lbs . I have 3.89 gears with 27.8 high tires. 60 mph is around 3150 rpm and 70 mph is about 3450 rpm according to my tach. He suggested changing the rear end to 3.00 gears, which I have. My question is with a 2800 to 3200 stall converter would this give me a noticeable difference and would that much less rpm hurt the converter/trans. The cams power band is 2700 to 6200.
Also it has a 'moderately worked' 850 Holley single pump vac secondaries, would a box stock 750 3310-4  improve the mpg?

Edit: it has a Mallory Unilite cent advance distributor.

Nope. If anything it could even cost you mileage. If you bring your cruise rpm below stall speed the converter is going to slip and build heat. At this point I would do some tuning on the carb. Make sure idle mixture isnt way over rich. Choke isnt staying on and or resetting excessively. Make sure jets are not too fat and the power valve is coming in at the right time.

kim wrote:Nope.  If anything it could even cost you mileage.   If you bring your cruise rpm below stall speed the converter is going to slip and build heat.  At this point I would do some tuning on the carb.  Make sure idle mixture isnt way over rich. Choke isnt staying on and or resetting excessively.   Make sure jets are not too fat and the power valve is coming in at the right time.

The additional heat is what I 'feared'  . My carb was 'worked' (not by me or my buddy) for performance. Rear power valve removed, jets changed from 82 to 88. vacuum check ball removed, 1/8" holes drilled in primary venturi plates, pump discharge nozzle changed from .040 to .042. Stock jets are 78/82.
Think I'll change the carb back to orig and go from there. I have gotten a 750 dp that I can use at the track (if I ever make it back, haha).
Thanks a lot kim

Check ball delete not a good idea. Causes secondaries to flop open. Spring change kit to facilitate a gradual open at desired vacuum/rpm better way to go at it.

kim wrote:Check ball delete not  a good idea.  Causes secondaries to flop open.  Spring change kit to facilitate a gradual open at desired vacuum/rpm better way to go at it.

Got the quick change spring kit but can't get the secondaries to come in right. Am putting the ball back in and reduce the jet size.

Side note: Found a formula to figure RPM.
RPM = MPH x Trans Ratio (in high gear) x Rear End Ratio x 336 divided by Tire Height. In my case (2800 to 3200 stall)
RPM @ 60 MPH is 2821
RPM @ 65 MPH is 3056
RPM @ 70 MPH is 3291
Darn, maybe I have the right combo after all .

Get yourself a wideband. Tune the primary main jet for best lean cruise. Increase PVCR to get WOT a/f ratio between 12.5-12.8:1.

kim wrote:Nope.  If anything it could even cost you mileage.   If you bring your cruise rpm below stall speed the converter is going to slip and build heat.  

NOT so ... realistically the cruise RPM can't be below ACTUAL stall speed occurring at that time and the rated "stall speed" is presuming Wide Open Throttle amount of torque going through it. At part throttle torque, the lock-up stall speed of actual converter lock-up, (which still will have a certain amount of slip unless it has a clutch in it), will not be up near that RPM.

rmcomprandy wrote:

kim wrote:Nope.  If anything it could even cost you mileage.   If you bring your cruise rpm below stall speed the converter is going to slip and build heat.  

NOT so ... realistically the cruise RPM can't be below ACTUAL stall speed occurring at that time and the rated "stall speed" is presuming Wide Open Throttle amount of torque going through it.  At part throttle torque, the lock-up stall speed of actual converter lock-up, (which still will have a certain amount of slip unless it has a clutch in it), will not be up near that RPM.

Soooo, I might be able to switch to a numerically lower rear gear if I chose? A 3.00 gear does seem a bit high and with that much weight it would be a bit sluggish from the 'git-go'. Would say 3.25 or 3.50 be better in my situation? I'm not attached to my 3.00 gear set over in the corner of the garage (next to my 4.30's ). I did have it set up as a spare but may be able to sell it are trade it. At 66 and counting I am thinking a little into the future, haha.
Thanks Randy

GT300TD wrote:

rmcomprandy wrote:

kim wrote:Nope.  If anything it could even cost you mileage.   If you bring your cruise rpm below stall speed the converter is going to slip and build heat.  

NOT so ... realistically the cruise RPM can't be below ACTUAL stall speed occurring at that time and the rated "stall speed" is presuming Wide Open Throttle amount of torque going through it.  At part throttle torque, the lock-up stall speed of actual converter lock-up, (which still will have a certain amount of slip unless it has a clutch in it), will not be up near that RPM.

Soooo, I might be able to switch to a numerically lower rear gear if I chose? A 3.00 gear does seem a bit high and with that much weight it would be a bit sluggish from the 'git-go'. Would say 3.25 or 3.50 be better in my situation? I'm not attached to my 3.00 gear set over in the corner of the garage (next to my 4.30's ). I did have it set up as a spare but may be able to sell it are trade it. At 66 and counting I am thinking a little into the future, haha.
Thanks Randy  

What you really need is to work toward installing a 4 speed G.M. 700R4 transmission. A 3-sometihing to 1 first gear ratio with an overdrive; (with a "lock-up" available if you keep the torque converter stall speed below 2,800 RPM).
To install one in a Ford with a 460, all it takes is a Powerglide conversion kit and a TCI valve body with total line pressure control.  And, of course a matching torque converter and transmission yoke.
Presently, a 3.00 rear gear will reduce your 60 MPH RPM level to about 2,450 RPM.