The Stovebolt Site Forums The Shop Area Engine Shop gear ratio = mpg

is there any type of formula to calculate mpg on gear ratio,i have a 1956 3100 with the 390? gear? with a 305 turbo 350, i get 12mpg at 70 mph, i wont to install a 300-350 gear, any way to figure it out, instead of lower # gear best mph and less get up an go

Not really, too many variables.
One problem is that at higher speeds, a higher proportion of the gas goes to wind resistance and less to engine speed, so slowing the motor down with a 3.00 axle won't have as much effect at 70 as a 50.
I'm sure you already know this, but the cure is a 700R4 and lock-up converter.

...part of the cure. You could also chop the top about 4"!

Panics right, but you might try backing into it. Find out at what RPM you get your best mpg now. This won't take into consideration wind resistance, and since your going to be slower your mpg will improve anyway, but there might jump. My 68 has a Quadrajet and if I keep the RPM under 2500 my mpg jumps from 8-9 to 10-12. I went from 4.56 to 3.73 it helped but I'm still over 2500 at 65 mph. Running the numbers rear axle ratio, tire size, on an RPM calculator will tell you what ratio to get to stay at your best RPM. On my 68 I would have better mpg with a 3.30 or 3.42. Hope that helps.

An overdrive would be the solution.

Larry

The impressive mileage figures newer cars and trucks get are a combination of factors, including aerodynamics, overdrive and computerized fuel injection.

Running the motor real slow can reduce consumption because it reduces the power lost to internal heat and friction. But unless the fuel metering is carefully controlled savings are limited by running too much fuel in order to be sure of running enough.

A better ratio in the rear will help but don't expect it to give a miracle cure.

I used to figure about 1/2 the the percentage of rpm drop to added fuel economy. For example a 30 % drop in rpm would equal 15% rise in MPG. This calculates if you are getting 12 MPG and drop the RPM by 30% such as a Warner OD you could realize 14 MPG at the lower RPM. You would have to drive a lot of miles before you would notice the difference in your bank account. These figures do not come from any scientific tests. Only observations from adding overdrives over the last 50 years with 50's engineered jalopies.

all good info, i have been driving it for 20 years now, not a show truck, typical daily driver,
maybe an overdrive trans first
i drive 75 miles one way to work, do not have a tach
i was thinking about rear end first because it has about 1.5" backlash at rear yoke, original rear end, 1984 trans, it is time to upgrade
thanks

The math presumes, reasonably, that slowing the engine down does not bring it below the speed where the torque curve begins (generally a function of overlap and intake valve closing - higher number for either raises the minimum RPM), or your stall speed (with automatic).
Stock cam + low stall or stick will run acceptably at under 2,000 RPM, hot cam and 2,500 stall won't.
Locking converters with computer sensors are very fussy about this, and will ratchet in and out of lock-up at certain RPM and throttle positions.
A V8 is probably safe, but some L6s are not happy at a specific point between 2,100 and 2,500 RPM (depending on the engine design) - it's a harmonic period. If it feels "buzzy", change the speed slightly.

I would say the torque curve is most important thing to consider, once you drop below that "ideal" point, you mileage will suffer just as much or more. It takes a lot of power to push a brick at 70 mph. The gear has to be matched to the engine, for example, our 2001 VW Beetle 1.8L runs 3000 rpm at 70 mph because thats where the torque is.

My trucks fuel mileage didn't drop much at all when I went from 3.42 to a 3.08, the engine runs less rpm and sounds better at 70, but it takes more throttle to hold it there. Now at 55 I do see a gain.

Other then chopping the top off and dropping the frame to the ground, you will have to find a happy medium and in gears and rpm.

Joe

did not think about power curve, will research my eng, it does not have a stock cam,will not run good on reg gas, diesels when turning off,premium all these years
it sounds like the overdrive trans will be first with a tach installed,it will be a non electric/computer overdrive, still just a daily driver,looking at the 200 or 700 series, looks like the 700r4 is an easier conversion, after those changes will focus on rear end next
thanks
stephen