bnickel said:basically if you drive only in town doing a lot of stop and go stuff and rarely get on the freeway or the highway a lower gear WILL get better mileage than the stock 2.79 somthing on the order of like a 3.25 or 3.50 would be about right maybe even as high as a 3.70 gear.
if you do use the freeway or the highway then the 2.79 gear you have will be better for mileage.
in town the lower gear allows the engine to not work so hard to get the car moving but on the highway the higher rear gear allows the car to stay moving with less work from the engine.
a good compromise for both in town and highway would be a 3.00 or 3.25 rear gear. i have a 3.25 gear in my car with a 351w and fmx auto trans. the last motor i built for the car before the current setup got an average 21mpg on the highway with a 26" tall tire, never measured in town though.
fasttback said:Yes, bnickel stated it well. If your worried about gas mileage (efficiency), stick with the numbers above. As always, there are other variables to toss in such as motor/weight characteristics.
My experience was from a 3.00 to a 3.50 gear on a 289. Since I did mostly city driving, this improved gas mileage and performance (I actually had to detune my carb in order to stop it from bogging off the line).
chromedog said:i did a tire change on my daily driver, (a 94 explorer) swaping 70 series tires for 75's... not much of a change, but a change, nontheless.this effectively raised my gear ratio slightly. i would have never noticed a difference, but my family and myself took off on a 1500 mile trip the day after i changed tires. when we returned home, i dug out my gas receipts, and averaged them together. my car, which normally does 24 miles per, had gotten just over 25, and i feel it was due to the tire change. the way i see it, the engine is turning a bit faster, but it is not turning as hard, due to the higher gear.
bad68coupe said:It just doesn't make sense to me. If it takes lets just say, 100 Calories of energy to turn the drive shaft once, and with that 100 calories....
ultrastang said:If you are running a C-4, C-6 automatic, or Top-Loader 3- or 4-speed manual transmission (all 1:1 in high gear), then you are greatly limiting the capability your car has in both performance and economy.
The best choice is to run a lower gear ~3.25-3.70:1 in conjunction with an overdrive transmission, such as an AOD or T-5 manual 5-speed (typically .68:1 in overdrive).
The overdrive will allow you to take advantage of off-the-line acceleration with a lower rear end gear, but at cruising speeds, the engine RPMs will drop drastically once the transmisssion goes into overdrive.
As an example, if you had a 26" tall tire, 3.00:1 rear end ratio, and non-overdriven transmission, your engine RPMs at 60 MPH would be ~2,326 RPMs. Given this very same senario, except coupled to an overdrive transmission, your cruising RPMs at 60 MPH would be 1,582 RPMs --744 RPM drop.
A T-5, in overdrive, coupled to a 3.50:1 rear end ratio would cause the engine to "see" the rear end ratio as being 2.38:1 (3.50 x .68 = 2.38:1). Even though the rear ratio may be 3.50:1, in overdrive, the engine thinks the rear ratio is 2.38:1 --That's even higher than a stock 3.00:1 ratio with no overdrive, so you would actually get better mileage with the 3.50s over the 3.00:1s, and still have much more accelleration in 1st - 4th gears with a T-5, or 1st - 3rd with an AOD.
http://www.ultrastang.com/Rearinfo.asp?Page_ID=3
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