mattengland

Summary:

How much gas mileage will I lose by switching from a 273 to a 327 gear in my '94 GT's rear differential?

Details:

I am getting my rear-diff fixed at the shop, including replacing a ring-and-pinion gear (forgive me if I'm buturing the terminology--I don't know much about cars) that has 3 chipped teeth (due to other problems I won't go into detail here--please ask me if you're interested). The existing, stock gear is a 273. I'm told (by my mechanic) that I can get better pickup with a different gear and that a 327 might be a good choice; the cost of the different gears is the same to me. I'm told that I might lose a couple miles per gallon in city driving, none with highway driving, and will gain significant pickup. The extra performance is mildly interesting to me as a driver, but I'm more interested in a potentially getting a better resale value when I eventually sell this car, hoping for a better "driving experience" at that time.

I'm basically trying to figure out how much extra it will hit my gas-paying wallet with the different gear. Can anyone confirm and/or provide perspective on the mileage changes with the gear change?

I'm also told that the spedometer will need to change to accomodate, and my mechanic apparently has that covered.

My car: 1994 Mustang GT Convertible 5.0 liter high-output with 85k miles (edit: it's a manual transmission). The car in general is in pretty good shape; I don't drive it in the winters. I'm not the original owner, but I bought it when it had only 6k miles on it.

Thanks for any help,
-Matt

mattengland

Does anyone have thoughts on this? I need to make a decision, probably by this morning.

fyi, I figured my option would be between a 273 (stock) or a 327 because the 273 is the stock gear, going to a 307/308 wouldn't make much of a significant difference either way, and going above a 327 would use to much city-driving gas.

Thanks for any help,
-Matt

Jugador

actually the general concensus i've seemed to find is that numerically higher gears are BETTER for city mileage because it's easier to get going and you can use more gears for cruising

mattengland

Interesting. So is there any tradeoff? eg, does a numerically-higher gear theoretically get worse mileage on the highway?

Jugador

typically it will get worse highway mileage because you have to operate at higher rpms. higher rpms is not always directly proportional to worse mileage, but in the case of the average 5.0 mustang, it would be.

mattengland

Is there any way I can attempt to guestimate a quantifiable difference in gas mileage for the highway miles? I tend to get a little over 20 mpg highway in the past. Could it go down signficantly? Possibly more than 10%?

Is there anywhere else I can research this?

My apologies if my continual questions are annoying.

mattengland

I don't think I've mentioned this yet: my '94 GT provides a manual transmission.

mattengland

I've seen a reference where the gear ratio is actually properly experssed as "2.73" or "3.27"...I think. Like I said, I know little about cars.