Compare gas mileage between 2.73 and 3.27 gear ratios in rear diff?

I should add that I will need to decide on this quickly, for the rear diff on my car will get sealed up soon, possibly by today (Tuesday

2007-05-08). The different gear ratios are all the same cost to me, so there installed-parts-cost-difference here.

-Matt

I don't believe you will see an return on the investment if you are doing this for resale. What sales monetary advantage you might (and that is a big MIGHT) see, is very unlikely to exceed the cost of the gear change.

What is the purpose of the gear change?

If you want the higher torque to the pavement-snapper take-offs and you drive it like you like it, you are going to in all likelihood see a drop in mileage. If you put a lower gear ratio in and drive like a little old lady, never exceeding more than 1/4 throttle, you mileage may not take a hit at all.

Many things come into play when addressing fuel mileage. Optimal engine RPM at a given speed certainly is important for straight highway mileage. Rolling resistance is huge, too tight front wheel bearings, dragging brakes, improperly setup rear end gears and bearings, and improper tires/inflation to name just a few. Driving habits are the most significant and ultimately the cheapest way to improve your fuel mileage in any given car. Easy starts, anticipate your stops, drive with the terrain, repetitive starting and stopping hammers the hell out of your fuel mileage.

Now as to you mileage differences between 2.73 and the slightly lower 3.27 gear ratios, everything else being the same, even your driving habits, I would be interested to know. If you do make the change, It would be cool if you could log average mileage before the change and average mileage with the same use patterns after the change.

As to which gear ratio, having run everything between 2.50/1 all the way to

4.30/1. And specifically changing from a mustang 3.08 to some 3.89s I would suggest you look at something more on the order of 3.73 or a tad lower 3.90s because for the time effort and money to change the gears, you really should be having to pry the grin off your face to remove it after you take a ride with your new gears. The differance from 3.27 to 3.73 is marked. To me 3.27's would NOT be worth the effort, if I were going through the hasseles of changeing the rearend gears I would go to 3.73s.

And I haven't even touched on how delicate and precise rear end gear set up is, and how hard it is to find a guy that truly does it well.

Good luck

To be clear, my rear diff is getting "overhauled" (my term) because of

3 chipped teeth on my original, 2.73 gear due to problems I have not discussed. (I am NOT simply changing gears for the fun of it.) Given that the rear-diff specialist (and they are a specialist in Chicago--I called all over to verify this--they are World World Auto Transmission in Chicago) is already tearing the rear diff apart, and the fact that I can (or could have--past tense now) got any gear I wanted for the same price (2.73 through 3+, due to the used parts that the shop has in stock--and yes, I'm using a used gear), I ended up choosing the 3.27, seems like best combo of gas mileage and power. (They changed the speedometer appropriately.)

How it's been determined the axel needs changing, too; I might be able to change the gear again in this process for minimal extra cost.

I haven't fully digested the note below, but I don't see much about mileage/power tradeoffs there, maybe there's not much one can say? I appreciate the thorough response anyway.

I'm basically looking for "ballpark" guestimates on mileage tradeoffs between these gear ratios (2.73, 3.27, 3.72).

If someone wants the details of what went wrong with my rear diff in the first place, let me know.

-Matt

3.72/2.73 more gas used per mile, dumbass.

If you were at 21 you will be at 15.4 mpg

Unless you go a lot slower 2.73/3.72 or instead of 60 try 44 mph,

Serves you right for not putting in a blower.

I discovered a typo above. I mean to comare a 2.73 to a 3.27, instead of a 3.72.

However, some qualitative analysis may still be pertinent.

-Matt

I don't think you will see a huge difference. Slightly lower on the highway and slightly higher around town.

My take on it is this, the automakers are trying to squeeze the most mpg they can out of their cars. Most of them are putting gears in around 3.3 ratio for any car similar to yours that claims to be performance oriented. If they could get better performance without taking a mpg hit they would go to a numerically higher gear but they aren't. So that tells me the 3.3 is around the sweet spot. A friend of mine just changed his from 2.8 to 3.23 in a mid 60's stang and he definitely felt the difference in performance. I think you'll like your 3.27 just fine.

just gears, you are only trading torque for speed, no performance is gained.

How the hell do you gauge performance? 1/4 mile times? stop light racing? Gears make a HUGE difference. If you can still go over 120 miles per hour with 3.90 gears, yet you get there in half the time who gives a shit...

-cept your engine is all wound up in high RPMs, get a rice burner.

What??? How long have you been around this group? Have you ever been around a group of people that race Mustangs and F bodys??? One of the first and biggest kick in the pants mods is lower rear gears. And no the engine is NOT wound up in high RPMs. This is one of the most idiotic replies I've seen in this group. Changing to 3.73 gears wil also increas top speed.

Yes.

in 1/8 or 1/4 mile. But not on the highway, it decreases it, instead of 140 mph, you can only go

100 mph. (not too many places you can do 140 anyway, expressways are too rough, and nobody is watching for a 140 mph car, not like in Europe on the autobons) It is just gears, why you want your engine wound up all the time going 70 mph down the highway ?

It all depends on what performance means to you. For most people who will never drive over, say, 100 and who have a modern four speed, a higher ratio will give them what most people want for improved "performance".

And along the same lines, do you think there is no difference in performance between using first gear and using forth gear? If you think so, then there is no reason to have more then one gear in the transmission.

Really!! Not according to my GPS

EEEEENNK!! But thanks for playing.

autobons?

You are totally clueless on this, aren't you? I had a 340 3speed Charger with 4.52 gears, that wasn't even "all wound up" at 70.

Exactly. 3.73 gears gives you the extra grunt to over come the wind resistance and bump the top speed up a bit. It's people that have never been there that do not understand that. And I do NOT advocate trying it, it's dangerous and illegal.

can I drive your car and try it out ?