jamon8

iTrader: (3)

I know that on the highway you get better gas milage with a 2.77 gear in the back

my question is with such a small gear that rotates the tires 2.77 times to each rotation of the driveshaft wouldnt you get better gas milage if the tires turned 3.73 times for every turn of the driveshaft

this has me stumped and I get different answers every time

84Zjustme

Just a thought. It makes more sense to say 2.77 turns of the drive shaft = 1 turn of the rear tires.

Darkshot

iTrader: (4)

You have it backwards.

2fast4u92z

Yeah if the motor turned 2.77 for one turn of the wheels car would be slow as hell.

84Zjustme

http://auto.howstuffworks.com/differential2.htm

KrisW

iTrader: (5)

And.....

Don't forget about engine load and efficiency when you're doing this. Most every V8 car that I had only got better mileage with 2.73 gears if there was no overdrive. My third gen V8 cars have gotten WORSE mileage with the 2.73s than they did with 3.42s.

I had never thought of that until my buddy who had a 5.0 Mustang showed me the difference in his car between 2.77s and 3.55s. I was floored because it went against all of the conventional wisdom. We were in CA at the time and we were talking with an aquaintance from the DMV referee station. He said that while the mileage is not as good with 2.73s in our cars, the EMISSIONS OUTPUT was better at the designated test points for CA standards!

Maybe it's a myth or a hoax or whatever, the truth is, the math doesn't lie. On my V8 cars that were stock and slightly modified, they got better mileage with 3.42-3.55 with overdrive trannies. They got better mileage with 3.08 and numerically lower ratios when I was running no overdrive.

This is only my experience, not some physics proof from engineering class. I can say that it consistently works out for me...

Nasty-Z

HUH ?

That's actually how it works , driveshaft rotates 2.77 times for every 1 rotation of the axle shafts . The other way around would be one hell of a bonneville gear . That is why RPM goes up with a higher(numerically) gear swap .

And for what it is worth , lower (numerically) gear ratios are not always the hot ticket for fuel milage , as you make an engine work harder to overcome less torque multiplication in the rear axle.

Usually a gear in the neighborhood of 3.42:1 to 3:73:1 for a stock or nearly stock engine with an overdrive trans has worked well for me .

Just my .02

TOM

sofakingdom

iTrader: (1)

Right.... the 2.73 gears ONLY get better gas mileage when driven according to the government's HIGHLY SPECIFIC driving schedule; which simulates your grandma lugging the car along in bumper-to-bumper traffic in too high of a gear, using no accelerator pedal. The mfrs have NO LEEWAY WHATSOEVER in how they operate the car to get those numbers... the car is started, idled for a certain period of time, then accelerated in a highly specific pattern (like, 0 to 30 in exactly 10 seconds or some such), then held at that speed, decelerated, accelerated to some other speed, and so on. Very very rigorously prescribed. That "number" ISN'T just some claim that the mfr decides they want to make.

The artificial and unrealistic nature of this legally mandated driving schedule, and the car mfrs' adeptness in fudging the cars so they make high numbers while running it, is well and widely known. IIRC there was even one mfr who was discovered to have included a special routine in their ECM to recognize that the car was being driven according to that schedule, and invoked a special "mode" for producing the max gas mileage "rating". Talk about CHEATING.

This is why you been hearing all this hoo-ha here lately about Toyota and Honda, among others, having to lower their "advertised" gas mileage estimates... they're bogus. Even though they may adhere to the "letter" of the law, the numbers they print are not representative of the RW.

So yeah, cars ONLY get better gas mileage with those terrible gears, if the driver "plays along" with the subterfuge. When the driver drives like a normal person, it's actually possible to get BETTER mileage with a higher number rear gear, because the engine gets to operate at a more efficient RPM. Think about it.... if you changed gears and got 10% higher RPM, but also got 20% better efficiency, you STILL made a 10% improvement in fuel consumption.

KrisW

iTrader: (5)

Exactly.

I knew there was some science in there somewhere to help out my case. By the way, sofa, what rear gear do you run with your T-56? Do you actually use both overdrives to your advantage is or is 6th just extra weight?

sofakingdom

iTrader: (1)

I just have the stock gears (3.73); the car could use more. 6th starts being useful at about 65 mph.

Shagwell

your intake, heads and cam shaft all create your engine's power band. A set-up like mine doesn't make any power below 3000 rpm, so cruising much below 2500 would cost me ungodly amounts of gas.

Proper gearing is different per car due to different engine power bands, different vehicles uses and different driver's demands.

Cadman

Just to put some balence here, I have run as low as 5.38s on the street and used to enjoy drag racing my 427, 64 vette with 4.56s. I have left the RS alone with it's 2.73s, 700 trans, 5.0 TBI and its run well for the 12 years I've owned it. With freeway (cruise control) at 77 MPH it has gotten 24.5 for the last few years. That is down from 25.4 when I first got it. I have driven about 1100 miles on visiting trips every 90 days all these years and that is where it got the 300,000 miles on it. Good car, still runs great.

KrisW

iTrader: (5)

I figured. I was thinking something in the 3.90 range would do really well with that 6 speed.
----------
I think the fact that you run at 77 mph (vs 65) speaks volumes here. I bet at 65 you would do better with 3.23 or 3.42 gears...

Cadman

could be, that little engine is only turning 1900 RPM at 77 so there's not much power below that. Interstate 5 has a 70 MPH limit and a lot of the traffic runs 80+. At 77 the CHP doesn't even look at me.