9" gear ratio question

[Erick]

We are looking at changing the gear ratio in our 9". It's currently 2.75 and third member is shot. It's setup with a 318 with 904 trans. Rear end is from a 58 Fairline 28 spline. Rear tires are 235/70/15. We are wondering what would be the best rear ratio to use for the rear end? Car will be used as a driver for city and highway.

Thanks,
Erick

[enjenjo]

I would suggest a 3.00, or a 3.25

[Learpilot]

I would suggest a 3.00, or a 3.25

I agree 3.00 works well most of the time. I would try the 2.75 and see how you like the performance. Remember a 9" is a poor man's quick change.

[C9]

I swapped to a 3.00 from a 3.70 diff (9") a few weeks back.
30" tall rear tires.
2400# 32 roadster.

I'm very pleased with the swap and in town performance didn't fall off as I thought it would.
It improved.

The original diff when I started driving this car 15 years ago was a 2.73.

Going to the 3.70 required a 1" longer driveshaft.

Reason being the nine inches have a couple different lengths of pinion shaft.

Changing to the 3.00 from the 3.70 required the 1" shorter driveshaft which I still had.

More than likely you have the longer pinion on both your 2.73 and 3.00 diff.

It may be due more to diff donor body style than ratio but something to be aware of.

[Erick]

Thanks a lot for the input. We would stay with the 2.75 gears but it howls terrible and the pinion seal leaks worse than the howl. Plus they want $60 for a pinion seal.

Thanks
Erick

[enjenjo]

I swapped to a 3.00 from a 3.70 diff (9") a few weeks back.
30" tall rear tires.
2400# 32 roadster.

I'm very pleased with the swap and in town performance didn't fall off as I thought it would.
It improved.

The original diff when I started driving this car 15 years ago was a 2.73.

Going to the 3.70 required a 1" longer driveshaft.

Reason being the nine inches have a couple different lengths of pinion shaft.

Changing to the 3.00 from the 3.70 required the 1" shorter driveshaft which I still had.

More than likely you have the longer pinion on both your 2.73 and 3.00 diff.

It may be due more to diff donor body style than ratio but something to be aware of.

I think you will find the length difference is in the yoke, and not the pinion shaft. At least on OE parts.

[Daveyboy]

Ok C9 why did you make the change?
Did you get better mileage?

[wayne petty]

it is posable to get better mileage with lower gears... if it moves the rpm range up off the idle transition ports .. so it is running on the primary venturi's      so if you are lugging the motor around town..  it may increase your fuel economy...

[C9]

I swapped to a 3.00 from a 3.70 diff (9") a few weeks back.
30" tall rear tires.
2400# 32 roadster.

I'm very pleased with the swap and in town performance didn't fall off as I thought it would.
It improved.

The original diff when I started driving this car 15 years ago was a 2.73.

Going to the 3.70 required a 1" longer driveshaft.

Reason being the nine inches have a couple different lengths of pinion shaft.

Changing to the 3.00 from the 3.70 required the 1" shorter driveshaft which I still had.

More than likely you have the longer pinion on both your 2.73 and 3.00 diff.

It may be due more to diff donor body style than ratio but something to be aware of.

I think you will find the length difference is in the yoke, and not the pinion shaft. At least on OE parts.

Correct.

Not enough coffee....

[C9]

Ok C9 why did you make the change?
Did you get better mileage?

Main reason for the swap was to get the engine (462" Buick) into a more reasonable rpm range on the highway.

70 mph with the 3.70's was 3000 rpm.
With the 3.00's it's 2400 rpm which works out nicely.

In-town performance - part throttle stuff - is better since the car runs through all three gears instead of dropping into 3rd right away.

Light car, plenty of power with the big engine so lugging wasn't really a factor.

It looks like mileage has improved.
Running down to the river to Laughlin, Nevada the road runs from 3300' altitude here in town to 4400' or so at the pass then down to 450' at the river.
About a 33 mile trip from the house and 12 miles of it is a serious grade coming up the hill.

Anyway, mileage on that trip usually shows 11-12 mpg with the 3.70's and the one trip wth the 3.00's showed 13 mpg.

I have yet to check mpg on a somewhat flat run like going from here to Hoover Dam, about a 100 mile trip, but November 2007 I got 16-17 mpg running at a pretty good clip.
Speed limits around here are 75 mph on the freeways and I ran 70-80 mph during most of that trip.

Idling 3-4 miles across Eldorado Dry lake and back probably put a dent in the mileage though.

[Daveyboy]

"Anyway, mileage on that trip usually shows 11-12 mpg with the 3.70's and the one trip wth the 3.00's showed 13 mpg. "

WOW i have a sbc 330 hp vortex heads New from chevy 370 gears. And i get 16 in town and 21 on the road with a 700 r4 trans
But at 70 i am running about 2800.
Would 300 gears give me more mpg?

[C9]

"Anyway, mileage on that trip usually shows 11-12 mpg with the 3.70's and the one trip wth the 3.00's showed 13 mpg. "

WOW i have a sbc 330 hp vortex heads New from chevy 370 gears. And i get 16 in town and 21 on the road with a 700 r4 trans
But at 70 i am running about 2800.
Would 300 gears give me more mpg?

Is the car you're driving the coupe in your avatar?

If so, your car is probably close to mine in the aerodynamics dept.

You're probably about as good as you're going to get unless you go to electronic injection.
A higher gear may lug the engine in OD.

How tall are your rear tires?



I'm running a mild cam and 750 Edelbrock jetted lean for the home town altitude and it runs good down at the river.
Hoping for 18-19 mpg in normal highway driving....

[phat rat]

"Anyway, mileage on that trip usually shows 11-12 mpg with the 3.70's and the one trip wth the 3.00's showed 13 mpg. "

WOW i have a sbc 330 hp vortex heads New from chevy 370 gears. And i get 16 in town and 21 on the road with a 700 r4 trans
But at 70 i am running about 2800.
Would 300 gears give me more mpg?

In order for that rpm to be correct you would have to be running a 22" tall rear tire. Either your tach or speedometer are reading wrong or maybe both. Running a 255/60X15 rear tire with 3.42 and a 700R trans I'm running 2100 @ 70mph. The same combo with a T-350 only ran 3000 @ 70mph