Gear ratio in Rear End

[AUSMHLY]

How do you find out what rear end is in the car, without pulling it out and opening it up?

I've heard there is a formula which involves, marking the tire and drive train. Then counting the revolutions and using that figure with some calculation. Is this true?

What do you do?
Would someone break down how to do it better than my guess.

Anyone?

Thanks guys,
Roger

 

[stevebn2bj7]

I have tried that many times and it should work. However, I never get it to. It should be simple enough, if you have a 3.90 the driveshaft should turn a little less than 4 times for one revolution of the tire. It works that way when the diff is out of the car. Hopefully someone can explain why it doesn't work when it is in the car.

 

[GregW]

Hi Roger,
There should be a number stamped on the pumpkin which will give you the ratio, assuming no one has changed the gears inside. 11/43 would be a 3.9 ratio. Whatever the two numbers are, just divide them to get the ratio.

P.S. It should be on the face with all the nuts (on the front side of the pumpkin).

 

[JamesWilson]

Should be able to see it:

 

[Michael Oritt]

Yes that is how you do it but since your differential is open you need to keep one wheel from spinning while you rotate the other. Get the car up on jackstands, lock one wheel either by putting a jack against the tire or lock up the brake adjusters, then make one revolution of the other wheel while counting the number of turns the driveshaft makes and you will have your answer. BTW if you have an OD car it should almost certainly be a 3.9 diff.

BTW when is the last time you changed your diff oil? I think the pulling a diff is a fun thing to do every so often just to check on the feel of things, see if there are metal parts floating around in the bottom of the center section that might get up into your hub bearings and seals.

Report back.

 

[AUSMHLY]

Thank you,
Steve,Greg,James and Michael.
My Mechanic and I are trying to figure out why my in town mileage (exclusive per full tank use) is 9-12 mpg and my freeway (exclusive) is about 20 mpg.
Combination is between 10-15??

We are wondering what the gear ration is in the rear end, for I've not rebuilt that nor opened it up.
The Transmission I had rebuilt and is correct, as is the overdrive.

I'll make this question another post.
Cheers,
Roger

 

[Bob_Spidell]

FWIW, I installed a 3.5 rearend in my BJ8 and the mileage barely budged (I expected 1-2mpg better/highway). It was pointed out to me, basically, that the engine requires the same amount of power to move the car at a given speed regardless of the gearing; i.e. the throttle will have to be open more to push the car the same speed with taller gears, offsetting the lower engine RPMs (i.e. more torque + less rpms = same HP). The advantages of the taller rearend are more relaxed cruising (a biggee for me), (arguably) less wear-and-tear on the drivetrain, (conceivably) a higher top speed and first and second gears are more useful overall (IMO). The only downsides I see are occasionally a little judder engaging first gear (but that encourages good clutch technique) and (theoretically) slower acceleration, but I haven't noticed that much.

I suspect your mileage issues are elsewhere (obviously, carburetion comes to mind). For some reason, your engine is getting more fuel than it needs. My BJ8 gets 23+mpg at 55-60mph, and 18-19mpg at 70-80mph. Around town, well, I try to stay out of town :wink:. On my last summer road trip the mileage was down, and I found the rear carb was running rich according to the plugs.

If your tach and speedo are accurate, something like this might help compute the gearing: https://www.angelfire.com/fl/procrastination/rear.html

 

[GregW]

I recommend eliminating the driver from the equation. Send your car down here and I'll drive it for a while to see what kind of mileage I get. Then we can compare. I'll even do some freeway driving for you, maybe take it to Vegas. Oh, we have some great canyon roads (Latigo Canyon photos). I can flog it there too, just to help you get a full picture.

 

[Keoke]

Well that Greg fellow may be funnin but maybe there is more truth than fiction to what he says,For example when you are doing in town driving if there are a lot of stop and goes. Then when the engine is idling at the numerous stops it is in it's most fuel inefficient operating mode . Consequently in town driving mileage is greatly influenced by the driver and the traffic conditions the car is subjected to.-Fwiw--Keoke

 

[GregW]

Then when the engine is idling at the numerous stops it is in it's most fuel inefficient operating mode.

That and I heard a rumor that Roger shifts into second at about 40 MPH. Goes through rear tires often too. :laugh:

 

[AUSMHLY]

Should be able to see it:

Yours says 11 over 43.
I'm trying to read my with a mirror, but I think it says
11 over 34 ?

 

[GregW]

I doubt it is 11/34, remember the image will be reversed. That would give a 3.09 ratio and I think the tallest gearing was 3.54. Thing is your freeway mileage isn't that bad, so I don't think it is the axle. At highway speeds, you're usually in top gear so the axle ratio would come into play more. At lower city speeds, there are more gearing options.

 

[Patrick67BJ8]

I doubt it is 11/34, remember the image will be reversed. That would give a 3.09 ratio and I think the tallest gearing was 3.54. Thing is your freeway mileage isn't that bad, so I don't think it is the axle. At highway speeds, you're usually in top gear so the axle ratio would come into play more. At lower city speeds, there are more gearing options.

Maybe the tallest offical gearing was 3:54, but I had dropped in my lap a Sebring, 2:9ish gear ratio for late model rear end when I lived in Florida. I was told that it was brought over by the Healey race team and they did not take things like that back with them. It was well wrapped in grease and brown paper. I sold it. Patrick

 

[Keoke]

Then when the engine is idling at the numerous stops it is in it's most fuel inefficient operating mode.

That and I heard a rumor that Roger shifts into second at about 40 MPH. Goes through rear tires often too. :laugh:

I think you may have hit upon the actual cause of this problem Greg solving the Gear Ratio mystery.--Keoke-- :laugh: