what rear gearing do you have with your m42?

[B-well]

I'm almost done with my 318 and its a vert with 4.27 rear gear.
I'm at 3100 at 65 and 3600 at 75.

This is the first time i'm changin the diff solely to get better mileage.
I've got a few diffs in the garage and id like to swap it once.

What ratio are you running?

What is the highest ratio you think i can go before it stops being beneficial mileage?

i have 4.10 and a 3.73 on hand and may have a 3.46 and 3.25 at my disposal.

Any input is welcomed.

Thanks

[Vtec?lol]

4.10 for city/hwy mix and 3.73 for mostly highway. anything else will kill mileage and drive-abilty.

[B-well]

4.10 for city/hwy mix and 3.73 for mostly highway. anything else will kill mileage and drive-abilty.

the car has no other purpose than cruising. no quick accelerating lolm42 or hard turns for this car.

3.73 it is.

do you have personal experience with the 373?

[Wanganstyle]

Probably 3.91 or 3.73.
No way to know without doing actual math though; Find an online gearing calculator and plug in your tire size and goal seek RPM for ideal cruising speed you want to be at.

[MikeDE]

I had a 4.27 in my Touring for almost 2yrs, it was hell.

Swapped a 4.10 into it and it's noticeably better, but def not the best for just hwy.

[KenC]

I rocked a 4.44 for a year or so. Made the car so much more fun to drive. Gave up 2-3mpg because of it though.

Drove from Portland to LA with it. 4400 RPMs for 14hrs :-/.

The car is slow no matter what but I think anything less than a 3.73 would make it unbearable on hilly terrain.

[B-well]

Probably 3.91 or 3.73.
No way to know without doing actual math though; Find an online gearing calculator and plug in your tire size and goal seek RPM for ideal cruising speed you want to be at.

ive already done the basic math to see where ill be for each ratio, but unless you get peak torque and optimal fuel comsumption RPM to add into your calculation you cant really get anything benificial.

Simply using a rpm/speed calculator gets you numbers but no good info. youd try to get the highest ratio if that was your only tool low revs = better mileage with that method.

I rocked a 4.44 for a year or so. Made the car so much more fun to drive. Gave up 2-3mpg because of it though.

Drove from Portland to LA with it. 4400 RPMs for 14hrs :-/.

The car is slow no matter what but I think anything less than a 3.73 would make it unbearable on hilly terrain.

im pretty sure a 373 is what im going to go with. and i have one on the shelf thats very fresh.

[Wanganstyle]

ive already done the basic math to see where ill be for each ratio, but unless you get peak torque and optimal fuel comsumption RPM to add into your calculation you cant really get anything benificial.

Simply using a rpm/speed calculator gets you numbers but no good info. youd try to get the highest ratio if that was your only tool low revs = better mileage with that method.



im pretty sure a 373 is what im going to go with. and i have one on the shelf thats very fresh.

Put the car on the dyno and you will know where your engine settings put out power.

then tell them to do part throttle pulls to see where non WOT has the best torque. AFR will tell you how clean burning it is for fuel economy-

Then graph some gearing charts and compare with the dyno charts.

I have had perhaps 6 different custom ratio differentials in my e30?

[B-well]

Put the car on the dyno and you will know where your engine settings put out power.

then tell them to do part throttle pulls to see where non WOT has the best torque. AFR will tell you how clean burning it is for fuel economy-

Then graph some gearing charts and compare with the dyno charts.

I have had perhaps 6 different custom ratio differentials in my e30?

This is a definite option but time/money consuming. ill see if there is any space open this week. hell id like to see what the the econom42 puts out lol

[Wanganstyle]

This is a definite option but time/money consuming. ill see if there is any space open this week. hell id like to see what the the econom42 puts out lol

If you really want to deviate from stock setting this is probably the best/only way to know for sure.

Worth it just for the amusement to hear your own car on the rollers!!

[B-well]

If you really want to deviate from stock setting this is probably the best/only way to know for sure.

Worth it just for the amusement to hear your own car on the rollers!!

my car, 318 convertible, came with 4.27, while 318iS came with 4.10

meanwhile M3 came with 4.10 and the convertible came with 3.25

so in my opinion the gearing for weight aspect is not important.

im going to go with the 3.73 as i have it and it has a fresh rebuild/paint from another car.

ill see about getting a dyno run with the 4.27 and then with the 3.73

[Toadleg]

I swap mine out to the 3.73 lsd. It lost a lil getty up but was worth it to have the lsd.

[B-well]

I swap mine out to the 3.73 lsd. It lost a lil getty up but was worth it to have the lsd.

still completely drivable with noticeably better gas mileage?

[Toadleg]

It was completely drivable and i didnt pay attention to the gas mileage I dont remember it making much of a difference. This was a 1.5 years ago and no longer have the m42

[Slovik]

In preparation for my s52 swap I ran a 3.46 for about 4 months and can tell you that the fuel economy over my 4.10 increased 3 to 4 mpg. 1st gear is really lethargic low down, but after you are moving you can almost be one gear lower than you normally would be. It is great on the highway at 75 or 80 which just sucked with the 4.10. I think the 3.73 would be best overall.