Diff Ratio Question

[rb1971]

I'm in the process of having my side loader rebuilt, and was wondering if anyone has thoughts on whether 3.64 is the right ratio for an S38 coupled to a 265/CR? I've got a 3.45 right now and it seems a bit low (i.e., every time I drive the car at its comfortable engine speed in top gear on the highway, I am the fastest car).

If it helps, I do a mix of city and side road driving, although I'd like to retain the option to drive US1 down to LA and the like.

As an aside, I've located a shop near LA that has a good reputation for rebuilding side loader diffs using more modern parts - the cost to do a pretty nice build seems very reasonable. Happy to share if anyone is interested.

 

[BarryG]

My 88 M6 had a 3.91 diff, and it's the same motor you have in your car.That's want bmw put in them.

 

[rb1971]

That car would have had a 280 Getrag OD box from the factory though, right? So wouldn't you need to step down the final drive ratio with a 265/5CR? I might have my math wrong but the final drive ratio on the 280 was 0.81 with your 3.91 box, which I think means that you would need about a 3.16 with a 1.00 diff ratio to achieve the same overall drivetrain effect in 5th. (Math not being my strong suit, I might have this somewhat wrong.)

I've still got the 280 I had in this car originally and I agree that it would need to be much higher ratio with that trans still in the car.

 

[BarryG]

We are running the s38 / 280 trans in our e30 and it has a 3.25 diff...
so I would say yes step down.

BG

 

[kasbatts]

I run 3.64 and c/r dog leg box and its awsome, but having said that we dont have the big long straight roads you have up in the US, for the more twisting/winding roads (maybe the same as your side roads? ) we have here in NZ its perfect.

 

[David]

Timely question as I need to replace the diff and I am looking at LSDs.

I have a stock motor and stock 265 tranny. My driving is a mix of city and highway. I like they way it drives, but wouldnt mind just a little more low end grunt.

From what Ive read, 3.45 is what I should use. Does that sound right ??

 

[x_atlas0]

I'm running a stock 3.45 unit with my stock wheel size and a Getrag 265 OD. At 70, I'm running at ~2500 RPM. The vehicle is still sprightly around town, and I usually get ~25 mpg on the highway.

 

[HB Chris]

David, a 3.45 will reduce RPM at speed but accelerate more leisurely. However, the difference in ratios is only 5%.

Chris

 

[rsporsche]

go to http://www.diffsonline.com/techinfo/index.shtml.

you will find an excel spreadsheet - first you enter the ring gear of the diff (3:45 or whatever). then you enter the tire size. then you can put in the gear ratios of the transmission ... the end that you will see the result of mph / rpm (in 250 rpm intervals) per gear. quite a handy tool.

by the way, these guys are BMW guys.

 

[rb1971]

For what it's worth, the diffsonline guys built my replacement diff in my M5 a few years ago (and did a fine job) so I went to them initially when I needed a diff for my E9. Not only were they unable to source the parts needed to buy a side loader after telling me they could and having me pay to ship it out there, they haven't returned it and did not answer my last emails or calls several months ago asking the same.