I have an engine swap so the transmission is geared differently and I need a rear end that is very low, as close to 3.000 as possible. What options are there? The lowest limited slip is 4.101 if I am not mistaken but can you gear it to the 3.909 and keep the limited slip?

Obviously I am not learned on the 3rd member and hope you guys can help me, do I have to weld the differential and stick with the 3.909 or is there another option? Thank you in advance, hope to hear some good options!!

 

My memory is shot but I'm pretty sure the 3.90 came LSD on the GSL model's.

 

Why do you believe that you need a lower geared rear end?

Your engine swap has nothing to do with the gearing of the rear end.

Lower gears are higher in number, not lower in number and the standard gears in most of the 1st gens is 3.90 range with the Se in 84 with a 4.076. The chart that Mazdatrix supplies is incorrect for the 85 SE models. The 85 SE models are still the 3.90. A lower gear set would be what you see at Mazdatrix's site-

4.10 27-110B-M068 4.444 27-110A-M054 4.875 27-110A-1312 5.125


http://mazdatrix.com/g5.htm

 

Because right now 70mph is 4.5k RPMs in 5th gear so I feel I need different gears to optimize performance and gas mileage.

 

it wont get much better and its a lot of work

get another car as your daily driver... a honda if you can

 

IIRC, there is a early 80's Mazda where you can get lower gears.

here you go:

It has come to our attention that the 3.636 gears from a '79-'82 RWD 626 will fit the '79-'85 RX-7 rear. The 3.308 ring & pinion from an '82-'84 Diesel powered B-2000 / Ford Courier will fit as well (Thanks Randy). For '79-'83 RX-7s, it is a direct replacement. '84-'85 RX-7s use a larger dia. axle, but the side gears can be swapped, allowing use of the higher geared 3rd member in the '84-'85 RX-7s as well.

http://www.grannysspeedshop.com/

 

Guys get the terminology correct.

Those are taller gears!
Taller gears will lower the RPM in every gear of the transmission which can bog down the engine if you have low HP and torque like our engines have.

Are you putting in a 13b and have a 12a currently?

If you want to lower your RPM in 5th gear then install a 2nd gen transmission from a 87-88 non turbo application. They have the tallest 5th gear. The swap for this is easy and there are threads on this.

At 95MPH with the 3.90 gears and my 2nd gen transmission I am only taching out 3800 RPM.

If you are interested in mileage then view this thread-

https://www.rx7club.com/1st-generation-specific-1979-1985-18/fuel-economy-report-since-recent-mods-se-656700/

If you are running a stock 12a or stock 13b Se engine then a tall 5th gear or taller gears will decrease, not increase performance.

 

If you read his signature, he is running a "-M50B25 engine, from 325is".

 

missed that until you mentioned it. BMW straight 6, eh? then perhaps he should consider the B2000/Courier diesel that was suggested as long as it checks out to be a bolt-in modification.

 

Correct, I have an engine and transmission from the 3-series so it has oodles of torque but I run out of room to run due to the gearing combination invovled. That's why I was looking to change what I could and the rear end seems the easiest and most cost effective, but I am all ears if someone has any other suggestions I even thought of mating up a 6-speed to it from the M-powered cars but that's WAY out of my price range.

Thank you for your time though guys, it gives me a much clearer picture of the options and problems I might run into.

 

Your best option would be to swap in a rearend of different size, i would go with a 8" from a toyota. Then youll have a more wide range of gear ratios.

 

live with what you got, if u wanted cheap and easy modification u shoulda bought a rustang.