rear end ratios

[Mayhem_J30]

I've asked some generic questions in the 240 section but the experience is limited with this subject.

I found some 4.364 rear gears that nissan motorsports offer for the 89-94 240sx so I assume they would fit the J's rear end and replace the current 3.92. Please comment on the other options as well: 4.083 and 4.636. Personally I think the 4.364 will be just right and I'm not worried about gas mileage since the J30 is no longer my daily car. This looks like about a 11.3% change so I would assume mpg to be decreased by a similar amount just as rpm to increase by a similar amount.

-Also they offer new speedo's to compensate for the different gearing. Which # of teeth is necessary to keep my speedo accurate with the new gear? Available is 17t-22t.

-Last thing, I am familiar with replacing driveshafts on other vehicles so that might give me some advantage, but will this job exceed the skill level of most DIY'ers?

[AZhitman]

Mayhem - Do a search for "4.08" - Dennis had some good insight on the subject.

That's SO NOT a DIY. That's why I said in the other thread that you'd be better off doing a bolt-in swap, unless you're hooked up with someone from a driveline shop (except that the 4.08 would do you no good). Call around and see what a ring/pinion swap would cost... Pretty pricey job.

Rearends are super-difficult to set up properly, even in an old domestic car with less specific tolerances.

[Mayhem_J30]

hmmm...which part is the hard part, pulling the diff out to work on or working on the gear swap?

[Q45tech]

The gears must mesh exactly as the load varies with acceleration cruise and deceleration.......to get 150k life --- a bad job can wear out the gear contact points in 1,000-5,000 miles not to mention the noise. A bad job lets the gear wobble and wear and explode.

11.3% more rear gear multiplied torque [cuberoot 2.2%] would be the approximate acceleration gain in gears for the quarter.

Lot of work for 0.2-0.3 seconds.

What if you tried to couple a 90-93Q transmission with its 5% lower 1st and 2nd gears........when you think about it this is what they did in the 90-95 turbo 300zx AT vs the normally aspirated 300zx.The ideal 1st gear x diff would be around 13 to 1 vs your current 10.8 or so [the Q is 9.85 to 1]. The new BMW 745 has a 14.1 to1 1st gear and shifts @ 37,66,102.

The 4.63 would be ideal [to maximize acceleration] for the Q/J [but who could stand the 30% lower gas mileage] and 60,000 mile engine life reduction from increased cruise rpm.

With any gear change you must evaluate the effect on the 50-70 mph pass as you can get yourself into problems if a slow shift occurs in the middle of a pass facing a semi. No gear willl make up for the [up to 1 second] time lost during the shift!

This is what's killing many of the new 5-6 speed AT they are optimized for zero to 60 and a near 100 mph quarter but must shift 62-72 mph vs the old 4 speeds which stay in same 2nd gear from 50 to [80-85-90-95].............everything is a trade off.

[Mayhem_J30]

I would much rather go a route where 1st through 3rd could be lowered and 4th stay the same for cruising purposes. But modifying a transmission rather then a diff sounds a lot more difficult although it also sounds like it would yield better results especially in the long run.

[Q45tech]

All the 90-01 AT have the same 1.000 3rd and 0.694 4th gear......the newer AT 5 speeds have a 0.8 gear for 5th so all the new diffs 2002 [G/Z/M/Q] are less than 3.4 to 1. [ 2.8 to 3.331].The new 6 speed manual uses the same as old Q 3.538 to 1.

There is no easy solution.

[diamondj30]

hey criss,when i did mine it was 1500.00 to put the 436 gears.glad i did.10 over on the speedo.the geasrs are about 500.gotta go.

[Mayhem_J30]

diamond gave me the idea to begin with but since he's been busy lately i haven't been able to ask him all of these questions. hang around more brother

[diamondj30]

hey criss,by your sig,it looks like you have a badass j30.