Ali SC3

iTrader: (10)

not sure about visually, but it sounds like it is the auto diff. the w58 even with the stock ratio feels kind of high.
most go for the stock auto tt lsd its a 3.76:! if they want lsd, its a good amount of labor so if a limited slip was in your plans then that would be the time to go for it.
alot also use the 3.92 its a little bit longer gears than the stock 5 spd as Khan said and it is easier to find from a sc400.
That would get you back to where you are supossed to be with a bit longer gear.
If you want really long gears you could swap in something with a lower number but you need the power.

Bleak

iTrader: (1)

So any sc400 diff is 3.92? I was looking around and some people wanted $350 for just a stock 5speed diff from a sc..

KahnBB6

iTrader: (5)

The SC300 5-speed diff is kind of rare but you can also find the exact same ratio with a 97-98 MKIV Supra diff and some years of the GS300 (96 or 97 GS was 4.27 I believe. Some were 4.08. There is a diff ratio bible posted somewhere).

All 92-97 SC400's had 3.92 ratio open diffs, yes.

Still, I think something is off. Even even verified by your GPS, those readings are still too high even for a W58 paired with a 4.27 auto ratio. FYI, 93-96 Supra MKIV non-turbo 5-speed (W58's) have the same combination that you do and their numbers aren't that high :/.

Bleak

iTrader: (1)

I don't know what else to check :\ I'll take some pics of the diff currently on my car tomorrow, hopefully that will clear a few things up.

KahnBB6

iTrader: (5)

Well, having messed with a changed diff ratio on a factory 5-speed car, it is usually the speedometer that is inaccurate because the speedo signal is taken from a sensor on the side of the transmission. You can cure that with a Yellowbox. That device changes the pulse to the speedo.

It's interesting because your speedo should be reading about 6% too high if you swapped in a W58 while staying on the factory 4.27.

This has to be an issue with your cluster, because the RPM signal does not have anything to do with any sensors on the transmission. Gearing changes can affect RPMs, yes, but there isn't a ratio available to make you spin at 5.2k @ 80mph. There just isn't. Your engine isn't spinning that high even if the dash says it is.

blk&blu*j

iTrader: (1)

Please read as I have only posted this 3 times!!!!!!!!!!!!!!!!!!!!!! My sons W-58 in a mk3 Supra with 4.30 lsd diff
is doing 3700-3900 at 80 in 5 gear. on a 16inch wheel. This is a bone stock factory car un molested!!!

He only verified the speedo he did not verify the true Engine revolutions tach speed!!!!!


Op think about this Lexus produced a stock Sc 300 with a W-58 and a 4.09 diff do you really think that this car off the showroom floor is doing 6000 rpm at 80mph??????
this car would have a life span of about a 2-4 mos for somebody who commutes 100 mile a day at highway speed. Think about this?????????????

blk&blu*j

iTrader: (1)

Your TACH is Wrong!!!!!!!!!!

1JZPWRD

iTrader: (59)

I say its the cluster itself. Its either the needle or the motor for the speedo. Even if you swap the rear end, its not going to help. Even adding a larger tire is not going to make it that much. I say swap the cluster or motor or reposition the needle and go from there. Think simple stupid here. Try the smaller things first then go big. The rear end wether an auto or manual or a SC300 or SC400 it does not make a difference. If the motor is screaming at this gear the. Your in 3rd gear, not 5th. Lol.. hope this helps.

Bleak

iTrader: (1)

-.- I'm in fifth. lol Also, I guess my next step is to get an external RPM guage and hook it up into the car.

KahnBB6

iTrader: (5)

I had to replace my physical gas gauge in my cluster because the little motor was becoming inaccurate. I wonder if a similar thing has happened with your tach gauge. You shouldn't need to get an external gauge. I'd see if the factory part is available or pull the cluster early and send it in for repair (but not to Lexus obviously).

Bleak

iTrader: (1)

The person who did my 1jz swap didn't use a tach adapter or anything, he said he just wired it in and it worked. (And for the most part, it does). I do actually have a MSD Tach adapter that I never used.. Could this be the issue? I am not getting accurate readings and need to install the adapter? Or the adapter won't do anything for me.. It's still new in the box. (I bought it for my swap and he said he didnt need it because he got it to work through wiring.)

1JZPWRD

iTrader: (59)

I think it is in the best interests of finding the issue by hooking it up.

KahnBB6

iTrader: (5)

^^ This. I didn't realize you had a 1JZ swap or I misread. You do need to modify the tach signal either with that MSD box or by modifying the gauge on the back of the cluster. Personally, I'd use the MSD *if* it does the same thing.

Here's a guess: the tach must have worked semi-accurately with the auto transmission (you swapped in the R154 yourself, right?) but something changed or was removed on the electronic side once the automatic was removed, perhaps because of an electrical signal the ECU needs from the automatic transmission or a factory difference in how Toyota engineered the automatic and manual 1JZ tachs to get their correct signal pulses. Are you using a 1JZ automatic ECU that came with your swapped car or did you switch to a 1JZ manual ECU?

Regardless, try the MSD. If that solves your issue then you're golden.

Ali SC3

iTrader: (10)

checked it on stock 5 spd its 3200rpm's @ 80mph and 3400rpms @90mph.

blk&blu*j

iTrader: (1)

Cool it took 30 posts to verify what I said from the beginning!

Buy the way the engine has nothing to do with the rpms, it could be a 12 cyl diesel an LS6, a V10

the final drive of the trans the tire size and the diff ratio are the determining factors.
not the Engine.

OP Hope you get it all worked out!