I am looking to improve my gas mileage a bit. The 430 has plenty of power for me already, so I dont mind a *small* performance loss if there is one. I dont think crusing at 80mph at about 3k is good for fuel consumption. Ive driven plenty of other cars that cruise 80 at 2.5k or below. I dont understand why it is so high with the stock 5speed auto!

1) Will the GS300 or 400 rear diff bolt right up to the 430?

2) Will the gear ratios give me at least 500rpm lower at 80mph?

Thanks in advance,

Brian

 

the 400 gear ratio is identical to the 430 (3.266). The 300 gearing is actually shorter (3.92). Switching to either one will be heading in the wrong direction.

 

will the GS300 diff make it have better acceleration?

 

Yes and No.

It will have more torque in each gear but the gear will run out sooner. This will have the car upshifting sooner in each gear. The stock gearing will then have MORE torque for the time it stays in the lower gear.

So, when the car with the 3.92 gears shifts from 1st to second the car with the 3.27 gear will still be in first and have more torque until it shifts to second. The 3.27 car will hit 60mph in 2nd gear while the 3.92 car will have shifted to 3rd. Two slow shifts plus the difference in torque advantage going back and forth between the two cars can actually have the 3.92 car slower to 60mph. Same in the 1/4 with the 3.27 car going through in 3rd gear with the 3.92 car requiring a shift to 4th in the low 90mph range.

The 3.92 gear car will FEEL faster/quicker and is in each gear BUT with the above going on it may not be quicker and could be slower to specific points. It will also have a top speed around 125mph while the stock geared car can hit 149mph.

 

I have the 3.92 gears in my gs400. I'm running 80 at 3k. When I had my 3.26 gears I was running 80 at 2500. The 3.92 really did make a difference in the acceleration. I take my car to the local 1/8th mile strip. I was running 9.3s to 9.4s with stock gears. After I installed the 3.92 gears I went back and my time improve .3 of a sec. On avg and highest about .4s of a second.

 

Something that Jbrady taught that will forever stick with me.

Flywheel torque x 1st gear ratio x differential ratio = wheel torque to the ground.

You'll get more torque to the ground at the wheels, but the engine itself isn't making more power.

 

LOVE IT! Glad that someone posted real-world results. I always said that shorter gears WILL make the car quicker...perhaps not to SPECIFIC points such as to exactly 60mph (due to the need for the tranny to shift sooner) but overall, it is quicker.


Good post!

 

Important observation, you can multiply torque but cannot multiply power. The higher the multiplication the slower the output. Net power remains (theoretically) the same. In practice measured power on a dynojet usually drops a small amount with an increase in torque multiplication.

Now, if you had an engine that could magically rev higher and higher without limit and had a flat power curve... then you would NEVER shift out of first gear and the higher the multiplication the better. In the real world MATCHING the gear ratios to the engine and the GOAL is the best solution. Some engines, like Mercedes turbo V12s, have so much available torque at low RPMs and do not rev past 6k do best with lower multiplication. Our vehicles with relatively low torque can benefit with some additional multiplication. This is seen in the 2004 introduction of the 6 speed trans and 3.769 diff on the LS430. No power increase but a solid improvement in performance. Now, if we could INCREASE the redline of our engines to 7000rpm (and maintain power) a change to the 3.769 diff gearing would show a much better result.

That said the change FEELS faster because the torque at the GROUND goes up in each gear. It is just that the car looses top speed in each gear as well.