OK, get ready to flame me. I don't care about doing 170 top end in my F. Does anyone know what the rear end ratio is on these cars and if shorter options are available? I'm not talking 4:11 here, so don't get your panties in a bunch.

 

the isf has a unusualy larege ( for toyota ) pinion gear so there are no other gear options available as of yet.. but a NA or auto TT supra differental will bolt right up in to the F and they offer 4.09 and 3.76 ratios and a real torsen LSD, all you have to do is swap the outer axle u joints from the F to the supra axles.. however when in M mode the F gets its manual shift point limits from wheel speed so there is a chance you can ove rev the engine with a numericaly higher ratio

 

Sounds expensive!

 

This has been talked about before and you can get different gear options right from lexus by using either a 250 or 350 gearing....there was a thread on this but I don't remember where it is, use the search function and you should find it. There was and IS-F that was running 373's maybe with this mod.

 

yah an isx50 is a posibility but you woul be stuck with the same open diff..

 

this thread:
http://www.lexusfforum.com/forums/showthread.php?t=460

btw stock is 2.93, my car has the 4.09 swapped in- too bad its not lsd tho

 

Axtellz, you should take it to the dragstrip with some drag radials and report your 60' times. I'm willing to bet you can get 1.7s!!!

 

do you have anny problems in M mode? does the car still up shift and downshift normally when in D and M mode?

im thinking of doing a 3.76 swap to my car...

 

Actually it's this thread:

http://www.lexusfforum.com/forums/showthread.php?t=1175

Lou

 

I considered this mod, I called Doug from Lexus of Dayton and he helped me with all the info but decided not to and wait until a better time.

 

Perhaps one day i will get the car to the track, but she's my baby for fun when its nice out. The car works perfectly in all modes- the only thing is that the gearing makes first gear kinda pointless.. Gotta shift mighty quick, as it just flashes by in an instant. And obviously the car runs at higher rpms at highway speed, but i still get 20-22mpg in "normal" driving.

 

Any idea what going to the 3.76 would do to gas mileage. It would suffer, but how much?

 

Contrary to popular opinion, gearing and gas mileage are not inextricably related. I doubt it would change much at all.

 

So only the NA or Auto Supra Diff bolts up?

I still have the OEM diff out of my old track car which was a manual 97 TT. Needless to say it would be great if it would bolt right up to the ISF.

 

Would not be a large difference if all of your mileage is around town, will change however with an extreme spread like 4.10's on the highway.
Copied from mpgenhance.com
Rear End Gears Fuel Economy

A commonly overlooked component of a vehicle’s drive train that has a large effect on fuel economy is rear end gearing. This is the ratio of gears in the rearend (differential) that you may have heard of in the numerical terms 2.73 to one, 3.23 to one, 3.55 to one, 3.73 to one, and 4.10 to one (commonly written in the following manner 3.23:1 with a colon). These numbers specifically relate the ratio of the speed at which the pinion gear rotates to that of which the larger ring gear rotates within the differential. This means, for a 3.73 to 1 setup, that your driveshaft will spin the pinion gear a total of 3.73 times before one full revolution is completed with the ring gear which in turn spins the rear tires through the rear axles.

What does this all mean you ask? Well the bottom line is that the smaller the pinion gear or higher the numerical ratio i.e. 4.10 to 1 (a typically high ratio) means that the quicker your acceleration will be and the lower your car’s top speed that can be reached will be as well. You will be going through the transmission gears more quickly while driving and achieving a faster acceleration but will be losing fuel economy as well. For example with a 3.41 gear you maybe be at 1800 rpm in 6th gear on the highway at 80 miles per hour but with a 3.73 gear you would be at say 2100 rpm at the same speed 80 miles per hour in 6th gear. So you will be burning more fuel at the same highway speed because you are at a higher rpm. See the manual transmission section for an explanation of how one wastes more fuel at a higher rpm.

Power Gain & Fuel Economy

Installing higher rear end gear numbers will not technically increase your horsepower. In fact it will slightly decrease your horsepower and torque on a dyno test. However it will greatly increase your acceleration and allow your engine to operate at a higher rpm throughout your normal driving range. So you will be utilizing more of your engine’s horsepower and torque all the time. For example, typical LT1 and LS1 powered Camaros and Firebirds at the race track can reduce their quarter mile times by up to 2 or 3 tenths in the quarter mile by switching from 3.23 to 3.73 or 4.10 gears.

If you are looking to boost fuel economy, installing a set of lower numerical gears would be your best bet. Switching from a set of 3.23s to 2.73s will lower your auto’s normal operating rpm range during driving which will translate to higher fuel economy for you the driver. The difference can be anywhere from a few mpg to several mpgs depending on how much of a smaller ratio you select.



Advantages/Disadvantages

The fuel savings or increased acceleration advantages can be quite dramatic with a change in differential gearing. And you can select how much of a change you want with the large variety of sizes available on the market today from such rear end gear manufacturers as Richmond, Motive, and Strange Engineering to name a few. However the biggest drawback is in installation and labor. Installing a new set of rear end gears is commonly known as one of the most difficult automotive tasks to accomplish. It requires precise measurements when lining up the contact area of the two gears and must be redone several times if they are not lined up properly. If the gears do not mesh correctly during setup, this will lead to premature wearing of the gears, excess friction in the driveline, and whining sounds coming from the rear end while driving. Having said this it is wise to select a rear end specialist with many years of experience in installing differentials to perform this task.