Ok, I was on the ISF forums and i saw someone had put our 4GS differential in their ISF to up their final drive ratio. Our 4GS ratio is 3.769. And the ISF ratio is 2.937. Am I crazy to say that if we were to put the ISF differential in our cars, we could get much higher gas mileage? (and oem lsd from the 10’+?) I know this would be kind of going backwards, but figured somebody might be interested to try it.

 

The GS 350 with 6-speed uses a Final Drive Gear of 3.615. The GS 350 with 8-speed uses a Final Drive Gear of 2.937.

The gear ratios of the 6-speed and 8-speed transmissions of course are different - that is the reason for the change in the Final Drive Gear ratio. Replacing a shorter final gear with a taller final gear will turn the engine fewer RPMs for a given speed in the same gear, which does not always necessarily compute to better fuel mileage. If it was more fuel efficient at all, it wouldn't really be noticeable under real world conditions.

 

Below are ground speeds at 6,500 RPM engine redline for all 6 gears (2013 - 2014) RWD - A760E 6-Speed Automatic:
(*Speed limiter is electronically governed at 142 MPH preventing max speed of ~170 MPH · **Engine will not reach redline in 5th or 6th gear)

1st Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
1st Gear Ratio: 3.52
Engine RPM: 6500 Redline = 41.6970892440055 MPH (Up-Shift Engages 2nd Gear @ 3940 RPM)

2nd Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
2nd Gear Ratio: 2.043
Engine RPM: 6500 Redline = 71.84226830097863 MPH (Up-Shift Engages 3rd Gear @ 4650 RPM)

3rd Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
3rd Gear Ratio: 1.401
Engine RPM: 6500 Redline = 104.76356469585963 MPH (Up-Shift Engages 4th Gear @ 4845 RPM)

*4th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
4th Gear Ratio: 1.000
Engine RPM: 6500 Redline = *146.77375413889936 MPH

**5th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
5th Gear Ratio: 0.717
Engine RPM: 6500 Redline = **204.7053753680605 MPH

**6th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 3.615
6th Gear Ratio: 0.586
Engine RPM: 6500 Redline = **250.46715723361663 MPH


6th Gear - 50% of Redline:
Engine RPM: 3250 = 125.23 MPH


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Below are ground speeds at 6,500 RPM engine redline for all 8 gears (2015) RWD - AA81E 8-Speed Automatic:
(Speed limiter is electronically governed at 142 MPH preventing max speed of ~170 MPH • **Engine will not reach redline in 6th, 7th or 8th gear)

1st Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
1st Gear Ratio: 4.596
Engine RPM: 6500 Redline = 37.66565457305995 MPH (Up-Shift Engages 2nd Gear @ 3855 RPM)

2nd Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
2nd Gear Ratio: 2.724
Engine RPM: 6500 Redline = 63.55042159243155 MPH (Up-Shift Engages 3rd Gear @ 4440 RPM)

3rd Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
3rd Gear Ratio: 1.863
Engine RPM: 6500 Redline = 92.92074525914309 MPH (Up-Shift Engages 4th Gear @ 5110 RPM)

4th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
4th Gear Ratio: 1.464
Engine RPM: 6500 Redline = 118.24545656952431 MPH (Up-Shift Engages 5th Gear @ 5465 RPM)

5th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
5th Gear Ratio: 1.231
Engine RPM: 6500 Redline = 140.62660310136764 MPH (Up-Shift Engages 6th Gear @ 5280 RPM)

**6th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
6th Gear Ratio: 1.00
Engine RPM: 6500 Redline = **173.11134841778357 MPH

**7th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
7th Gear Ratio: 0.824
Engine RPM: 6500 Redline = **210.0865878856597 MPH

**8th Gear:
Tire Size: 265/35 x 19" = 82.6" Circumference
Final Drive Gear: 2.937
8th Gear Ratio: 0.685
Engine RPM: 6500 Redline = **252.717296960268 MPH


8th Gear - 50% of Redline:
Engine RPM: 3250 = 126.35 MPH

 

Thanks for the info @bclexus

 

You bet - glad to help!

 

I'm curious to why there are so many MPG discussions and crazy ways to try to increase MPG. You bought a luxury sedan with a pretty powerful V6 that takes premium gas. You really want to swap out a very technical item that was specifically engineered for the transmission gearing to save a few bucks a month?

 

Yeah a bit off to do all that to save on MPG. But I would also say the GS350 is comparatively weak and also very fuel inefficient. The only good thing about the engine is that it is potentially more reliable than some others.

 

Must be something in the water

https://www.clublexus.com/forums/is-...nto-350-a.html

Lou

 

So since I have a 8 spd, if I were to swap in the ISF torsen, id keep the gearing normal and get LSD?

 

we all want to get most bang for our buck....not everyone bought this at sticker price. If we could afford anything, we would get {place brand - model here}, but we liked what the Lexus GS offered and are now trying to see if we can make our experience even better.

 

I agree- I went used so I could get a great car and save some money. Sure, I wish my MPGs were a little bit better, but I dont wish that the performance of the car was anything less than stock.

Do the math, if you currently fill up, say 5 times a month, and you were able to squeeze out a few MPG to make it be 4 fill ups a month:
($40/fill)(5 fills/month)(12months)=$2400/year
($40/fill)(4 fills/month)(12months)=$1920/year
So you can save ~$500 a year if you could manage an increase of a few miles per gallon. How much is the diff gonna cost? fluid? how much is your time worth?
To me I'll keep my fast VIP and just go easier on the throttle- driving style can save you some.