PaulC

OK you probably think Im crazy for having done that but my rear end had problems and was turning itself into junk. And the 3.90 was free

But I just finished off my first tank of gas and driving in Dallas traffic to and from work. I got 28.7 mpg, thats 363 miles on 12.x galons of gas. I was lucy to get 300 miles a tank with the old rear end.

With gas prices expected to go up again here shortly since OPEC has announced another reduction in production and drilling. The 3.90 rear end might become a sought after piece.

Then again I might be having flashbacks too , just kidding.

DoubleRX7Dee

I actually have the same exact project in mind....I think anyways

I'm hoping to put my 1984 GSL first-gen (FB) 3.909:1 rear end gears into my 1987 rx7 base model(or SE I guess...) gas prices steadily climbing is killing me in my probably 20 MPG at best 4.10:1 rear end.....and I have the turdy first gen car sooo....
all I want to know is will the pumpkin out of the first gen car just unbolt and drop right into the second gen's rear end? giving me a nice little clutch type LSD and a better gear ratio for gas milage??

ericgrau

iTrader: (1)

I was actually wondering about this for a while. Cool, I'll keep it in mind.

Best bet is probably the convertable rear end.

311unity13B

jesus christ, i only get 200 miles out of a tank

kungfuroy

iTrader: (6)

no, the 2nd gen uses a long pinion gear

HotRodMex

iTrader: (3)

did you change the speedo gear in the transmission?

ericgrau

iTrader: (1)

In case anyone gets mislead by this thread, most people go in the opposite direction for better acceleration. With the 3.90 you get better gas mileage but your acceleration suffers. Mainly in 1st gear I think, but it also makes it more similar to driving a 4 speed transmission in terms of acceleration in other gears.

slo

iTrader: (1)

going for 4.1 to 3.9 is only going to make about 200 rpm of difference, you could make more of a difference to your mileage by driving 3 MPH slower.

Ex-Eg-Driver

iTrader: (1)

X2

haha

03/08/87

Let me guess, you don't have power steering or AC.


that's the only way I could imagine getting that kind of mpg


Did you cruise at 60 the whole way?

03/08/87

Awesome man.


I like to measure how 'well in tune' a motor is by how much mpg you can get from it.

Icemark

wow you don't often see a thread from 2001 brought back by a newbie.

The first gen GSL uses too small of a differential for you to use a gear from one on a FC.

And the 3.9 rear end is available from any FC coupe with a auto tranny that would fit and is a bolt in.

rotarybeat1287

iTrader: (16)

Yeah my FC was an auto and when we swapped everything over to a 5-speed, we just left the 3.90 in there. I do get pretty good gas mileage for what the car is. But I'm not like obsessive over mpg numbers or anything.

1991 coupe btw.

HotRodMex

iTrader: (3)

HA, didn't notice the age on the OP. Guess I'm not going to get a response to my question

Are you sure the difference is that small? I know cruising at 80 the difference between a 4.1 and a 4.3 is like 600 rpm or something.

slo

iTrader: (1)

I used a java based gear ratio calculator, and came up with less than a 200 rpm difference in 5th at 75.

Easy math shows the change is about 5% worth, thats very small in practical terms.