Question about installing new rear
Thread Starter
Heel & Toe
Joined: Dec 2003
Posts: 23
Likes: 0
From: York PA
Question about installing new rear
Hello! I have an '87 automatic coupe. We originally bought the car with a 2.59 rear, but in the hunt for quickness, we bought a used 3.07 and had it installed, which dropped the gas mileage from 24.6 mpg to 18. I don't drive the car as much these days and gas is never cheap, so I'm thinking about installing the original 2.59 and replacing the U-joints in the drive shaft at the same time. The only issue is that the 2.59 is sitting up in my attic with no cover which has left the gears exposed. It's warm and dry up there, so I have no fear about rust or anything like that. I've never done a rear-end swap on my own and only asked the shop to do it for me. Does anyone have any suggestions on what I should do to clean out the inside of the old rear before I attempt to install it?
Thank you!
Thank you!
Burning Brakes
Joined: May 2002
Posts: 1,143
Likes: 5
From: CASTLE ROCK CO
Hello! I have an '87 automatic coupe. We originally bought the car with a 2.59 rear, but in the hunt for quickness, we bought a used 3.07 and had it installed, which dropped the gas mileage from 24.6 mpg to 18. I don't drive the car as much these days and gas is never cheap, so I'm thinking about installing the original 2.59 and replacing the U-joints in the drive shaft at the same time. The only issue is that the 2.59 is sitting up in my attic with no cover which has left the gears exposed. It's warm and dry up there, so I have no fear about rust or anything like that. I've never done a rear-end swap on my own and only asked the shop to do it for me. Does anyone have any suggestions on what I should do to clean out the inside of the old rear before I attempt to install it?
Thank you!
Thank you!
Billy
Race Director
Joined: Apr 2003
Posts: 17,700
Likes: 1,667
From: moraga ca
Take a new mileage reading. There is no way the mpg would drop 25% by switching to a 3.07 rear gear. These were optional gears with some c4 automatics. I was running a 3.73 rear with a built 383 and an automatic and got better mileage at the street and strip than you are getting now with your 3.07. Leave it alone and enjoy the new performance set up. This is in a Corvette right? 
Team Owner
Joined: Sep 2003
Posts: 25,394
Likes: 2,741
Take a new mileage reading. There is no way the mpg would drop 25% by switching to a 3.07 rear gear. These were optional gears with some c4 automatics. I was running a 3.73 rear with a built 383 and an automatic and got better mileage at the street and strip than you are getting now with your 3.07. Leave it alone and enjoy the new performance set up. This is in a Corvette right?
I'm guessing maybe the speedometer was never correctly calibrated and that can be redone. Check the VSS in your car for color, it's either white or black. Which is yours?
In your particular location I'd be surprised if maybe the 3.07 if driven rationally might produce equal to the previous 2.59 or potentially better. York PA - I'd think maybe better.
Last edited by WVZR-1; Sep 15, 2014 at 07:53 PM.
