'77 Spider rear-end

[corndogpilot]

Anyone know how to obtain a different rear-end or at least different ratio's for the existing rear-end? I do a lot of highway cruising in the 75-80MPH range, and worry about the longevity of the engine running at 4000 + RPM all day. I believe the stock rear-end is a 4.30, correct? Is it possible to get a 3.90 or below for the car?

Thanks for the advice.

[So Cal Mark]

the 79 and later rear ends are 390, and the automatic version cars are 358

[MIGHTY 8]

So out of curiousity, I have an '80 spider with the manual transmission. I'm on my third diff. now, so for future planning what would I notice if I went with a lower ratio such as on the automatics? Anyone completed this conversion before by chance?

Thanks

[So Cal Mark]

slower acceleration, lower rpm at cruising speed, potentially better gas mileage

[MIGHTY 8]

Thanks!

Think I'll stick to the stock set-up. Spent so much on getting the acceleration to go up, would hate to take steps in the opposite direction.

[corndogpilot]

Thanks for the suggestions everyone. So if I upgrade to a '79 or later, I am assuming everything else matches up the same as my '77?

[So Cal Mark]

everything matches up, with the possible exception of driveshaft length. Some combinations require the matching driveshaft for the same era differential

[brad131a4]

Mark how hard would it be to drop a lower 5th gear into the tranny instead of changing the rear end.

[Jim DeShon]

I wouldn't be too concerned about running in the 4000 rpm range for extended periods. I ran mine as a commuter car for years with a 4.30 in it with no problems. I think changing the rear end ratio would reduce torque at the rear wheels enough to be a problem unless you gave it some more motor. Just my humble opinion. ...J.D.

[So Cal Mark]

these motors are just fine cruising at 4k all day long. Unless you can find an optional 5th gear that came in these trans, it would be extremely difficult to change