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I have a 77 280z with an R200 rear end, 354 gears. It has a 4-speed manual transmission.

Questions:

1) Would it be smart to put 3.9 gears in the car for more accel. (because it's a 4speed not 5)?

2) Would it be SMARTER to put 3.9 gears AND a 5-speed tranny?

3) With the 4-speed and 3.9 gears, anyone happen to know the approx. RPM @ 60mph?

(if it helps for #3: right now at 90mph, in 4th gear, with a 3.54 ratio and 215/60R14 tires, my tach reads exactly 4000 RPM)
 

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Answer

The 3.90 would be a great option, but you will need to get a 5 speed. The 81 zx 5-speed has a great 5th gear for highway cruising.

> I have a 77 280z with an R200 rear end, 354
> gears. It has a 4-speed manual transmission.

> Questions:

> 1) Would it be smart to put 3.9 gears in the
> car for more accel. (because it's a 4speed
> not 5)?

> 2) Would it be SMARTER to put 3.9 gears AND
> a 5-speed tranny?

> 3) With the 4-speed and 3.9 gears, anyone
> happen to know the approx. RPM @ 60mph?

> (if it helps for #3: right now at 90mph, in
> 4th gear, with a 3.54 ratio and 215/60R14
> tires, my tach reads exactly 4000 RPM)
 

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Registered
Joined
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1,067 Posts
> I have a 77 280z with an R200 rear end, 354
> gears. It has a 4-speed manual transmission.

> Questions:

> 1) Would it be smart to put 3.9 gears in the
> car for more accel. (because it's a 4speed
> not 5)?

> 2) Would it be SMARTER to put 3.9 gears AND
> a 5-speed tranny?

> 3) With the 4-speed and 3.9 gears, anyone
> happen to know the approx. RPM @ 60mph?

> (if it helps for #3: right now at 90mph, in
> 4th gear, with a 3.54 ratio and 215/60R14
> tires, my tach reads exactly 4000 RPM)

Here's a Z-car algebra lesson:
1. take a tach reading at 60mph
2. divide the ratio of the rear you'd like to use by the gear ratio of your present rear
3. multiply the [email protected] by the number you get and you can see the effect of changing rears

example:
you have a 5-speed '77 280Z with a 3.35 rear and you want to change to a 3.9 rear for better acceleration. In fourth gear you are going 60mph and turning 3000rpm.

do the math...
3.9 / 3.35 = 1.164 (conversion factor)

3000rpm x 1.164 = 3492rpm

Assuming you have a 5-speed (5th gear had a ratio of 0.85 for 1977), in fifth gear you would be turning...
0.85 x 3492 = 2968rpm (this is about what a stock 4-speed '72 Z was running in fouth gear.

You can also think of using an automatic Z differential. the ratios for these were usually 3.75 and 3.5 depending on the year. Use them for minor changes and, if used, they aren't as beat up as the diff.s out of a stick shift.
 
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