Originally posted by stuart barrie
...snipped..cjb...
So to reduce RPM at cruse speed I will need Diff and speedo gear to match.
Hi Stuart (everyone):
You can play with the numbers (all on ZHome.com), but it looks like you have a few options.
1. Change the rear gear from 3.9 to 3.364 (stock in the US) and that should drop your RPM (holding constant actual speed)... by about 14%. So instead of 4000RPM at an actual speed of 71mph, you would be turning 3440 RPM. (if I'm thinking about this the right way).
However this might feel a little doggy around town in first and second gear with your present 5spd.
2. Change the 5spd from the Euro Spec. with its .864 overdrive, to the US Spec. 81/83 280ZX 5spd. with its 0.745 overdrive 5th gear. Again that's about a 14% reduction. The advantage to this approach is that it leaves your Z feeling a little more peppy around town because of its slightly lower first and second gears (numerically higher ratios).
3. You could do both #1 and #2 if lowering your highway cruise RPM is the main goal. That's what I have in one of my 1972 240-Z's. It's the car I take on longer road trips, and it's set up for mainly that purpose. I wanted to be able to hear the stereo at cruise, without the engine noise. I wanted to be able to hear my passenger if she was speaking to me, without the roar of the engine.
With the Euro 5spd. you have fairly tall first and second gears. (lower numerical ratios -2.906 & 1.902).. with the 81/3 US Spec. 5spd you have a bit lower first and second gear (higher numerical ratios 3.062 and 1.858), which helps off-set the taller rear gear at lower speeds.
I would guess, and it's only a guess - that if you rebuild your 30+ year old 5spd., you could swap in the 0.745 overdrive 5th without any problems. As I recall all the stock 5spd.s from 72 Model Year forward through 83 were the FS5C71B with only different gear ratios.
just some thoughts...
good luck,
Carl
Carl Beck
Clearwater,FL USA
http://ZHome.com