yes - when I put in the L31, i may need a 300km/h speedo...
Assuming I have the early 80's 280zx box, which I'm pretty sure I do, with my standard diff at 7,500rpm I can on paper reach 325km/h.
With a 3.7 diff it comes down to 310km/h.....
Even at 6000 rpm with standard diff and assuming I have the gearbox I think I do, I can do 260km/h.
Obviously wind resistance and rolling resistance will pull it down a bit, but I reckon the L31 should be able to get pretty close to 300.... Standard speedo only goes to 240. Hmmm.
Oh yeah, and Gav, whats your reasoning behind the 260 5 speeds being better than 280 boxes? according to http://zhome.com/ZCMnL/tech/GearRatios.html, the late 260 boxes (77 - 79), assuming they are the same as US 280z's, will give you quicker acceleration in lower gears, but the earlier 260 box and to a smaller extent 280zx boxes may be better with a higher diff ratio....