What trannies, if any, can be swapped with the original 5 spd on a 1989 Civic Si? Are there any with taller fifth gears? Interested in keeping performance but also knocking back the rpm on the top end (at 75 mph or so) to the mid/low-2000rpm range or so for improved gas mileage. Please also advise how difficult any possible swaps might be--mods and such.

Thanks in advance for the help. Dave.

 

.702 92-00 5th gear or .694 HF 5th gear, keep the rest of the stock gears.

You'll be at about 3000-3100 rpm @ 75 mph, perfect sweet spot for best mpg.

D15B7 with cam and I was getting 53 mpg.