Higher mpg with different tranny in 1989 Civic Si?
 

It looks like I'll have to put a different transmission in a 1989 Civic Si I recently bought. Would it give me any mpg improvement if I put in another tranny and if so, which one? Would a VX tranny fit for instance?

I VX tranny will not easily fit, I'm pretty sure you have a cable operated clutch, vx is hydrolic, I think the vx might have an extra mounting point as well, so it would be a bit of work to make it fit.
I don't have the specs of gear ratios in front of me, but if you search around you can find them, redpepperracing.com has them if you search there.

You probably don't want to go with any higher gearing than a DX tranny if that. The VX would be too high of gearing with a si engine. For example the point of maximum torque in 5th would be at 130 MPH. Unless you live by the autobahn. The DX tranny might out perform the SI for MPG but only if you do a lot of freeway cruising. In a hilly area it with stop signs every four blocks it would probably do worse.

If you want to try to see how your car would run with a HF or VX tranmission try driving around town with only second, forth and fifth gears

I *think* the user Stormin Matt swapped an HF transmission into his Si. Guess what his gas mileage was? EXACTLY the same. There's a post in the EF forum on Honda-tech about this exact topic - I believe stormin matt posts here too though. The overwhelming majority of the posts I have seen on this topic say that the only increase *may* come from swapping a DX transmission in. Even then, the increase will be marginal. At one point I had a '91 Si and also a '90 DX. Both of them bone stock - gas mileage between the two was nearly identical and only fluctuated marginally week to week.

I have an older civic (87 DX 1.5 CVCC) and did an HF trans swap. I gained 2-4 mpg at 65 mph on the interstate (38mpg with stock trans, 40-42 with HF trans) but I feel that it really hurts my mpg when driving around town. My engine has decent low end torque at 1500 rpms, so it works fine with my motor, but if your engine is a dog below 2000 rpms, then you probably won't like it/won't see a benefit IMHO

The only difference between the Si and DX transmissions is the final drive. 4.25 for the Si and 3.88 for the DX. One through fifth gear ratios are the same.

I have a '90 Si. With oversize 195/60R14 tires I'm getting 35-36 mpg at 65 mph (3000 rpm) per the speedometer, it is 2.1% slow. If I slow down to 57-60 it'll do better, but I tend to drive 65 mph as of late.

The VX trans. uses hydraulics to move the clutch fork. There is an aftermarket company making a cable-to-hydraulic clutch conversion but I don't know anything else about it.

Another thing to think about. Swapping the CRX HF's .694 5th gear in place of the Si's .771 would give you a similar final drive ratio as the DX's .771/3.88 combination (2.949 vs 2.991).

Actually, I swapped an HF transmission into a DX that originally had an SI transmission. And I would guess that, if anything, a DX motor will respond better to an HF transmission than an SI motor. So, quite honestly, I would no go this route. You could try swapping a DX transmission to see how that works. After all, you can get them dirt cheap, if no free. But I would say away from REALLY tall gearing unless you go with an engine that is designed to run with it (ie D15B6 or B15Z1).