I feel the same way...although I guess you can get approximately the same result by putting in a taller axle ratio, as long as the difference between 1st and 6th is the same. My project has a .72:1 5th and with its tall rear end it's going to go 70mph at 1700 RPM. In fact I'm a bit worried about it being too tall, but I'll see once I get it on the road. Torque peak is at 2000RPM so I'm hoping it'll be ok.
it all depends on how fast you will be going. if you plan on going 80mph all the time you will be set! otherwise i would agree...
seeing how most car companies compare their 5speeds with the 6 you are probably right, but i also think the fit would benefit from having the gears a little closer together. 1st feels too short and 2nd feels like it bogs no matter how late you shift into it. the si never felt this way since it was much shorter. in the si 1st gear went to 40mph, then 2nd was right behind it going to 60 and 3 to 85ish. in the fit, 1st goes to 30, 2nd 55, and 3rd 80ish. i am sure the division of the actual ratios could be worked out into percentage drops in rpms. however the point is that on paper the si only goes few mph faster in each gear, but the real world results are profoundly different.
you probably could put a d15z1 in a fit, but i dont think you would accomplish much by it. the improvement in fuel economy would be marginal compared to the cost of the operation. there is a lot more that goes into the vx being amazing on fuel beyond the engine and transmission: factory tire size, as you said weight, creature comforts like power steering, the frontal area of the civic vs the fit, probably a more restrictive exhaust, etc. its not that any of these things make a huge impact on the fuel economy but it all adds up.
don't waste your time or time will waste you
The Fit seems to have a very short first gear and isn't as easy to be 'smooth' compared to Civics. I only drove a manual of a friends a couple of times, though.
I just now tried driving a '99 Civic Si of a friend today; comparing it to the '92 VX, I definitely could get into the feel of the Si easily. I could get the car to 5th gear so much faster in the Si than in the VX. I wonder if its because of having more HP and torque? It rode as well, or better, in 5th at 30 MPH in the Si compared to 5th at 40 MPH in the VX. I did like the ride comfort improvement to boot. The VX just wants to die compared to the Si if you try to drive it very easily. I'd imagine its because of going in/out of lean burn.
Though, I must say my memories are from the VX with the bad 02 sensor until just recently, and; since it was replaced the other day, the ride has changed drastically. It still is a little 'sensitive' but I reckon it all goes back to it being a lean burn motor.
I certainly wouldn't recommend trying to go all out racer with a D15Z1. Leave it to a non-lean burn engine. It'll last longer at least as a ricer, let alone performance gains.
I used to have a '99 Si. It had a 4.40:1 final drive, and was running 4400 rpm at 80 mph in 5th. My VX had a 3:25:1 final drive, and only turned about 2700 rpm at the same velocity.
Still, that (lightly modified) Si was the fastest car I drove until recently. I got pulled over running almost 140 in that car in 2005. Yet, it would still get 33-36 mpg (premium, of course) if you kept your foot out of it.