Ok guys I have some basic knowledge and I've tried to do some research but I'm not finding exactly what I'm looking for. I'm trying to find out if switching my tranny is gonna be worth doing or not. I have a 94 si w/ a z6 that's completely stock aside from suspension and tranny (it's an s20 I believe from a vx). Doing 70mph in 3rd I'm at 3k rpm, 4th is about 2300 rpm and 5th is about 1900 rpm.

What I want to know is if I get an si or ex tranny and drive it easy would I get better, worse, or the same mpg? Also would it be basically the same (mpg wise) to just stay in a lower gear like 3rd as compared to an si/ex 5th? If not y? I know that in theory the longer gears should get better mpg, but I also know that gears can be too long and force the engine to work harder than it needs to decreasing the mpg. So if my 3rd runs at 3k rpm at 70mph that should be better than a 5th at 4-4.5k rpm at 70mph right?

Anyone that can help I would greatly appreciate it. I know it depends on certain factors but assuming city, highway, up/down hills, straight aways and everything else is involved what would be the best?

 

I don't think you can really theorycraft mpg that easily. Lower gearing could result in operating at higher rpm, which is theoretically bad because it means the throttle is less open and the injectors are cycling more quickly. But on the other hand you can run leaner. And the head is more efficient at higher rpm in these cars.

But it's all a moot point if you might find yourself shifting earlier, which could mean you're actually operating in a taller gear than before. For example I used to cruise around town mostly in 3rd and 4th in my GS-R, and now with a Type R transmission I cruise mostly in 4th or 5th. So I'm actually using taller gears than before. If anything I think that might hurt my economy because Honda engines are really happy when they are revving.

In practice all this cancels out and fuel economy should be similar, so you're free to base an upgrade on other factors.

 

I know it's kind of a hard question and there may not be any definitive answer but all input is appreciated. I had another question, I'm not sure if it belongs here or not though. If I were to turbo the z6 (for efficiency not just power) would the longer gears help me more or would it pretty much be the same story?

 

Going into boost is much like VTEC, it will require more fuel to compensate. It's all sort of the same theory though I suppose.

 

for a single cam, it more depends on how much you really have to shift, and if you don't care about getting up to speed quickly. If you drive in more flat areas, and it's just a DD car that your tranny will be fine. But if you drive on a lot of hills, you have to downshift more with a longer tranny, to get more power to make the hill. As for a shorter trans, you would most likely not need to downshift because the shorter gears will have more overall "power".

Everyday I have to drive up a somewhat long steep hill on my way home, With my GSR and an LS trans, I would have to drop it to 4th, if not 3rd to make the hill, and with a Type R trans, I can make it in 5th.

With my single cam and a LONG gears trans, I can barely make it in 4th, but if I have an SI/EX trans, would have no problems in 4th, and could possible make it in 5th.

MPG between a VX and SI trans wouldn't change a whole lot, you may not even notice it.

 

I wouldn't count on a turbo doing much good for your fuel economy. I have been told there is a theoretical window they can operate in where the turbo runs so efficiently that it actually uses less fuel than if the turbo were absent. That window is small and relies on precise engine speed and throttle position conditions.

The modern use of a turbo as a fuel-saver by automakers is more about reducing displacement and maintaining power levels than making engines of a given displacement more efficient. And while they can rig the turbo to be efficient on fuel economy cycles, in real world driving my impression of their implementation is not positive, at least regarding fuel economy.

That said a well-tuned turbo does not have to be much of a fuel economy penalty.

 

For the moment the turbo is just an idea I'm playing with. I've read some very positive things though, some sounding a little to good to be true..... However even notaracist has said he turboed a d series and his mpg went up into the 50's if I remember correctly. All of this however is with good tuning and staying out of boost. In general though guys I know that have boosted 8-10 psi on turbos like t03/04 and have seen around a 5 mpg gain when they drive easy and they aren't tuned for fuel efficiency. So I would kind of have high hopes for a turbo tuned for efficiency with the a longer geared transmission (might help stay out of boost).

 

Notaracist got 58mpg on a turboed y7, just looked it up

 

On the subject of gearing vs economy. I had an ls 5 speed with a bad 5th gear so I used to scream in 4th on the freeway cruising around 70mph, but, I actually got slightly better gas mileage than when I had a good 5th gear. Not a ton, but I think its because the stock ls engine is really happy around the 4-4.5k region

 

4th gear in an LS isn't screaming until you have a 98+ JDM ITR trans, @ 60 I'm at like 3500 or so

 

4.058 FD, 1.000 5th, stock Y7, SMSP header/exhaust = 47mpg

Unless you have a D15Z1 or D16Y5, lower (shorter) gearing will net you better mileage if you keep your foot out of it.