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Looking at swapping a manual trans from a '97 OB into a '98 legacy GT, but the OB trans has a different 5th gear than the the GT, ,y friend who will be helping me seems to think it could result in lower MPG, any insights on how much of a difference this might make? Also, is it possible to swap in a gear that might actually get better MPG than the car did stock? sorry to be so vague, I'm unsure of the numbers he was quoting me on this.
Thanks

According to my chart, the Outback is .871 and the GT is .780, both 4.111 FD. You will be revving higher on the highway, using more gas. You're looking at 3385 vs. 3031 rpm @ 70 with the tire diameter of 24.88 that I was too lazy to change in my calculator. The larger tires of the Outback probably make it a wash in the respective cars, but will end up costing you gas with GT diameter tires. The tallest 5th gear I know of is .738 on WRXs, which also have a gas-friendlier 3.90 FD. To figure out what it would cost you in mileage exactly is impossible, but the theoretical would require CD, frontal area, and injector duty cycle at the different load each gear would require at a given speed. A more broad guess could be made using the aero information if you had an engine dyno plot listing horsepower using the BSFC formula (which I alter to reflect lean burn state. I forget what I change, but it matches fairly well when compared to published mileages of various cars).

Just try it and find out. The cost to change 5th gear would buy you a years worth of fuel.

The Outback transmission was "designed" with larger diameter tires in mind so you may have speedometer issues.

I ran this exact transmission. It was awful on the highway - in practice that short 5th gear translated to ~3800 RPM at 72 MPH. Terrible for mileage.

Well, I drive 600 mi/wk, so its clearly not going to work out unless I switch something. Thoughts?

Hmmm, sorry to thread jack but how bad do you think it's going to be for me with a 4.444 5spd and rear diff in my 91? Gonna be rockin stock wheels with whatever size tires came on the car.

I'm guessing it's going to be horrid.

Horrible. I wouldn't drive a car like that, personally, but I hate short ratios. I keep looking for a higher gear and the idea of burning way more fuel than I need to cruise is like nails on a chalk board to the pragmatist in me. After 4 years with my Mazda, I still sometimes think I'm in 4th because 5th runs 23 mph/1000 rpm (same as 5th in my 1983 GL). The Legacy ran about 27 mph/1000 rpm and that wasn't bad as much taller would have left it with no power if I needed it. I wish I could cheaply run over 30 mph/1000 rpm in Tigershark, but unsprung weight will already be a concern (ruling out taller tires) and the reverse-cut FD I need only comes in the 3.7 my Legacy has (at least not 3.9 or worse).

I had a 05 WRX transmission and VF8 turbo in my Legacy. The gearing was perfect, and the turbo was large enough that it wouldn't spool up unless you were trying. I averaged about 30mpg.

Compare that to my other setup - Legacy outback trans with a VF10 turbo. 70mph is getting close to 3500rpm and the turbo spools and hits boost with just a light tap on the throttle. I average about 22mpg.

I miss my old setup.

Doubt I'll be running a turbo, so that's not really a concern. I'm hoping to use the OB trans, what would be my best option for getting the MPG up?

Getting another transmission instead would be the cheapest. You could get Outback tires and struts to fit them, but that will also hit your acceleration in the other gears as you're essentially putting in a taller FD (as well as having to accelerate bigger wheels), so that will make all your gears longer. I suppose you could make aerodynamic mods to make up for the difference, but i think it would depend on how low you can get the load on the engine and low the IDCs go in lean burn. If you don't reduce the load on the engine below the level it already would attain, you wouldn't see any gain and extensive aero mods will cost you money and time to make and experiment with and look "funny" to most people.

One thought I had is to get a trans out of a 95-99 L which has a 3.9 FD ratio. That way he can keep the 3.9 rear diff he already has and perhaps gain a few mpg on the highway. Anybody have any practical experience with this setup?

Why not just run larger diameter tires? It readjusts your final drive and you get some extra ground clearance as well.

I would have guessed someone would have called me on my mistake by now. He has a 4.44 rear diff in the car now, not 3.9. Where did I get the idea that auto trans cars have a 3.9?

98 GT w/ MT should be 4.111 FD.

It's an AT, I'm swapping an MT in for him.

IF MPG is a big deal, then find a WRX 06/07 Trans and rear end. 3.70 final drive

Sounds like you need to do some more research, as there are few MTs that work with the 4.444 FD ratio that the car has now.
SO, you will end up having to do the rear end AS WELL AS the transmission.
( Early WRX would be the best choice for ratios and cost)

dscoobydoo wrote:IF MPG is a big deal, then find a WRX 06/07 Trans and rear end. 3.70 final drive

Then the question would be: Is a lower first gear possible?

Lunatech wrote:Then the question would be: Is a lower first gear possible?

Smaller diameter tires

See how it just keeps going in a circle. Lower gears:taller tires, higher gears:smaller tires. End results essentially wind up pretty much where you started. LOL, I've had this discussion with myself on many long drives.

Tire diameter affects all the gears, but short of changing transmissions/internal parts, that's the only option.

Automatic transmission gear ratios:
1st - 3.027-----------------------1st - 3.454
2nd - 1.619-----------------------2nd - 1.947
3rd - 1.000---------VS-----------3rd - 1.366
4th - 0.694 ----------------------4th - 0.972
reverse - 2.272 -----------------5th - 0.738
final drive ratio - 4.44 -----------final drive ratio - 3.70


How low of a first were you looking for?
To get the same ratio with the AT as you have now with a 3.7 FD, you would need the first gear from an STI
Your top gear would be better for MPG, but you lose it on the bottom end.
Unless you want to hand build a 6-spd, you are not going to get the ratios you want .