Gas Mileage FAQ

Dang, I didn't know this was a sticky. I need to get in the loop. That's an insane amount of stuff you typed up there.

When all is said and done, I would need to extremely alter my setup to get better mileage than your final drive ratio...

[Sinister]

Originally Posted by Wrinklechops

When all is said and done, I would need to extremely alter my setup to get better mileage than your final drive ratio...

We need to double check that. Sometimes I've heard that the actual ratio between engine speed and wheel rotation is the same because they changed not only final drive, but also the gear's ratio's themselves. So we'll have to double check where you and I are at at a specific Speed.

Originally Posted by Sinister

We need to double check that. Sometimes I've heard that the actual ratio between engine speed and wheel rotation is the same because they changed not only final drive, but also the gear's ratio's themselves. So we'll have to double check where you and I are at at a specific Speed.

Yea, I suspect it's not far off. I am curious if Subaru changed the drive ratios when they went with the 2.5 engine in the WRX because they had to sort of compensate for the gas mileage - do you think? Because if everything else was exactly the same, you'd think a 2.0 would get better mileage for the simple fact that it doesn't have the volume to use air and fuel like a 2.5, hence the lack of power

[mycologist]

According to Cars101.com

2003 WRX:
1st - 3.454:1
2nd - 1.947:1
3rd - 1.366:1
4th - 0.972:1
5th - 0.738:1
Final - 3.9

2006 WRX:
1st - 3.166
2nd - 1.882
3rd - 1.366
4th - 0.972
5th - 0.738
Final - 3.7

2008 WRX:
1st - 3.166
2nd - 1.882
3rd - 1.296
4th - 0.972
5th - 0.738
Final - 3.9

So for all but 2006-2007 (I assume 2006 = 2007) you get 2.8782 engine revolutions per wheel revolution; 2006 gets 2.7306

At 3000RPM in 5th with 840rev/mile tires:
2003 & 2008: 74.45 mph
2006: 78.46 mph

[Sinister]

Originally Posted by mycologist

According to Cars101.com

2003 WRX:
1st - 3.454:1
2nd - 1.947:1
3rd - 1.366:1
4th - 0.972:1
5th - 0.738:1
Final - 3.9

2006 WRX:
1st - 3.166
2nd - 1.882
3rd - 1.366
4th - 0.972
5th - 0.738
Final - 3.7

2008 WRX:
1st - 3.166
2nd - 1.882
3rd - 1.296
4th - 0.972
5th - 0.738
Final - 3.9

So for all but 2006-2007 (I assume 2006 = 2007) you get 2.8782 engine revolutions per wheel revolution; 2006 gets 2.7306

At 3000RPM in 5th with 840rev/mile tires:
2003 & 2008: 74.45 mph
2006: 78.46 mph

Interesting info.

I wonder why the 1st and 2nd were changed...

[mycologist]

Originally Posted by Sinister

Interesting info.

I wonder why the 1st and 2nd were changed...

More torque = easier to get it going with longer gears

I wonder if that 3rd gear difference between 06 - 08 is a typo on their site.

[Sinister]

Originally Posted by mycologist

I wonder if that 3rd gear difference between 06 - 08 is a typo on their site.

I dont think so... previous years haven't been as concerned about 3rd gear nearly as much as 06/07's... the additional torque of the 2.5L mixed with the extra load of the 3.7 final drive, and then full boost in 3rd... it's the first gear to blow on an 06/07... so hopefully it was changed in 08 to help sort that issue out!

[cecilia]

Originally Posted by Sinister

Also, because resistance is not linear, I'll bet if we were cruising at 85mph, that the resistance would be a lot greater from windows being down, and the load would be closer to the top of our torque curve so it would make more sense to cruise with AC.

I don't drive 85 on the highway too often, so if anyone does... feel free to let me know the differences!

So. Just drove from Boulder to Jackson Hole and back, decided to experiment a little. No AccessPort, just the car's mpg reading, so may not be the most accurate ever, but... tried each combination for 20 mins. I-80 is SUUUUUUPER boring, so there were long stretches of really similar road conditions. I had a bunch of skiing/camping gear in my car, and no other people.

Driving to Wyoming:
75 mph, windows closed, no AC, just the fan - 29.7 mpg
80 mph, windows closed, no AC, just the fan - 27.6 mpg
85 mph, windows closed, no AC, just the fan - 24.9 mpg

sunroof cracked and front windows about 1/2 open:
75 mph - 28.8 mpg
80 mph - 26.1 mpg
85 mph - 23.7 mpg

windows/sunroof closed and AC on low:
85 mph - down to 20.4 mpg, didn't get to try 75 or 80, it started hailing. Pretty big difference for 85 though!

Driving back to CO - front windows 1/2 open and sunroof cracked the whole way. On the 85 mph stretches i was still over 30 mpg. (Guess it's slightly downhill overall?) Averaged 30.9 over 500 miles, overall averaging about 75 mph, lots of 85 mph, couple stretches of 115 mph and flooring it a bunch of times to pass people on 287. I'm sure it would have been lower overall mpg with AC. I'll definitely try it next time and add to the list.


I think that even at 85 mph the windows don't reduce mpg as much as AC.. which does sound weird because of all the articles/whatever talking about wind drag. I didn't try with all 4 windows open, so maybe that's the key?

[Sinister]

Awesome info!

[RayfieldsWRX]

The interesting thing from that info is the massive drop as you set your cruise for higher speeds. Much more significant than the AC factor, IMO.

[theangrysamoan]

I couldn't agree more those are some great tips

Thanks for taking the time

[Sinister]

Originally Posted by RayfieldsWRX

The interesting thing from that info is the massive drop as you set your cruise for higher speeds. Much more significant than the AC factor, IMO.

Definitely. I'm curious if it has to do with speed/gearing or drag more. I remember that cars when I first started driving were calculating highway mileage at 55.... and now it's 65. I'm wondering if they changed gearing to be good at 75 if we could get the same gas mileage, or if it has more to do with the friction than the gearing... Any engineers in here??

Originally Posted by theangrysamoan

I couldn't agree more those are some great tips

Thanks for taking the time

Thanks for taking the time to read it!

[RayfieldsWRX]

I tend to believe that it has much to do with drag and turbochargers. Once you get the brick cruising down the road at 85mph, it may be that you're now into boost territory. It may be that the way our cars are mapped, there would tend to be a substantial drop in mpg's. It would be nice to have steady-state boost readings to go with those figures provided by cecilia.

[Sinister]

Originally Posted by RayfieldsWRX

I tend to believe that it has much to do with drag and turbochargers. Once you get the brick cruising down the road at 85mph, it may be that you're now into boost territory. It may be that the way our cars are mapped, there would tend to be a substantial drop in mpg's. It would be nice to have steady-state boost readings to go with those figures provided by cecilia.

true. Boost is developed by load though. And gearing has a lot to due with load, as well as the drag. So I'm wondering if 5th/6th gears were changed if we'd be able to stay out of boost more and get decent mileage.

[RayfieldsWRX]

Originally Posted by Sinister

true. Boost is developed by load though. And gearing has a lot to due with load, as well as the drag. So I'm wondering if 5th/6th gears were changed if we'd be able to stay out of boost more and get decent mileage.

Probably so! I guess the engineers have to find a balance between having some power on tap for hills, and not having the rpms TOO high for fuel economy.