lower final drive mod - no difference in MPG
 

went from 4.11 final drive ratio to 3.23. highway cruising rpm dropped from 2500 to 2100 rpm. also when accelerating now engine load has increased considerably. very easy to accelerate with 80% load now. highway load increased from 35% MAP to 50%. overall fuel consumption has stayed exactly the same at 10l/100km 80% city. thinking of going back to 4.11 to bring back the peppy feel. I wonder if this has something to due with all wheel drive system?

Do you monitor if you are in closed loop or not? My guess is you might be hitting fuel enrichment.

i monitor my AFR...fuel enrichment starts at ~95% manifold pressure and I usually aim for 80%. its kinda hard to hit above 90% load without substantial throttle (I have a linear drive by wire map). timing is obviously lower under more load to prevent knock but I thought lower rpm would make up for that.

Is this in the Subaru? You changed all 3 diffs? (I've never heard of a 3.23 for a Subaru).


If you are 80% city, it wouldn't surprise me to see no increase. In my car with the "short" gearing, I spend much of my commute barely going fast enough to keep it in top gear (~35mph). If I dropped the final drive, I'd spend a lot more time in a lower gear and the gains would likely cancel out. ** this is in an automatic... with the manual I could hold 5th down a little lower.

Is there a change in mpg at a steady 55, 60 or 65 miles per hour on flat roads vs the old gearing?

Shucks, i was contemplating a 6th gear swap for my STI to help with highway crusing since that is where most of my driving is.

are you able to swap out just your 5th gear vs. your final gear?

Just a thought : where does the odometer pick up its signal ?

It might think you're doing less miles than you're really doing after swapping the final drive.

+1

Just changing the gear ratio, and not correcting the speedo/odo error, results in what you see - no change - because you're still pumping exactly the same amount of air/fuel volume per indicated mile.

At an indicated 60 mph, you're doing 76 mph, and only recording 78% of your actual miles. If this is the case, equal calculated MPG is a 27% improvement.

Have fun,
Frank

sorry the final drive is 3.73 not 3.23. no highway cruising mpg has actually stayed same if not worst. the engine is definately working harder. but i never trust instruments they are mostly fed from the MAF sensor and it does not compensate for enrichment. my instruments are always off at least by +/- 2%. i used pen and paper at the gas pump. in terms of speed sensor, the pcm was updated by dealership to make up for different ratio. gps verified speed and its good.

just confirmed its about 2200rpm at 65mph (300 rpm lower and 10% higher load). I can also cruise at 1200rpm now going 37mph and get about 40mpg. kind of hard to measure difference in mileage at crusing still as its so variable but it seems the same. The car runs pretty lean checked AFR logs too. im suspecting that the AWD is working harder. this project cost me 1500$ so far.

I have added some engine mods as this had been going on but I thought they would increase or not affect mileage:

- same diameter exhaust and headers but better flow (no resonator and high flow catalyst).
- delta camshafts, oneset for low rpm and other for high rpm use to keep bottom end friendly
- aggressive spark advance table (48 deg at 2500rpm cruising)
- leaner cruising rpm AFR (but richer full throttle)

dyno indicates more low and high end torque with BSFC at 2500rpm (full throttle).

1. you should be using a gps to track actual miles
2. Scangauge is then set to the gps
3. never use speedo after making changes.
4. do you run a baseline????
mpg at 45, 50 ,55, 60 ,65, 70?
mpg at 2000rpms, 2300 rpms, 2700 rpms......

If you had....then you would already know the answer BEFORE spending $1500