i always put my car into 5th gear when im cruising at 40 mph. its 1 bar under 2k rpm. i wanted to save gas by shifting at 2.5k rpm, but now that i read this post, i will start driving in 4th gear.

 

Do you notice a difference on gas mileage though? have you compared it to when you don't shift that early?

I read that the boost gauge is a pretty good meter on how much gas you are actually using. Higher the gauge reading, the more gas you burn.

Thats kinda why i wanted to see what you guys thought. At 40-45 in 5th, on a flat run, my guage definetly reads higher than it does when i'm in 4th.

Unless i'm totally off about the whole gas/boost thing that is..

 

IMO I think it definitely helps, cause you're not at an RPM where the turbo will spool easily. According to my boost gauge I can barely build boost fast enough in 5th gear at 40-45mph.

I usually shift at 3,000 rpms while driving around the city normally. The boost gauge usually just barely builds enough boost to get me going and that's it. I can usually get 20-22mpg per tank driving like this in the city.

 

I've been using the Greddy E-01 for 8 months now. its the best for horse power increase and fuel saving. i can go as low as 12 psi for economy, or 21 psi for tearing ***!

 

60mph is where I'll go to fifth when crusing. Its a little too weak to pass anything below that speed. Of course, I come from the opposite camp than the Mustang guy, as I have a Subaru and Honda background.

 

Once I get over 3k rpms on 4th on a normal driving day then I switch to 5th.

 

Not quite. Fuel economy (mpg) is basically dependent on three things, which essentially multiply together:

- Engine revs per mile (depends on what gear you are in)
- Air intake per engine rev (depends on engine speed, throttle position and boost)
- Fuel consumption per volume of air (air/fuel ratio, on a stock Evo this generally gets "worse" -- richer, more fuel used -- as RPM increases)

At 70 MPH, full throttle in 5th gear, you will have a LOT of air going into the engine on each cycle -- lots of boost. But there are fewer engine cycles per mile, and the air/fuel ratio is not particularly rich yet. There is a lot of fuel being injected per cycle, but the cycles are long and the fuel is being efficiently burned.

Same speed, full throttle in 4th gear, you will have less air going into the engine on each cycle (less boost and a faster intake stroke), but more engine cycles per mile and the air/fuel ratio will be richer. Will that give better fuel economy than 5th gear -- it's hard to say, but I would guess not.

At 70 MPH, full throttle in 3rd gear, you will have even less air intake per cycle (boost is tapering off and the intake stroke is very fast). The engine is truly gasping for breath. But there are lots of revs per mile, and the ECU will be dumping fuel into the engine way beyond what is needed for combustion, just to help cool the cylinders. Fuel economy will not be good.


So yes, boost is one measure of fuel economy. If you want to save fuel, stay out of boost. But gear selection is probably a more important measure of fuel economy. Don't shift from 5th to 4th in order to decrease your boost and save fuel; it probably won't have the desired effect on fuel economy. Instead, use a light foot on the throttle.

 

I think "Noize" hit it on the head...we have no ability to ACCELERATE out of trouble if we're in 5th gear at 45-50 mph range. I will shift to 5th if on an open road going over 50 mph. 4th gear is the highest gear I'd ever use if driving in any kind of traffic between 35-50 mph.

 

Everyone is different, but I will not use any gear until I hit 3000 rpm's or higher. You have to remember that this is a small engine and as long as your not using boost your not really use that much gas. I shift higher in this car than any other I have ever owned or driven.