Hi,

This may be a stupid question, but humor me for a moment, okay?

While driving my WRX to work today, I found myself in 3rd gear (30mph @ ~2250rpm) behind a long line of traffic. I changed to 4th for no particular reason, and the revs dropped to ~1700rpm. So long as I was driving at a constant speed on a flat road, keeping in 4th was fine. However, I found myself changing to 3rd when approaching slight inclines since the car gets groggy.

This got me to wondering about fuel economy.

When driving along in the scenario presented above, which do you suppose is more economical - driving at 30mph in 3rd @ 2250rpm or driving in 4th @ 1700rpm? Taking it a step further, what do you suppose is the most ideal gear/rev to be in for maximum MPG? I'm assuming 5th gear @ 55mph, but perhaps that's not not the case.

Anyone have any ideas or opinions about this?

Curiously yours,
Bradus

higher revs equal more friction, equal more gas used.

For high MPG driving the idea is to be in the highest gear possible that can maintain your present speed without feeling groggy as you put it.

If you're under 2000, perhaps even 3000rpm, normally you feel like you need to downshift on an incline, but on a flat surface where momentum is quite important you can normally run quite a high gear comfortably.

If I'm around 60mph, I'll normally choose 4th. Any higher than 65 and 5th.

i think the 10-20-30-40mph shifting on flat roads yields good mileage, if you're willing to bear with the sluggish acceleration. for cruising, wouldn't the lowest speed in 5th gear give you the best mileage? i think that would be around 40, maybe a little less.

of course you may get rear-ended if you cruise at 40.

raj

I remember watching on TV a fuel-economy competition in the early 80's. Regular Japanese econo-boxes in rural Malaysia, I believe. This is what they do - start in 2nd gear, shift to 4th, then to 5th, all at the lowest rpm possible without severly bogging down the engine. Then cruise at around 40-45 in 5th. And of course, neutral when doing downhill or slowing down. But the main thing is very very gentle on the throttle and brakes.

Hey, I was only about 12 when I watched that, but it was so fascinating that I still remember it. :lol:

-Ray

I always thought that for the best MPG, shift as soon as possible without bogging the motor.

The thing that I've found saves the most gas on my WRX is keeping the needle at or under 0psi, but where's the fun in that? :D Really though, I think because when the motor pulls boost, the ECU richens up the mixture. This REALLY kills fuel economy.

seems to me you'd have to go a little deeper than "slowest speed without being groggy"

There has to be some kind of optimum engine speed is producing the most torque per rpm. That would be the optimum wouldn't it?

Originally posted by Bradus
but humor me for a moment, okay?
Okay, I know I'm getting off-topic now, but another way to get good fuel economy on the highway is to draft a semi-trailer. Drive really really close. But please DON'T DO IT! :o I used to be a truck driver, and it's very dangerous to follow a truck for various reasons.

-Ray

:lol: Don't do it!

Plus, it's a good way to coat all your intake hardware with diesel soot, and pick up rock chips in the paint and windshield. Not like anyone needs more of that, eh?:rolleyes:

I was reading up on the hybrid vehicles, and I remember them saying that the computer will only start the IC motor when it is able to run it at full throttle. Something about how they are most efficient when running with 0 manifold vac. Obviously this doesn't completely apply to this particular argument, but you would be running a lower manifold vac in a higher gear (more engine load). I believe this is the same principle that turbo-diesels use. Better fuel economy with higher cylinder pressures. Somebody correct me if I'm wrong.

Since we're mentioning diesel-turbos, I just realize that our WRXes are not the only turbo vehicles that don't come standard with a boost gauge. The 14-liter Detroit Diesel turbo inline-6 powered Freightliners I used to drive don't come with boost gauges either. :( Sigh... It'd be interesting to know what kind of boost those big single turbos give.

FYI, best fuel economy on those engines are at about 1500rpm, while fuel cutoff is at 1800rpm. And normal usable range is 1200 to 1500.

-Ray

Higher revs means more work done which means more fuel consumes... friction is not the KEY motivator for fuel consumption.

I've been giving this some more thought.

From a MPG point of view, town driving is always going to be less economical than highway driving so I guess my question about 30mph in 3rd vs 4th is not that relevant.

A more interesting question to other WRX owners is this:

What is the furthest distance you have travelled on one tank of gas?

Cheers!
Bradus

The best I've seen was 371.1 miles between Green River Utah and Wells Nevada, all at about 75-80 in 5th gear.

I have only about 4 other fills that were over 300 miles, typically I get more like 250-280 from a tank.

I drive one of those Honda Civic econoboxes that averages ~40MPG. Instead of a tach, the car has a shift light which tells you when to shift for optimum gas mileage. Unless the car is cold, the light always tells you to shift up, even when in lower gears at low RPMs. Seems to be consistent with what everybody else is saying here.

Question: How much does it cost all you WRX'ers to fill up with 91 octane when the tank is practically empty?

-Chris

Filled up this afternoon at 275 miles. It took 11.9 gallons @ $1.72 = $20.47.

Years ago Car and Driver did a test to find out what works. In all cases Wide throttle openings and low RPM produced better fuel economy. I have been practicing in the rex as it gets horrible gas mileage. I shift out of first almost imediatly. Just to get the car moving. Then I shift at 2900-3200 rpm. Dont be afraid to give the car throttle... The ECU is not dumping fuel just based on your right foot... A carb would but with EFI this is not always the case... I try not to go above 3200.. I can feel a distinct change in power at exactly that rpm. I presume this is the tumbler valves opening. You do not want to take the car into boost... So its a balance in RPM and your foot to keep boost down.

Low RPM and wide throttle has produced about 30% better mileage over light throttle and 4000 RPM driving.

kind of of topic:
P.S... On the detroit series 60 engines you should see about 30psi of manifold pressure. There is no wastegate on the turbo so this varies with load and temp and exact model. I once watched a 16 cyl quad turbo Cat diesel on an engine dyno. Actualy it was two water brake dyno's bolted inline. The screen said 1450hp and 4250 ft lbs of trq at 950 rpm with 45 psi boost. Water temp was 165 degree's and oil temp was 170 degree's. It was held at that load for over 2 hours for a break in procedure. Sounded like it was at idle. Operator said "man, this things weak, should do 2000" and shook his head. I used to modify Cummins engines in Ram's. Stock they would do 140 hp and 280 ft lbs at the wheels on a dyno jet. After they made 250-300hp and 650-900 ft lbs depending on how hot the owner wanted. I never took the auto trany trucks over 550ft lbs for fear of killing the tranny's. Yes.. This is at the wheels.. 1 tons ran low 15's at the strip and on the highway would pull away most cars.

CT.

rkkwan suggests: "But the main thing is very very gentle on the throttle and brakes." The WRX Owner's Manual agrees: "Always accelerate gently until you reach the desired speed. Then try to maintain that speed for as long as possible."

On the other hand, AZScoobie reports: "In all cases Wide throttle openings and low RPM produced better fuel economy." This advice agrees with recent research in Europe (see http://www.nfia.com/html/news/articles/news010607.html). Here's an excerpt from that article:People saved the most gasoline when they pushed down on the accelerator briskly and then shifted quickly, keeping the revolutions per minute low – not by accelerating very gently.

"It's not commonly understood by people who drive," Dr. Dougherty said. "They think that the way to get best fuel economy is to accelerate very gently, but that proves not to be the case. The best thing is to accelerate briskly and shift.

"Don't give it everything the car has, but push down when you're going to shift, using maybe two-thirds of the available power, and change through the gears relatively quickly."And, for reference, here's a shift table from the WRX Owner's Manual:

I asked subaru a question very simular... I asked "at what rpm is the 2002 WRX most efficient running at?" They replied 3000RPMs

Now just because you keep it at 3000rpms in first gear doesn't mean you'll get better milage than 1500rpms in third gear.

But from my experience I just set the cruise at 3000 and stay in 5th gear and adverage 28MPG mostly highway, 42 miles one way to work.

Originally posted by tbhuey
Filled up this afternoon at 275 miles. It took 11.9 gallons @ $1.72 = $20.47.

Filled up this afternoon at 302 miles. It took 13.956 gallons @ $1.559 = $21.76.

Bradus