Should I switch gas stations to get better gas mileage

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May 22, 2006 | 09:51 PM

#16

wu!

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Joined: Jan 2004

Posts: 963

From: CA

Car Info: '18 WRX Premium, '14 320i xDrive

Quote:

Originally Posted by Wingless Wonder

You're not going to maximize fuel economy at speeds over 80 mph, guys. The power needed to move a car increases almost exponentially, not linearly, at highway speeds. If you like driving 80+ to get to your destination faster, than you'll have to live with less than maximum possible fuel economy.

Short shifting can work as long as the load isn't too great, like climbing a steep grade. Try to minimize shifting and speed variations by looking ahead in the road. Every time you lift to shift, then plant your foot back on the go-pedal, the injectors shoot a shot of fuel to cover the bog that would result from the sudden throttle opening (same as an accelerator pump on a carburetor).

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0==WW==0
"…axles of evil…" - george w. bush

All else being equal, will I get better fuel economy at the exact same RPM in a higher gear or in a lower gear?

May 22, 2006 | 10:07 PM

#17

Wingless Wonder

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Joined: Dec 2002

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From: Sacramento CA

Car Info: 02 Impreza WRX sedan

All things being equal, lower RPM at a given speed will give better economy, unless you're climbing a grade that requires you to keep the pedal mashed to the floor just to maintain the speed limit - in that case, it would be better to downshift to keep the engine in its torque band. Lower RPM cruising speed is good for economy and engine life since the number of engine cycles has a long term effect on durability. F1 engines can rev to 20,000 RPM but only last for two races, if that.

--
0==WW==0
"…axles of evil…" - george w. bush

May 22, 2006 | 10:42 PM

#18

meilers

VIP Member

Joined: Mar 2003

Posts: 2,023

From: Phoenix, AZ

Car Info: Subaru Impreza WRX 2002

Quote:

Originally Posted by ldivinag

in the last SCC street car shootout, the heavily modded STi got like 24mpg...

still trying to figure that out...

my stock 04 STi gets 22-23 on the highway.

When I added a turboback exhaust and reflashed my ECU, my WRX gained 3-6 mpg, even though I gained almost 65 horsepower. Both WRX and STI are tuned extremely rich from the factory, perhaps to prevent detonation with poor-quality fuel. My injectors are only running 78% duty cycle (I like a safety margin) but I was able to advance timing to almost 35 degrees with zero knock and lean out the a/f like crazy. Very conservative factory tune!

May 22, 2006 | 10:44 PM

#19

meilers

VIP Member

Joined: Mar 2003

Posts: 2,023

From: Phoenix, AZ

Car Info: Subaru Impreza WRX 2002

Quote:

Originally Posted by Wingless Wonder

All things being equal, lower RPM at a given speed will give better economy

See the other thread on the forum about this... reading the a/f gauge I found that this is true, but only above 2200-2600 RPM. Below 2000 the car DUMPS fuel into the cylinders when the engine bogs and needs torque. The engine seems most "happy" and fuel-efficient at 2600 to 3000 RPM, vacuum (negative boost) and just a feather touch on the pedal. This of course requires a really flat road with good conditions.