Decent Highway Gas Mileage...

[nquillen]

Originally Posted by serialk11r

The ECU does adjust. When the air is thinner, you lose power so you need a bigger throttle opening = less pumping loss. In a more abstract setting, having a lower starting pressure increases the maximum possible theoretical thermodynamic efficiency by increasing your "pressure ratio" although there's way too many things affected to say how this really affects the engine.

If the cruising power requirement were exactly the same you could expect maybe a couple percent efficiency increase. Since the air being thinner also reduces your aerodynamic drag (0.28 is pretty low for a production car but drag power is still pretty high at highway speeds) by 17%, your engine load just dropped like 9% (obviously depends on speed, which determines the ratio of aero drag to tire resistance). Since your thinner air is robbing more potential power from the engine than your cruising load has decreasing by, I think ~10% is probably a good estimate.

~10% sounds reasonable.

A small part may be related to the fact the at altitude premium gas is lower octane, having a higher BTU per gallon. Of course most gas sold at higher elevation has 10% ethanol which would partly counteract this.

[serialk11r]

Originally Posted by nquillen

~10% sounds reasonable.

A small part may be related to the fact the at altitude premium gas is lower octane, having a higher BTU per gallon. Of course most gas sold at higher elevation has 10% ethanol which would partly counteract this.

As far as I know trying to guess which fuel has more BTU per gallon is a total crapshoot, fuel varies so much and no oil companies are saying exactly what they're putting into the fuel.

[MRZ415]

45mpgs thats AMAZING, I barely get that number in my GOLF TDI (DIESEL)

[WRXGuy1]

How many miles since you cleared it? I can clear my average MPG and go down a hill for it to show 127MPG and take a picture of that...

Having said that, I have seen excellent mileage with this car. The best I've seen was 39MPG on the display (37.8MPG actual), which is still amazing. This was over the course of ~400 miles as well.

[DeeezNuuuts83]

Originally Posted by nquillen

I have also wondered how accurate the pump auto click off point is, I have had random occasions where the pump would click off after 2 or 3 gallons even though I was on an empty tank.

Right, though again, as long as you always fill up, it'll even out. I know what you mean though, but it's usually more of an issue if you fill up and don't need much gas, such as after driving 50 miles rather than going on a 300 mile trip (in which the gas pump shutting off too early or too late wouldn't skew your mpg too bad).

You just have to be consistent. Personally I fill up at the same gas station >95% of the time and do the same thing -- fill up, let it click off, then fill until it clicks off again.

[Jidonsu]

It probably did help that Colorado Springs to Longmont is a ~1200 feet drop in elevation, but still, this is pretty dang good.

[Those Meddling Kids]

Originally Posted by Jidonsu

It probably did help that Colorado Springs to Longmont is a ~1200 feet drop in elevation, but still, this is pretty dang good.

Very true. I have been averaging around 30mpg so far.

[totally]

Originally Posted by Jidonsu

It probably did help that Colorado Springs to Longmont is a ~1200 feet drop in elevation, but still, this is pretty dang good.

That's a significant drop. You should try the other direction too as a round trip route will produce more accurate results.

[gchin]

That would be great, but cannot take the MPG number unless you drive round trip. Isn't it EPA rated at 30 MPG highway?

[DEnd]

Originally Posted by nquillen

I have also wondered how accurate the pump auto click off point is, I have had random occasions where the pump would click off after 2 or 3 gallons even though I was on an empty tank.

Not all that accurate. They work by a diaphram and venturi effect. Basically if you look in the pump nozzle you will see a tube. The tube goes past a diaphram and back into the nozzle. As the fuel flows past the tube in the nozzle it creates a vacuum that pulls air into the tube (the Venturi effect) past the diaphram. When fuel reaches that tube then the vacuum is lost which causes the diaphram to move sutting off the pump.

Basically if you get the nozzle of the pump into the fuel filler hole slightly askew, where there is not a free flow into the gas tank, then gas will back up to the nozzle and cause it to shut off before the tank is full.

Originally Posted by nquillen

A small part may be related to the fact the at altitude premium gas is lower octane, having a higher BTU per gallon. Of course most gas sold at higher elevation has 10% ethanol which would partly counteract this.

Lower Octane gas may or may not have a higher or lower BTU. While there is a relationship between BTU and Octane it only really applies to the pure forms of each different chemical, and is only a casual relationship not a correlation. Since Gasoline is a mixture of many types of Chemicals, two mixtures can have the same BTUs but different Octane ratings.

[RAWR BRZ]

wow nice for a 10mi drive I did with cruise control set at 65 I get only 37

[Longhorn248]

After a 180 mile trip, the average read 38.1mpg. Not bad for a sports car.

[Leonardo]

I have 6813 miles on mine. My overall avg is 33.8.

[RoryTate]

I've managed a calculated 42 MPG on an entire tank of gas several times this summer (not just 42 over a relatively short distance, but 42 for the entire tankfull). This was mostly highway driving, manual trans, with two adults packed for vacation.

I could have had better mileage if I babied it, but I wasn't driving for mileage necessarily.