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View Full Version : Amazing Gas Mileage!


magnumforc
06-07-2011, 06:33 PM
We just returned from a 900 mile round trip to Northern Arizona and the fuel milage was startling to sy the least. With only 3K miles on the clock, we found the following:

Average mileage on all fuel used (two and four lane highways with speeds of 65-80 including slowdowns through some small towns) was 28.2 MPG!

Best mileage on a single fillup was 30.2 MPG on Shell gasoline. (Used a half tank...that was on approximtely 10 gallons and all on 4 lane interstate at 75+)

Worst mileage on a single fillup was 22.9 MPG and that was mountain driving and on Texaco gasoline. Only used about 5-6 gallons and then refilled with Shell and gas mileage went to 25.8 on the remaining fuel. I believe the gas MAY have been formulated for higher altitude but don't have a clue why because Prescott, AZ is 5300 feet but all around is lower so it doesn't make sense to reformulate for high altitude for those local stations only.

To say the least, I am very pleased with the mileage. All figures are from the vehicle itself, so the actual numbers might vary I suppose. Regardless, it's in line with last years 2600 mile trip with my 328i and the 31.7 mpg on most of the highway portions of the trip. Bigger heavier car, but I did keep my foot out of the "turbo zone"!

CC Brown
06-08-2011, 05:56 PM
My computer is almost always 1 mpg too high when compared to actual miles/gal calculations. Once I got almost 32 on the computer but it turned out to be barely over 30. Still that's amazing. I do see my mileage go down when travel 75 and above. The speedometer is also off by 4%. When showing 79 it really is around 75. This has been measured again radar and the speed shown on my Garmin. I wonder about actual miles shown on odometer. I am going to measure that on my around the country 7 week trip. Anyway at 79 it get 27-28 real mile/gal.

magnumforc
06-08-2011, 06:04 PM
Every BMW I have owned has been off by about 4 MPH at 80. Much to my benefit when I sailed past the CHP radar trap on I-5 at what I thought was over 80 and find it was just over 76. Whew. Guess that's why I get passed a lot but also why I see lots pulled over while I continue on my merry way.

As for the odometer, there has been long contention that if the speedo is off then the odometer is off and we're not actually getting 50K miles out of the warranty. I never got involved in that fray, as I change cars too often to get near that figure, but suppose there is a good argument somehwere.

11gt535
06-09-2011, 08:46 AM
I tend to get between 18 and 22 mpg with a mix of city/highway driving. Pure highway driving I see 27+. Stop and go is murder on mpg in the GT, the mileage quickly starts to plummet when I get off the highway close in to the destination. It's hard to get excited about 20 mpg, but it is surprising perhaps given that it's about 15% less than my prior car even though it weighs 75% more and has roughly double the torque. My next experiment is to compare driving in DS versus D and see how much of the magic is just by tuning the performance down.

magnumforc
06-09-2011, 09:53 AM
I found driving in DS ate fuel pretty heavily due to the long shifts and the vehicle never wanting to make it to the upper gears unless through a forced shift. If you shift manually, you can grab the gears pretty efficiently but in DS alone with no manual help it just sucks fuel mightily. I might add that I drive in the :Sport" mode with only the chassis tuned most of the time. This reduces the throttle tip in and helps save fuel, but also increases turbo lag and reduces instant acceleration. It's a give and take game. Mileage for performance. But the nice thing is that it's your choice at the press of a button. How many can say that?.

Capobranco
06-09-2011, 10:25 AM
I found driving in DS ate fuel pretty heavily due to the long shifts and the vehicle never wanting to make it to the upper gears unless through a forced shift. If you shift manually, you can grab the gears pretty efficiently but in DS alone with no manual help it just sucks fuel mightily. I might add that I drive in the :Sport" mode with only the chassis tuned most of the time. This reduces the throttle tip in and helps save fuel, but also increases turbo lag and reduces instant acceleration. It's a give and take game. Mileage for performance. But the nice thing is that it's your choice at the press of a button. How many can say that?.

Hmmmm....somewhat counter intuitively - I have found if I dial in the "comfort" setting - my mileage seems to go down noticeably on my 20 mile commute to downtown DC. If I stay in "sport" mode I seem to get better mileage. I usually drive in sport mode but the last few days - just to feel the very compliant ride - I have been driving in "comfort". I usually get about 18.1 mpg on my commute and I now have been averaging 15.9 mpg. I thought it might be the gas I bought so I switched brands - no difference. Maybe the really hot - 100 degree weather is taking its toll. :dunno:

magnumforc
06-09-2011, 10:44 AM
As I noted, I drive in "sport" but with the chassis tuned rather than with the engine tuning set. Seems to provide the best overall mileage. And you are right on point. The higher the temperature, the lower the fuel mileage. Colder air has a higher density than warmer air, thus provodes somewhat better fuel mileage. However, that effect is offset a bit by turbocharging where we don't have super intercoolers to bring air temperatures down as they pass through the assorted mass of piping asociated with the intake syatem. When racing, we used to pack the intake manifolds with dry ice on hot days between runs to keep them as cold as posssible, so the air charge would be as cold and dense as possible.

11gt535
06-10-2011, 11:55 AM
I need to do a little bit more testing, but I found through some math and staged tests that my car in DS accelerating briskly to 40mph uses 0.013 gallons of gas, burns about 0.029 gallons per mile at 40mph steady, and while idle stopped burns around 0.014 gallons per MINUTE. It turns out gas burned while idle is a huge proportion of my total commute gas usage and flipping on DS really doesn't make all that much difference.

My assumption going in was that acceleration was a hog. The display encourages this as you watch the MPG drop while the pedal is pressed. The way it is displayed you are penalized as if you burned that fuel to go the distance you've already gone, only because you can't count the miles you haven't gone yet.