Gas Milage

[partyman121266]

I am sitting here at work having a discusion about gas milage. I have noticed that my 2006 330i gets its best gas milage when I am driving about 80 to 90 miles a hour. The guy I work with says this is not possable. He claims that all cars get there best gas milage at around 50 miles a hour. He also says there is no way I can get my best milage at that speed. So guys and girls. What is the real truth. I am not too smart on the math for this may someone can help. [driving2]

 

[Big Daddy]

THis article says it better than I can, How Stuff Works

 

[Kirby]

There are a lot of variables that affect gas mileage. But the biggest "gas eating" factor is wind resistance. Wind resistance is exponential, specifically resistance = velocity squared.

In easy terms, that means that if you double your speed (50 mph vs. 100 mph) the wind resistance is 4 times greater, and it takes more gas to overcome that wind resistance.

Certainly, gear ratios affect the actual mileage. In a silly, extreme example, driving 50 mph in 1st gear will certainly use more gas than 80 mph in top gear.

You said you get better mileage at 80 mph. How do you know? From the fuel consumption indicator? That is really not a overall indicator. The only accurate way to really know is to fill up, record the starting mileage, drive at 80 mph for a while, fill up and record the finishing mileage, to get your true MPG. Then repeat at 50 mph.

My guess is that you'll actually see better mileage at 50 - 60 mph than 80 - 90 mph, but I could be wrong.

 

[CosmosBlack]

I believe you definitely do get better mileage going at 50-60 than at 80-90mph.

 

[Doughnut]

I believe you definitely do get better mileage going at 50-60 than at 80-90mph.

Just like Kirby said, resistance is X at higher speeds, 80-90mph is alot greater than 50-60mph

 

[frolf]

wind resistance and rpm's are the heavy weights in fuel economy. lower both = better mileage

 

[Big Daddy]

And in a lot of cases there is nothing you can do about wind resistance. On my e30 318is I clocked it with radar at 133mph, after a good waxing and adding a splitter I got 135mph and that was all there was, up against the wall, as they say in racing! I'll bet my mileage was really really poor! At 55- 60 driven on the highway in 5th I got close to 40 mpg, drive it hard like at the track or autocross and it dropped to 20 - 23.

 

[Xhippee]

Gas

What does everyone recommend as far as type of gas and octane rating? Book on my 02 325ci says 91 octane; but I hear consumer-ists say to go ahead and use 87

 

[Big Daddy]

The argument about using lower octane has been beat to death. Yes your car will compensate for a lower octane rating by adjusting the timing etc. However this will reduce your performance and over time damage the engine. Is it worth the 5 - 10 cents per gallon?

 

[Average Jae]

you sprung a premium for your car,
so spring for premium in your car