Car Forum World

The following quote is from the Government web site fueleconomy.gov for info to enhance your Gas Mileage:

" While each car reaches its best fuel economy at a different speed or range of speeds, gas mileage typically decreases quickly at speeds above 60 mph.

As a rule of thumb, you may be able to assume that each five mph you drive over 60 mph is like paying an extra $0.20 per gallon for gas. "



Here is my question

What the hell are they TALKING about??????

It is a regular and well recognized fact that gas mileage INCREASES considerably on the highway. My car gets about 20mpg red light green light and about 26 30mpg on the highway. it is common sense that it takes more gas to move a two ton car from zero 65 than it does to keep a two ton car that is already moving at 65 AT 65.

So why could they write that?

it is because the car gets less economical once it passes it is best efficiency point. The major reason is because with extra speed comes extra force needed to push the car through the air in front of it. Squared up trucks and cars fare worse, since the air has to move at sharp angles to get out of the way. More aerodynamic vehicles can "cut" through the air, allowing the air to flow at shallower angles, requiring less energy to displace it. This is why you never see wind tunnel tests of truck on TV. The air has to climb up an over the cab and it becomes evident that there is lots of energy wasted on that.

Also, every car has a peak power point and once you pass it, you are pouring more fuel into the combustion chambers and to get less energy out. There is not a car on the road that gets better mileage at 70 than 55, regardless what anybody tells you.

Your arguments about fuel efficency do not really apply to the web site declaration. you're correct in stating that it takes less energy to preserve 65 mph than speed up to it, but that is because you are overcoming inertia and this requires extra energy input. It most likely takes less energy to get from zero to 65 than it takes to preserve 90 mph in a big pickup, although. This is why it only takes about 150 hp to get any car to 100 mph, but it takes about 500 hp to get a aerodynamic car to 200 mph.

There is a difference between cruising on the freeway, which will as you have shown, will give you better fuel economy, and racing your engine, which will chew through fule like it's going out of fashion.
It isn't so much how fasst you go, it's more how fast the engine revs, up to about 2500 revs in top gear cruising on the freeway will give you good fuel economy, but if you take the revs too high, forcing the engine to work hard with little result other than going fast, say around four to 5000 revs, you'll be racing the engine, the fuel air mixture will travel through the cylinders too fast and won't burn so proficiently, so effectively you'll be dumping approxamately 20% unburnt fuel out the exhaust, cars set up for racing do not dump this much fuel as they're set up for racing, different computer mapping, gear ratios, tyres etc.
your car isn't set up for racing, so in a nut shell, too higher revs = too much unburnt fuel = less power = less milage per gallon = more cost to run your car.

If you read the damn question what they said was you get the best MPG at about 60 mph. The faster you go above that your mileage decreases. nobody disputes the fact hghway driving gets better mileage than in the city. So.what the hell are you TALKING about?