2006-10-01 07:58:44 · 8 answers · asked by Pattar 1 in Cars & Transportation ➔ Commuting
its all to do with your gear ratios... some cars are more efficient in different gears at different speeds....
its like comparing a hummer to a civic...
2006-10-01 08:01:15 · answer #1 · answered by radiohead5953 3 · 0⤊ 0⤋
That's a tough call! I would tend to believe the 70 KMPH would get better fuel economy, simply because wind resistance goes up geometrically as speed increases. Of course this depends on how much difference there is between 4th gear ratio and overdrive ratio. 100-200 RPM isn't that signifigant:
It also would depend on how streamlined the vehicle is. A Porsche, etc. is a lot "slicker" than a van, or one of those ugly box cars they make today.
It works like this: up to about 70 KMPH wind resistance is not a big factor. One of those boxy Scions or Honda Elements will have about the same wind drag as a Porsche!
Of course 70 or 80 KMPH is fairly slow: I'd bet you'd get almost identical fuel economy, unless you are driving a very box-like vehicle!
Above 70 kMPH wind resistance becomes more and more of a factor. If my memory serves me correctly, wind resistance DOUBLES every 20-40 KMPH or so.
In fact in drag racing, the Funny Cars are running almost as fast as the Top Fuel guys, even though they weigh quite a bit more and can't come off the starting line as fast.
They make up for it at the end of the track, where their more aerodynamic bodies go faster, especially when they exceed 400 KMPH.
A drag racing friend of mine experimented with different rear end ratios on his car, which he also drove off the track. He said the higher gear ratios gave him very slightly better mileage, but not very much: less than 1 mile per gallon. He was going anywhere from 3:1 to 4:1 ratios; way more than the difference between overdrive and 4th.
What is the ideal aerodynamic shape for a vehicle?
You've seen it: look at an airplane!
There's a spec called "drag coefficient". Sports cars like a Porshe have a coefficient of about 4. The very best on the road today have in the mid to upper 3's
The lowest drag coefficient car was made back in the 1920"s!!!
It had a drag coefficient of about 2.8!!!
But this "car" looked more like a Cessna Airplane with the wings cut off!
In short, the ideal shape is a teardrop. Think about it: water can form any shape as it falls through the air: Whatever shape it forms will be the easiest path!
2006-10-01 15:21:51 · answer #2 · answered by econofix 4 · 1⤊ 0⤋
well if the question is short the answer is short, none of them, you should put more information cause there's a lot of things that can affect the petrol economy in high way driving, like if the A-C is working? the windows of the car? what kind of cars ? what kind of engines? the aerodynamics of the car?
2006-10-01 15:46:32 · answer #3 · answered by compadre_sv 3 · 0⤊ 0⤋
other driving at 70 in fourth gear
2006-10-02 00:51:39 · answer #4 · answered by bandaruvamsi 2 · 0⤊ 0⤋
the one at 80
2006-10-01 15:00:51 · answer #5 · answered by GLOBAL WARMING! 3 · 0⤊ 0⤋
one more factor to consider; the size of the engine.
if both autos are the same, the one going slower burns less fuel.
It's more energy that you must add to go faster in any gear.
2006-10-01 15:01:55 · answer #6 · answered by Anonymous · 0⤊ 0⤋
I drive in 4th at 70km/h and I get an optimal fuel consumption. I usually change 3rd at 30-40km/h, 4th at 70km/h and 5th at 90km/h.
I have a 1,4l 98HP engine on my car.
2006-10-01 15:01:57 · answer #7 · answered by Λиδѓεy™ 6 · 0⤊ 0⤋
There are two many variables gear ratios drag coefficient sorry.
2006-10-01 15:21:30 · answer #8 · answered by mick 6 · 0⤊ 0⤋