better mpg 75 or 85?
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Originally Posted by SanchothePanda,Feb 28 2006, 02:14 PM
Yes, the more surface area the more drag, but that's like saying drag makes your car wider as you increase your speed. Aerodynamic drag/downforce increases because the faster you go the more energy you impart to the fluid through which you are moving and you are encountering more of the fluid in the same time frame as before. Therefore it makes sense that as you double your speed (V), your drag quadruples v^2. The power required to gain and maintain that higher speed increases as a cube of the velocity, v^3.
Here's a blurb from wiki, http://en.wikipedia.org/wiki/Drag_%28physics%29 gives a full explanation of the drag formula.
"Another interesting relation, though it is not part of the equation, is that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power. However, with a doubling of velocity, the time taken to cover a given distance also halves. This means that the total energy used to overcome drag over a given distance only increases with the square of velocity."
All other variables being constant, going faster means more resistance which means more power needed to maintain that speed which means more fuel and less mileage. It also means you're at a higher rpm so you'll still be burning more fuel than at a lower rpm with the same throttle. Other variables that play into this are: Are you on a flat plane or are you going up/downhill, cruising or constantly braking and accellerating, do you have the air conditioning on, do you have the top up or down and all sorts of other little fun variables.
Another interesting thing to note is that the national speed limit of 55mph was instituted to force fuel conservation during the oil crisis...
"Immediately before the 1973 energy crisis, the highest posted limit was 75 mph (120 km/h) (Kansas's turnpike speed limit had been previously lowered from 80 mph), and Montana and Nevada posted no speed limit on most rural roads. Congress imposed a nationwide 55 mph (90 km/h) speed limit in 1974 by requiring the limit as a condition of each state receiving highway funds, a use of the Commerce Clause of the United States Constitution[29].
It was estimated that vehicles traveling 55 mph used 17% less fuel than at 75 mph (120 km/h). It was also believed, based on a noticeable drop the first year the limit was imposed, that the 55 mph limit down on highway deaths. After the oil crisis abated the following year, the 55 mph speed limit was retained mainly due to the safety aspect. Later studies were more mixed on this point."
Here's a blurb from wiki, http://en.wikipedia.org/wiki/Drag_%28physics%29 gives a full explanation of the drag formula.
"Another interesting relation, though it is not part of the equation, is that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Since power is the rate of doing work, exerting four times the force at twice the speed requires eight times the power. However, with a doubling of velocity, the time taken to cover a given distance also halves. This means that the total energy used to overcome drag over a given distance only increases with the square of velocity."
All other variables being constant, going faster means more resistance which means more power needed to maintain that speed which means more fuel and less mileage. It also means you're at a higher rpm so you'll still be burning more fuel than at a lower rpm with the same throttle. Other variables that play into this are: Are you on a flat plane or are you going up/downhill, cruising or constantly braking and accellerating, do you have the air conditioning on, do you have the top up or down and all sorts of other little fun variables.
Another interesting thing to note is that the national speed limit of 55mph was instituted to force fuel conservation during the oil crisis...
"Immediately before the 1973 energy crisis, the highest posted limit was 75 mph (120 km/h) (Kansas's turnpike speed limit had been previously lowered from 80 mph), and Montana and Nevada posted no speed limit on most rural roads. Congress imposed a nationwide 55 mph (90 km/h) speed limit in 1974 by requiring the limit as a condition of each state receiving highway funds, a use of the Commerce Clause of the United States Constitution[29].
It was estimated that vehicles traveling 55 mph used 17% less fuel than at 75 mph (120 km/h). It was also believed, based on a noticeable drop the first year the limit was imposed, that the 55 mph limit down on highway deaths. After the oil crisis abated the following year, the 55 mph speed limit was retained mainly due to the safety aspect. Later studies were more mixed on this point."
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From: Charlotte NC
I drive 75% highway use and drive 80-85MPH the whole way as much as I can. I fill up on citgo 93 gas, use regular honda dealer oil, and redline whenever possible. Top is down anytime over 50 degrees. Seldom have passengers
I consistently get 325 MPT, and when I fill up, it takes 12.2-12.5 gallons according to the pump
I consistently get 325 MPT, and when I fill up, it takes 12.2-12.5 gallons according to the pump
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From: Durango, Colorado
Well, I of course have what is known on this board as "the Dolebludger Mod" on my intake, which involves an AUT CF CAI plus a ram air scoop in front of the radiator to its intake, with which 75 or 85 makes no difference in mpg, because the ram air compensates for the speed difference in this range. 29 mpg either speed. But I did notice a drop in mpg on one trip I made at 110, to 26.5 mpg.
Thanks,
Richard
Thanks,
Richard
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From: Gie
Originally Posted by dolebludger,Mar 1 2006, 05:57 AM
Well, I of course have what is known on this board as "the Dolebludger Mod" on my intake, which involves an AUT CF CAI plus a ram air scoop in front of the radiator to its intake, with which 75 or 85 makes no difference in mpg, because the ram air compensates for the speed difference in this range. 29 mpg either speed. But I did notice a drop in mpg on one trip I made at 110, to 26.5 mpg.
Thanks,
Richard
Thanks,
Richard
You can't argue with physics and your car will still require almost 50% more power to go 85 than 75. More power requires more fuel. Unless your snorkel makes your engine a LOT more volumetrically efficient at higher speeds (which I doubt), your car will most definitely require more fuel the faster you go.
If you were able to run a very controlled test with the only variable being your speed (constant at 75 or constant at 85), your car WILL get better gas mileage at 75. So many people have claimed the opposite, but no reliable test has been performed.
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From: York, PA
Let's keep in mind the car isn't producing a lot of power until it get's into the higher RPMs so it requires less effort to accelerate or maintain speed at higher speeds.
When you hit the throttle at 2k rpms it takes more throttle to get the car moving as it would if you were in a lower gear and say at 4k rpms. You could give it 1/4 throttle in the higher rpms and get the same response you would get at 3/4 throttle in the lower rpms.
more throttle = more air = more fuel
less throttle = less air = less fuel
so someone needs to come up with an equation that would figure in the aerodynamics of the car with the efficiency of the motor (where it is producing maximum fuel efficieny for power output.)
When you hit the throttle at 2k rpms it takes more throttle to get the car moving as it would if you were in a lower gear and say at 4k rpms. You could give it 1/4 throttle in the higher rpms and get the same response you would get at 3/4 throttle in the lower rpms.
more throttle = more air = more fuel
less throttle = less air = less fuel
so someone needs to come up with an equation that would figure in the aerodynamics of the car with the efficiency of the motor (where it is producing maximum fuel efficieny for power output.)
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From: York, PA
Originally Posted by DrDre1443,Mar 1 2006, 12:07 AM
...when I fill up, it takes 12.2-12.5 gallons according to the pump
How big is our gas tank anyway?
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From: Sewell, NJ
campbuds you are talking about acceleration, not steady state cruising. I believe all internal combustion engines are most efficient at WOT at whatever RPM is the hp max. Most efficient does not neccesarily mean best mileage. Your equations for throttle, air and fuel are correct. But it requires more throttle to keep the car going at 85mph than at 75mph or 65mph, etc.
DrDre1443 the tank in your car is only 13.2 gallons, 1 gallon in the best case scenario is 30 miles.
DrDre1443 the tank in your car is only 13.2 gallons, 1 gallon in the best case scenario is 30 miles.
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Originally Posted by vAnt,Feb 28 2006, 02:22 PM
I really hope you don't do this as a daily thing. Eventually you'll meet that 'pissed off' guy who brake checks you.
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