dmz

Anyone who passed their physics class will know this claim is bogus. They just happen to be cruising at a very steady speed and got better MPG than their city driving. I bet if they travel the same trip at 60MPH they will get even higher MPG. It's not that diffcult to figure that It takes energy to move and more energy to move faster.

XclusiveAutosports

I would assume that you would get better gas mileage at 75mph instead of at 85, but what the hell do I know.

slimjim8201

Drag on an object increases with the square of speed. The power required to sustain an object's velocity increases with the cube of speed.

Therefore, your car is experiencing 28.44% more drag force at 85 mph than at 75 mph. Furthermore, your car requires 45.57% more power to travel 85 mph than 75 mph.

For shits an giggles, a car requires over 95% more power to travel 250 mph than 200 mph. So if you have a 500 hp car that can go 200 mph, you would need almost 1000 to go 250!

Lice Locket

With my CRX, I always "cruise" at 80-85 mph to go to work,since I work really far from my house. I get a consistent 350 miles per tank (10 gallons). The trick is drafting...

C&D did the same thing with their Insight to win this fuel economy competition. They cruised 85 mph while drafting another car and got over 100 mpg.

dyhppy

i understand common sense, but there are many guys making the claim.

SpitfireS

Last vacation in Sweden I managed the highest milage ever. (for me)
About 30.5 mpg.
Driving at 50-53 miles per hour.
As constant as possible.
One tank in one drive.
(okay, I stopped maybe twice to rest for a bit and take a picture)

If you want proof i'll have to scan the fuel receipt.

iRkennethMe

i must argue that drag increases from surface area. the more surface area the more drag assuming that the speed is constant

slimjim8201

I'm not sure who you are arguing with

DRAG = Cd * (1/2) * ro * U^2 * A

where

Cd = Drag Coefficient
ro = fluid density
U = fluid velocity
A = characterstic area (typically frontal area)

If you double the characteristic area, you double the drag.
If you double the fluid velocity, you quadruple the drag.

So you aren't really "arguing", you are just "adding"

Blackhawk

i agree with slimjim, by the way i've seen your S around campus .....with out a doubt you will get better mpg at 75 than 85. I'd say for the s2000 ideally its probably best at like 50 or 55 in sixth

SanchothePanda

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."