Fuel contains only a certain amount of energy for a given volume. Therefore a given amount of fuel will only do a certain amount of work. The amount of work, and therefore fuel, needed to move a vehicle from A to B is determined primarily by the weight that needs to be moved. The weight directly affects the kinetic energy and the rolling resistance. This was first quantified by Sir Isaac Newton over 300 years ago:

Fuel Energy = (n*KE) + (RF*d)

where,
n = number of stars and stops
KE = kinetic energy
RF = rolling friction
d = distance travelled

Expanding this formula in more detail gives:

Fuel Energy = (n*0.5*m*(V^2)) + (u*m*g*d)

where,
m = mass
V = velocity
u = coefficient of friction
g = gravitational constant

So you see vehicle mass is a factor in both terms of the equation. While reducing speed affects only one part of the equation, reducing vehicle mass affects both.

While there are other factors that can improve fuel efficiency, reducing vehicle weight has the greatest effect. Compared to a basic sedan such as the Chrysler Sebring, the FuelVapor Ale achieved 240 percent improvement in fuel economy through weight reduction alone, while other factors such as their patented fuel vapor technology only gave them an additional 60 percent improvement.

Factors that might reduce weight in any car:

- a car with one seat instead of four need only be 1/4th the size and weight
- frame and other parts made of lightweight material such as aluminum tube, aluminum honeycomb, aluminum-magnesium alloy, magnesium, plastic, or carbon composite
- 2-cycle engine has more power per cubic inch, and therefore less weight, than a 4-cycle
- engines with less horsepower tend to be lighter
- two or three wheels instead of four eliminates tires

Even Supermileage vehicles that get upward of 2000 mpg depend mostly on reducing vehicle weight to achieve most of their performance goals. However, these are all prototypes that operate under impractical track conditions, so other factors besides weight reduction can be significant under these unusual situations.

Additional factors include:

- high compression ratio improves carnot efficiency of the engine
- single speed transmission reduces losses between engine and wheels
- better lubricant improves mechanical efficiency of drive train
- properly balanced parts reduces vibration losses
- low frontal and planform area reduces aerodyamic drag
- laminar airflow over vehicle surface improves aerodynamic drag coefficient
- constant speed engine can be more easily optimized for high efficiency
- pure chemical fuel narrows combustion temperature range for more complete combustion
- removing catalytic converter improves fuel economy
- removing muffler improves fuel economy
- turbocharger improves fuel economy
- engine-off during overspeed, such as downhill stretches, reduces fuel consumption
- slick tires can reduce coefficient of friction
- high tire pressure can reduce coefficient of friction
- in-wheel electric motors can nearly eliminate mechanical losses
- regenerative braking using an electric, gas-charged, or hydraulic accumulator can reduce losses during starts & stops
- a non-mechanical auxiliary power unit can replace an inefficient alternator
- no air conditioning or other power-hungry gadgets improves fuel economy

Generally though, it is the vehicle weight reduction that gives the best results.