I often indulge an informal study of how different variables affect fuel economy. I’ve gone so far as to monitor in real time key engine and vehicle parameters from the car’s computers to see how they relate to overall fuel economy . The evidence—albeit anecdotal observations of empirical data—is so consistent and repeatable, that I find it absolutely conclusive. Excellent fuel economy really comes down to just a few key ideas that matter more than all the others combined.
1) Stay in the highest gears of your transmission to the greatest practical degree
2) Coast as much as possible before applying brakes.
The implications of the first point cannot be overstated. There is no acceleration rate or vehicle speed that will give you good fuel economy in first gear. Full throttle in first gear, you might be momentarily getting only 1-2mpg. Steady speed you might only get 4-5mpg. Conversely, the highest gear of your transmission might give you 20-25mpg even at full throttle, and 35-50mpg at a steady state on modest speeds around 60mph.
In other words, full throttle in the highest gear of your transmission will still deliver many times better fuel economy than mere idling in the lowest gear of your transmission. The higher engine loading from the taller gears drastically increases efficiency.
This is why it is it is possible sometimes for a larger engine to actually provide better fuel economy under some conditions. Consider the case of two pickup trucks, one with a powerful V6 and another with a yet-more powerful V8. The window sticker indicates that the V6 model should get about 2mpg better in EPA testing. The EPA cycle, however, does not include towing or steep hills.
If greater engine torque of the V8 at speed prevents or delays downshifting to a lower, less efficient gear, then it’s quite possible for the V8 to get better fuel economy. Experienced drivers who tow large trailers are well aware of this phenomenon, having seen small engines deliver terrible MPG because they must drop down several gears to have enough torque to climb the hill. The loss of mpg from the kickdown is greater than the small gain in MPG from selecting the smaller engine for running on level ground.
This is a major reason why semi trucks that need about 450 horsepower use a large engine (15L) spinning at low RPM and delivering massive torque (1500+lb-ft) when they could just use a late model Corvette engine and rev it up. The Corvette engine would not only consume much more fuel, but it would wear out very quickly compared to the million-mile longevity of the large diesel.
I’ve also found interesting the degree to which the importance of the relative factors changes with the conditions. For example, tire pressure is key factor, but goes from being entirely unimportant at rest, to being super important at around 30-40mph, to being only moderately important at 70mph.
Having your air conditioning operating can go from being dominant at slow speed to having only a 1-2mpg effect at highway speeds. Recent experiments in my own car, with a smallish 2.5L engine, have shown almost 3 mpg difference in stop-and-go city driving between using or forgoing air conditioning. At idle, the air conditioning almost doubles my car’s fuel consumption based on real-time monitoring. My minivan sees far more drastic results because the air conditioning must work so hard to cool a large interior, and this vehicle has both front and rear evaporators. Driving in town went from 17.5mpg to almost 22mpg just by turning off the air conditioning. (OK, it actually went out, but the effect is the same, lol!)
It all matters, of course, but focusing on the most important things is the best way to reduce your fuel consumption.