How to get the best possible gas mileage

[Eric]

Out of your particular car, that is.

Each vehicle is different; some are going to use more gas than others no matter what you do. But there are a few things you can do to get the highest possible mileage out of the vehicle you're driving.

Here's how:

* Keep each tire inflated to the recommended psi. (In some cases, inflating them to the maximum rated safe inflation pressure will save you some fuel - but at the expense of a rougher ride and - probably - more rapid tire wear.)

* Replace all lubricants (engine oil, manual transmission lube, axle lube, wheel bearing grease, etc.) with synthetic lubricants. Synthetics reduce friction, especially at very low temperatures, and this can save you both gas and wear and tear.

* Keep the car clean; a dirty vehicle's aerodynamic drag is measurably greater than a clean one's - and the more drag, the more fuel used.

* Keep the trunk empty. Carrying around junk just means the car's heavier - and a heavier car will use more fuel than a lighter one, all else being equal.

* Keep the car in overdrive as often as possible and as soon as possible. Overdrive cuts engine RPM to just above idle speed - even when you're driving 40, 50 or 60 mph (or even faster than that - though if you want to optimize gas saving, you should avoid driving faster than about 65 mph).

* Keep the engine in perfect tune; with modern cars especially, it's easy to neglect the engine until the "check engine" light comes on or there is some other obvious problem. But gradual deterioration of things like spark plug wires, ignition cap/rotor and even spark plugs (don't believe that "100,000 mile" advertising stuff) can noticeably eat away at gas mileage many thousands of miles before the engine begins to noticeably run rougher or the "check engine" light comes on.

[swamprat]

65 mph? Beam me up! That's a jail sentence. I never drive that speed.

On a long trip, though I tested the old Saturn at 70 mph and then returned at 80 mph to test the theory that slower gives you better mileage. Intrinisically, that is true, but the degrees are different for each car. The mileage was within 1 mpg both times. Big deal. I prefer to drive 80. You would think that the difference would be greater at the higher speeds, but not necessarily so.

[Eric]

65 mph? Beam me up! That's a jail sentence. I never drive that speed.

On a long trip, though I tested the old Saturn at 70 mph and then returned at 80 mph to test the theory that slower gives you better mileage. Intrinisically, that is true, but the degrees are different for each car. The mileage was within 1 mpg both times. Big deal. I prefer to drive 80. You would think that the difference would be greater at the higher speeds, but not necessarily so.

Well, you know I don't drive that speed, either! However, if you want to eke as much mileage out of a car as possible, keeping the speed on the highway to around 60 mph will definitely improve mileage in most vehicles by a not-small number relative to driving 70 or faster. I can't recall the equation offhand but aerodynamic drag increases rapidly at speeds over 45 mph, maybe even geometrically.

This is why, for example, it takes a lot of additional horsepower to get a car that has a top speed of say 140 mph to 160 mph.

I'm pretty sure the optimum speed for absolute best-possible economy is about 45 mph - but of course that's just not feasible (or desirable) on a highway. But 60- 65 is doable....

[Ken]

I can't recall the equation offhand but aerodynamic drag increases rapidly at speeds over 45 mph, maybe even geometrically.

This is why, for example, it takes a lot of additional horsepower to get a car that has a top speed of say 140 mph to 160 mph.

I'm pretty sure the optimum speed for absolute best-possible economy is about 45 mph - but of course that's just not feasible (or desirable) on a highway. But 60- 65 is doable....

IIRC, double the speed, square the drag. I think most cooking engines are at their most economically efficient at around 2500/2800 rpm - the range which gives my car speeds of 62/70 which, strangely enough correspond to our two NSLs for single carriageway and dual carriageway/motorway driving.

Ken.

[swamprat]

IIRC, double the speed, square the drag. I think most cooking engines are at their most economically efficient at around 2500/2800 rpm - the range which gives my car speeds of 62/70 which, strangely enough correspond to our two NSLs for single carriageway and dual carriageway/motorway driving.

Ken.

The formula is correct, but it doesn't take into account differing thermal efficiencies of engines at different rpms, differences in gearing, and other phenomena that go with real world highway conditions. If the formula was the sole determinent, my gas mileage would be far poorer at 80 than it is at 70. That just isn't the case.

[Ken]

The formula is correct, but it doesn't take into account differing thermal efficiencies of engines at different rpms, differences in gearing, and other phenomena that go with real world highway conditions. If the formula was the sole determinent, my gas mileage would be far poorer at 80 than it is at 70. That just isn't the case.

Agreed, hence my caveat 'most cooking engines'. Volumetric efficiency, thermal efficiency, valve timing, state of tune, drag - there are many variables all of which play their part. It is a fact though, that the last few cars I have driven - including one which had the annoying habit of telling me when to change up and change down - were all within the rev range I gave for cruising at our two main national limits.

Ken.