So, for most cars, the “sweet spot” on the speedometer is in the range of 40-60 mph. Cars with a higher road load will reach the sweet spot at a lower speed. Some of the main factors that determine the road load of the car are:
Coefficient of drag. This is an indicator of how aerodynamic a car is due only to its shape. The most aerodynamic cars today have a drag coefficient that is about half that of some pickups and SUVs.
Frontal area. This depends mostly on the size of the car. Big SUVs have more than double the frontal area of some small cars.
Weight. This affects the amount of drag the tires put on the car. Big SUVs can weigh two to three times what the smallest cars weigh.
In general, smaller, lighter, more aerodynamic cars will get their best mileage at higher speeds. Bigger, heavier, less aerodynamic vehicles will get their best mileage at lower speeds.
If you drive your car in the “sweet spot” you will get the best possible mileage for that car. If you go faster or slower, the mileage will get worse, but the closer you drive to the sweet spot the better mileage you will get.
That’s a good general statement! The Coefficient of Drag is a function of the frontal area AND the SHAPE of the frontal area, as well as the design of the trunk area.
Extensive wind tunnel testing arrives at the best practical Drag Coefficient.
However, driving the slowest speed in the highest gear will give the best mileage in most cars.
Not to be smart but on the freeway I drive 70-75. Around town I drive 30. On the two lane I drive 59. When I get to 1/2 tank I fill it up and look at what it cost ($10-13) then I hit the “no” button for no receipt and drive on. Whenever I check I’m getting somewhere around 27 on the highway and 24 overall, so who cares? Even when I was working and paying $500 a month for gas, I didn’t change a thing.
I do the same thing, living in Tornado country you never know when you will lose power. Also with the web hacking that 1/2 tank might get us to where the utilities are still on.
Slower = higher mpgs in most cases. That’s one reason hybrids get such good city mpgs: they recover the braking energy by charging the battery, thereby reaping the rewards of high mpgs at low speeds. Non-hybrids don’t do well because of all the stop/start cycles.
The force needed push the car is a function of the square of the speed. Frontal area matters as does coefficient of drag but speed is the biggest contributor. Weight has very little do with drag at a steady speed. Its greatest affect is stopping and starting.
Generally driving just beyond the speed that the car will shift into the top gear will give the best mileage. You want the engine turning slowly, the throttle as open as much as possible and as low a speed as it takes to hold that highest gear. That is BELOW the speed limit of most highways and may be below the minimum speed (45mph) for many cars.
So that may be 40 - 55 mph or so depending on the car. But you my not be saving much more than 65-70 mph and you’ll tick off fewer other drivers.
My car I never really noticed enough of a difference to care from cruising the highways at 55 or 75. My car usually kicks into overdrive just after 60.
No, just until it automatically shifts or you can shift a manual into top gear and not bog the engine. If it just automatically shifts into 6th, you need a few RPM more or it will downshift into 5th with a light gas pedal press. Just a little faster than that so it stays in 6th gear.
Good description there Mustangman. The wind force effectively pushing backwards against the car’s forward motion is 4 times greater at 60 mph than 30 mpg.
