I know a lot of people say higher speeds waste gas. Well that is not a 100% true, it depends on what you are driving. My last 4 Vettes were some of the best gas mileage cars I ever owned.
We did a bunch of driving visiting relatives and hitting large car shows for many years and I proved out that speeds up to 80 the mileage was just as good there than the lower speed limit. The aerodynamics of the car allowed that. Of course if your driving a box car your putting a lot of gas through your engine
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I cleaned out my desk and lost a bunch of paper I had, but remember getting close to 30MPH on a long trip with my 98 Vette and getting better than 25 with 3 more Vettes even one being a 427CI 505HP 07 Z06.
So its not always a higher speed uses more gas, its what you are driving and how your driving it.(not talking legal here)
It’s more than just clean aerodynamics, it’s also about how the car is geared and the manifold pressure versus rpm.
It’s more obvious in aviation. Most everyone knows who Charles Lindbergh is and that he was the first person to fly solo across the Atlantic. Fewer people know that he pioneered and surveyed long distance routes to South America and across the Pacific for Pan American and that during WWII he served in the Pacific as a technical rep with USAAF squadrons flying P-38s and USN and USMC squadrons flying F4U Corsairs.
At the time the conventional wisdom was that operating an aircraft engine with high manifold pressure and low rpm would damage the valves. Lindbergh demonstrated that was not the case and the much greater fuel efficiency that resulted at those low rpm, high manifold power settings allowed for greatly increased range. When USAAF P-38s shot down the G4M Betty carrying Admiral Yamamoto, it was made possible by Lindbergh’s range extension techniques.
Fast forward to today and “over square” engine operation in general aviation piston engines is still a often incorrectly viewed as being harmful to the engine. Pilots will use a higher power setting that involves higher rpm and low manifold pressures. They’ll use a square power setting such as 2500 rpm and 25” of manifold pressure, or an under square setting such as 2400 rpm and 24” of manifold pressure, but many pilots won’t use an over square power setting like 25” of manifold pressure and 2200 rpm.
That’s unfortunate as fuel economy is better at the over square setting. For example you can operate a 235 hp Lycoming O-540 at a maximum under square 65% power cruise setting of 21” MP and 2575 rpm at sea level. If you do that you’ll burn 13.8 gallons per hour. You can also operate it at the same 65% power and get the same cruise speed at a maximum over square power setting of 25” MP and 1875 rpm. If you do that you’ll burn 12.1 gallons per hour. That’s a 13% reduction in fuel burn. Plus the propeller tip speeds are lower and it’s quieter. You also put fewer tach hours on the engine. In car terms you are using a higher “gear” to use lower engine rpms and higher manifold pressures to get better fuel economy.
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In a car it works more or less the same way, except you don’t have quite as much control over the gear ratio and resulting manifold pressure.
Still, in my Tacoma I can drive 75 mph and do it in either 4th gear with a 1.0:1 ratio or in 5th gear with an overdrive .72:1 ratio. At the same speed, I burn a lot less fuel with the lower rpm and higher MP that comes with the over drive gear ratio.
If I buy a new Tacoma it has a 6 speed transmission with .69:1 and .58:1 over drive gears that give even more efficient operation at highway speeds.
The effect is ore pronounced with corvettes and the combination of low drag, 6 speed transmissions and taller gearing to allow for higher top speeds, low engine rpm, and efficient high manifold pressures at 80 mph cruising speeds are possible.
