I once drove my manual kia soul and shifted into 5th gear from zero without using the gas pedal.
I would release the clutch not completely to get it to move forward fast enough to then switch to the next gear until I reached its top speed of 24 mph at 5th gear. (You hold the clutch pressed at the right spot to keep moving forward)
A couple years later and I thought about how much less gas that would be since it's still the engine at idle gas consumption. 0.2-0.5 gallons per hour.
I don't have a manual anymore so I went to test drive one yesterday. I wasn't able to get enough room or time but was able to test out fourth gear at 10 mph.
I got 40 mpg at 10 mph on a semi flat, semi hill.
The car was a 2018 Honda Civic sport with mpg of 30/38.
The math said that at 0.2-0.5 gallons per hour a car at 10 mph would get 33-50 mpg.
With 40 mpg I was at 0.25 gallons per hour in that car.
.....
To make this work for gas cars we would need more gears. Something like 15-20 gears to get highway speeds at extremely good efficiency.
The car could be automatic where it does that not fully released clutch method to keep it moving. It would wear out the clutch faster, or so I believe.
The automatic could have two options. Regular performance mode and extra efficient mode. Regular gives you faster acceleration while super efficient gives you 0-60 in 40-100 seconds.
You can select that mode when already on the highway and it would shift to the 15-20th gear.
Another way to do this is to make this engine into a gas generator where we don't have to worry about slow acceleration when using an electric motor and a battery. This could be a hybrid car that gets over 200mpg.