A car would not run at all in a vacuum. Gasoline needs oxygen to burn. Assuming that the engine was given enough oxygen to burn its fuel but was otherwise still in a vacuum, yes. There would be no wind resistance to slow it down. No. The Internal Combustion Engine that powers cars needs air (specifically oxygen) in order to run. Without air, then the fuel cannot combust and the engine will not run.
However, just for the sake of discussion let's assume the car has an onboard air supply with which to fuel the engine. In that case, the answer would be yes, cars would get excellent gas mileage in a vacuum. Normally, when a car is moving forward it has to push the air in front of it out of the way. The energy required to push the air has to come from the car. Energy spent pushing air obviously cannot be used to power the car. Therefore if you remove the air by placing the car in a vacuum, then the energy that would normally have been wasted pushing air out of the way can be used instead to power the car. That is assuming, of course, that the car has a supply of air for use in the engine's combustion process, as noted above.
No. A car will get better mileage with the correct tire pressure.
for any car to get better gas mileage get a better flowing intake and/or exhaust or get it tuned
They get slightly better city mileage then conventional cars. They would help reduce foreign oil dependence if people drove them.
get a better car
better gas mileage
Many times a high mileage newer used car is much better to buy. The miles are most likely highways miles which are way easier on a car than city driven miles, which is mostly stop and go traffic, and the book price is reduced according to mileage making them an even better deal.
A free flow muffler can improve mileage but by not by much. It would take a long time to recoup the cost of a new muffler.
The best gas mileage from unleaded gasoline or premium gasoline depends on the car. Old cars get better mileage from unleaded.
Diesel.
The best tires for a small car would be the ones that the car manufacturer recommends you use on the car. If you keep good tires on your car and check them every now and then you will get better gas mileage.
Yes, the Cadillac STS will get good gas mileage on the highway. It does however have a reputation for unreliability so another more reliable car would be a better choice.
It varies from car to car. My Honda Insight gets about 37 MPG city and 54 MPG highway. Toyotas tend to get better city mileage than highway.