What are 5 factors that effect fuel consumption in a vehicle?

displacement (how big the engine is), engine rpm, fuel quality (octane, water content, contaminants), carburetor/ fuel nozzle size, internal resistance (friction of moving engine components), external resistance (wind resistance), engine load (amount of weight being pulled/pushed).

turn engine off if you are going to idle for 15 seconds or more
accelerate at a smooth and steady pace
coast down hill, coast when you see a red light ahead
keep vehicle in correct gear for the given speed (applies mostly to manual vehicles)
reduce vehicle weight by getting rid of junk.
keep vehicle clean (its a very small amount, but a clean waxed vehicle offers less resistance than a dirty vehicle)
keep vehicle at or near 55 MPH

engine age
the amount of times you stop and start
wind resistance on vehicle or drag eg if you drive with windows open
speed driven at
driving aggressively

there are many factors, so I’ll cut to the chase and limit it to the top 5:
1) driving style (spirited will yield worse mileage; includes all freeway/city driving; steady RPMs will give better mileage)
2) whether A/C is on or off
3) maintenance (properly inflated tires, no oil/gasket/exhaust leaks, no alignment issues, no fouled sparkplugs)
4) giving your car the correct octane rating fuel
5) overdrive on for freeway cruising

there are many other smaller contributors, but these are the ones I think listed from greatest affect to least.

Five factors affecting fuel consumption:
1) driving habits
2) engine displacement
3) gear ratio (transmission and final drive)
4) friction (drivetrain friction, rolling resistance, aerodynamic drag)
5) engine tuning / maintenance

Five techniques to save fuel:
1) drive like there are eggs on your pedals
2) plan trips (combine errands)
3) never idle, ever (okay, idle at stop lights or you’ll ruin your starter)
4) drive the speed limit (not plus 5 or 10)
5) keep your car maintained