Depends on which engine it has and if it is an automatic or if it is a 4WD. It is not going to get good mileage no matter what engine and transmission it is equipped with. The best you will get it around 16 highway.
It is important to know the gas tank size for a car when driving. For a 1966 Chevy Suburban C10 the gas tank size is 40 US gallons.
Change the air filter and spark plugs, and try to drive at a constant speed.
I would reckon about 12 or 13mpg highway, maybe a little more
Yes
yes
timing fire order or mechanical failure
Driving gently you might be able to get up to 15 or 16 on the highway, around town would be less.
Use any name brand gasoline treatment then top off with fresh gasoline and you'll be fine.
My 1969 Chevy pickup has it in the fuel tank behind the seat
More fresh air mixed with the gas. Better performance.
If it is a 4.3 then it will get around 17 street and 21 or 22 highway
Startoff and brake to stop slower