Tire Rack likes to compare selecting tires to buying shoes.
Just like going to a shoe store you need to know what size you have, what purpose you have in mind (i.e. cross-trainer, running shoe, oxford, or fuzzy bunny slipper), and what budget boundaries you have.
Many customers I speak with tend to like this illustration as it eases their selection process. There are three steps I like to break it down into: What size do I have? What is the purpose of the tire? What am I willing/ able to pay?
Before you to go too far into your tire search, check your size first. Our website has the most popular sizes listed for your vehicle in the Upgrade Garage, but it is always good to double check. The tire size can be found on the inflation placard either on the door jamb or gas cover depending on your vehicle. Do not fully rely on the tire size currently on your vehicle, as a different size may have been installed at some point. 
Another good thing to be mindful of is the load index of your vehicle. If you have a 3/4-ton pickup such as a Ford F250, you will also require a tire with a higher load index, or if you have a vehicle such as a Chevrolet Corvette, your car may have come with light load tires and going to a tire with a higher load index would make the vehicle ride firm.
Next Post: Part Two -- What is the Tire's Purpose?

original tires- LT245/75/16
I have a new set of rims 18" x 10
what is the correct size/hgt tire to use?