Odds are the aftermarket wheels fitted to your car when you bought it are very close in size to the factory wheels, meaning the tire requirements are most likely not all that different.

If you look on the sidewall of the tire it will have a number along the lines of what you've seen posted here - something like 225/40R18 up front and 255/35R19 in the rear. You can decode this number like this:

Looking at the first number, 225/40R18, it breaks down to:

• 225 - this is the overall width of the tire, measured in millimeters. In this case, 225 millimeters (22.5 centimeters or 8.58 inches wide
• 40 - this number relates to the sidewall height of the tire (think distance from the wheel to the ground) and is technically called the aspect ratio. In this case, it tells you that the sidewall height is 40% of the overall width of the tire. So 40% of 225 = 90 millimeters or 3.54 inches tall.
• the "R" just means it's a radial tire (a type of construction method common to all modern passenger car tires)
• 18 - this is the size of wheel the tire fits, in this case a wheel with a diameter of 18"

Using this same decoding method you immediately see that the rear tire fits on a 19" wheel rather than an 18" and that the rear tire is 30 millimeters (or 1.18") wider than the front tire (255 vs 225 width). But while the aspect ratio of the rear tire is 35 compared to the front's 40, you're looking at very similar sidewall height when you do the math. 40% of 225 is 90, 35% of 255 is 89.25.

Aspect ratios of 35 and 40 are pretty low profile when compared to traditional family sedans, but not too bad when compared to a lot of very high performance sports cars. Lower profile tires tend to be more expensive than higher profile tires, and tires that fit larger wheel sizes are more expensive as well.

If you go to Tire Rack - Your performance experts for tires and wheels you can browse tires based on size and compare prices, performance etc.