|Tire sizing is misunderstood by many. Some think that a
"50 series" tire is a wide tire. Not necessarily. Modern radial tires will have
a reading of something like this : 225/60-15. Broken down, here's what they mean : 225 is
the section width in millimeters, meaning the maximum width of the tire (which may or may
NOT be at the road surface - depending on manufacture). 60 is the aspect ratio - meaning
the sidewall (in this case) is 60% of the section width (in this example, 60% of 225mm, or
135mm). 15 is the wheel diameter in inches.
It is possible (and common) to have a 60 series tire that is wider than a 50 series tire. A 225/60-15 is wider than a 195/50-15 by more than an inch! Considerations must also be made as for the width of the wheel that a tire goes on to. A rule of thumb is that the wheel should be about 80-85% of the section width of the tire. So a 225 section tire (8.8") should be mounted on a 7 or 7.5" wheel.
Plus sizing refers to wheel diameters larger than what the factory installs on the car, while keeping the rolling diameter the same as stock. The advantage of this is that there is less tire sidewall, and therefore less sidewall flex, which gives more accurate feel and improved traction because there is more tread surface contacting the road. The key to plus sizing is changing the ASPECT RATIO to keep the diameter the same as stock. For example, if you're factory tire size is 185/70-13, and you want to run 14" wheels ("plus one"), you need to run a 185/65-14. This gives you a shorter sidewall, and changes the diameter about 1%. You can go as many "plus" sizes as you dare - as long as the math works out. You can even run wider tires AND bigger wheels. In the example above, you can go to a 205/50-15 ("Plus two"), and have less than 1% change, and pick up nearly and inch wider tire. The downside to "plus sizing" is increased ride harshness (remember a tire's sidewall flex is a factor in effective "spring rate", as far as ride quality is concerned). A general rule of thumb is to drop 5% aspect ratio for each inch of increase in wheel diameter - assuming tread width remains the same.
Below are the tradeoff's of plus sizing :
Speed ratings on a tire specify the ability of a tire to
withstand heat buildup during high sustained speeds. It also generally refers to how
sticky a tire is just due to the better design & materials that must be used to give a
tire a higher speed rating, but keep in mind this is a general rule - a BFG V-rated tire
will generally be stickier than a BFG H-rated, but not necessarily a Yokohama H-rated, so
keep with the same brand when making the assumptions of "sticky". Here is the
breakdown of the various speed ratings :