This not all inclusive of every tire manufacturer's available sizes and should be used as a guide only.

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Tire Terms |  What Tire Numbers Mean  |  Conversion of Old Size to New Metric Size | Speed Ratings 

Calculating Tire Height/Width  |  Tire Diameters for 14" to 18" Wheels | Tire Conversion Chart | Diameter  Comparison

Tire Terms

• M+S: A standard for all-season tires indicating traction capability including mud, snow, rain, and slush.
• Sidewall: The portion of the tire between the tread and the bead.
• Contact Patch: Also known as the footprint, this is the portion of the tread that is in contact with the driving surface.
• Bead: The portion of the tire that is retained by the rim, strengthened by a steel wire wrapped in tire plies.
• P-Metric: The most common currently-used system of tire sizing for passenger vehicle tires.
  
• LT-Metric: The most common currently-used system of tire sizing for light truck tires.
  
• Traction: A is best, C is "acceptable", and it represents a tire's ability to stop on wet pavement in a certain controlled situation.
  
• Temperature: A is best, C is "acceptable", and it represents a tire's resistance heat generated by friction between the tire and roadway.
  
• Tread Wear: 100 is the baseline number against which all tires are rated, the higher the number the better. A tire graded 300 should last three times as long as one graded 100 under exactly the same conditions. These ratings are NOT comparable between different tire manufacturers.

What The Numbers Mean

• P = passenger . If it said "LT" it would mean "light truck".
• 275 is the section width (in millimeters) from one sidewall to the other at the tire's widest point (the bulge) when mounted on a specifically sized wheel and inflated to the recommended pressure. The section width varies approximately 0.2" for every 0.5" change in wheel width.
• 60 determines the sidewall height. On this example, the sidewall height is 60% of the section width (275 mm) or 165 mm based on the proper wheel width and inflation pressure.
• R means Radial. I suppose there are designations for Bias Ply, but those tires are very rare.
• 15 is the wheel diameter in inches.
• 107 The 107 represents the tire's load index rating. As the number goes up, the load rating of the tire goes up. Each number (i.e. 107, or 93, or whatever) represents a certain weight that particular tire can accommodate.
• H The H is the speed rating for the tire and represents the maximum sustained speed the tire can withstand. There is a lot of fine print regarding that subject such as the load on the tire and the pressure inside the tire, so check on that before subjecting your tires to such "abuse".
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Convert Old Sizes to New Metric Size

• E78-14 is equivalent to P195/75R14
• F78-14 is equivalent to P205/75R14
• G78-14 is equivalent to P215/75R14
• H78-14 is equivalent to P225/75R14
• E78-15 is equivalent to P195/75R15
• F78-15 is equivalent to P205/75R15
• G78-15 is equivalent to P215/75R15
• H78-15 is equivalent to P225/75R15
• L78-15 is equivalent to P235/75R15

Speed Ratings

• Q rated capable of speeds up to 100 mph.
• R rated capable of speeds up to 106 mph.
• S rated capable of speeds up to 112 mph.
• T rated capable of speeds up to 118 mph.
• U rated capable of speeds up to 124 mph.
• H rated capable of speeds up to 130 mph.
• V rated capable of speeds up to 149 mph.
• W rated capable of speeds up to 168 mph.
• Y rated capable of speeds up to 186 mph.
• Z rated capable of speeds above 149 mph.

Load Index Rating

Tire Height and Width Conversion

The way to figure out the width and height in inches from your P-metric tire designation is as follows: Take the section width, in this example we'll use 275 mm, and divide by 25.4 because there are 25.4 millimeters in an inch. Multiply that by the aspect ratio, in this case 60%, or .60. That will give you the height of one sidewall in inches. Multiply by two because you have two sidewalls from the ground to the top of the tire, and add the wheel diameter. Presto. Piece of cake!
Example: The diameter of a P275/60HR15 tire is: 275mm / 25.4 x .60 x 2 + 15 = 27.99" diameter.

Tire Diameters

Simply find the wheel diameter in which you're interested, then cross the section width with the aspect ratio and you'll get the diameter in inches.  Actual tire sizes may vary slightly from manufacturer to manufacturer. 

Tire Diameters for 14" Wheels

Tire Diameters for 15" Wheels

Tire Diameters for 16" Wheels

Tire Diameters for 17" Wheels

Tire Diameters for 18" Wheels

Tire Diameters for 19" Wheels

Tire Diameters for 20" Wheels

Tire Size Conversion Chart

While today's P-metric passenger tire sizes have existed since the early 1980's, restoring classic muscle cars and pony cars has kept yesterday's Numeric and Alpha Numeric tires from disappearing. The chart below is intended to help you determine their equivalent P-metric tire sizes.

Two versions of numeric tire sizes were used as original equipment on vehicles between 1949 and 1970. The early numeric tires had the equivalent of a 90-series aspect ratio, while later tires offered a "lower" profile equivalent to a 80-series. These tires typically featured tread widths ranging from 3.5" for the smallest 13" wheel diameter tires to about 5.5" for the largest 15" wheel diameter tires.

To convert Numeric sizes to today's P-metric sizes, it is important to remember that early cars were not only equipped with narrow tires, they were equipped with narrow wheels as well. In most cases, numeric tires should be replaced with today's 80- or 75-series sized tires. This is especially important if the original wheels are to be used. Today's lower profile sizes will usually result in too wide a tire with too much gap between the wheel well opening and the top of the tire.

For example: A 1967 Chevelle 7.35-14 might be replaced with a 185R14 or P195/75R14.

Alpha numeric tire sizes were introduced as original equipment in the late 60s and became widely used in the early 70's. These tires were identified with a letter which indicated the tire's load capacity, followed by a "R" if radial ply construction, the tire's aspect ratio and wheel diameter. So while G78-15 (28.01"), G70-15 (27.5") and G60-15 (26.4") sized tires are all rated to carry the same load, their different aspect ratios resulted in tires with the overall diameters indicated above.

To convert Alpha Numeric tires to P-metric sized tires, it is important to identify the original tire's aspect ratio. The 78-series Alpha Numeric tires should be replaced with today's 80-, or 75 series tires. If the vehicle was equipped with the low profile 70-, 60- or 50-series sizes, the P-metric substitution should be selected from the P-metric size column that offers the equivalent aspect ratio as the existing tire.

Listing in chart does not imply complete interchangeability.

When changing tire sizes, dimensional clearances must be checked.

Each row of the above chart is roughly the same diameter tires, but the width varies by column, and the diameters are not exact matches.

Diameter Comparison

While this chart certainly doesn't guarantee a specific tire will fit your vehicle, it can give you an idea of the tire sizes with the same diameter.

Source: https://www.tirerack.com/tires/tiretech/45_conversionchart.jsp