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Tire Information

Tire TermsWhat 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
A very good link to calculate tire sizes

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

P275/60R15 107H is a typical label you'd see on a tire's sidewall and it breaks down as follows:
  • 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".

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

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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

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
 RATIO

          

75

24.9

25.5

26.1

26.7

27.3

27.9

28.5

29.1

29.6

30.2

70

24.2

24.7

25.3

25.9

26.4

27.0

27.5

28.1

28.6

29.2

65

23.5

24.0

24.5

25.0

25.5

26.0

26.5

27.1

27.6

28.1

60

22.7

23.2

23.7

24.2

24.6

25.1

25.6

26.0

26.5

27.0

55

22.0

22.4

22.9

23.3

23.7

24.2

24.6

25.0

25.5

25.9

50

21.3

21.7

22.1

22.5

22.9

23.3

23.6

24.0

24.4

24.8

45

20.6

20.9

21.3

21.6

22.0

22.3

22.7

23.0

23.4

23.7

40

19.8

20.1

20.5

20.8

21.1

21.4

21.7

22.0

22.3

22.7

35

19.1

19.4

19.6

19.9

20.2

20.5

20.8

21.0

21.3

21.6

Tire Diameters for 15" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

25.9

26.5

27.1

27.7

28.3

28.9

29.5

30.1

30.6

31.2

70

25.2

25.7

26.3

26.9

27.4

28.0

28.5

29.1

29.6

30.2

65

24.5

25.0

25.5

26.0

26.5

27.0

27.5

28.1

28.6

29.1

60

23.7

24.2

24.7

25.2

25.6

26.1

26.6

27.0

27.5

28.0

55

23.0

23.4

23.9

24.3

24.7

25.2

25.6

26.0

26.5

26.9

50

00.3

22.7

23.1

23.5

23.9

24.3

24.6

25.0

25.4

25.8

45

21.6

21.9

22.3

22.6

23.0

23.3

23.7

24.0

24.4

24.7

40

20.8

21.1

21.5

21.8

22.1

22.4

22.7

23.0

23.3

23.7

35

20.1

20.4

20.6

20.9

21.2

21.5

21.8

22.0

22.3

22.6

Tire Diameters for 16" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

26.9

27.5

28.1

28.7

29.3

29.9

30.5

31.5

31.6

32.2

70

26.2

26.7

27.3

27.9

28.4

29.0

29.5

30.1

30.6

31.2

65

25.5

26.0

26.5

27.0

27.5

28.0

28.5

29.1

29.6

30.1

60

24.7

25.2

25.7

26.2

26.6

27.1

27.6

28.0

28.5

29.0

55

24.0

24.4

24.9

25.3

25.7

26.2

26.6

27.0

27.5

27.9

50

23.3

23.7

24.1

24.5

24.9

25.3

25.6

26.0

26.4

26.8

45

22.6

22.9

23.3

23.6

24.0

24.3

24.7

25.0

25.4

25.7

40

21.8

22.1

22.5

22.8

23.1

23.4

23.7

24.0

24.3

24.7

35

21.1

21.4

21.6

21.9

22.2

22.5

22.8

23.0

23.3

23.6

Tire Diameters for 17" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

27.9

28.5

29.1

29.7

30.3

30.9

31.5

32.1

32.6

33.2

70

27.2

27.7

28.3

28.9

29.4

30.0

30.5

31.1

31.6

32.2

65

26.5

27.0

27.5

28.0

28.5

29.0

29.5

30.1

30.6

31.1

60

25.7

26.2

26.7

27.2

27.6

28.1

28.6

29.0

29.5

30.0

55

25.0

25.4

25.9

26.3

26.7

27.2

27.6

28.0

28.5

28.9

50

24.3

24.7

25.1

25.5

25.9

26.3

26.6

27.0

27.4

27.8

45

23.6

23.9

24.3

24.6

25.0

25.3

25.7

26.0

26.4

26.7

40

22.8

23.1

23.5

23.8

24.1

24.4

24.7

25.0

25.3

25.7

35

22.1

22.4

22.6

22.9

23.2

23.5

23.8

24.0

24.3

24.6

Tire Diameters for 18" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

28.9

29.5

30.1

30.7

31.3

31.9

32.5

33.1

33.6

34.2

70

28.2

28.7

29.3

29.9

30.4

31.0

31.5

32.1

32.6

33.2

65

27.5

28.0

28.5

29.0

29.5

30.0

30.4

31.1

31.6

32.1

60

26.7

27.2

27.7

28.2

28.6

29.1

29.6

30.0

30.5

31.0

55

26.0

26.4

26.9

27.3

27.7

28.2

28.6

29.0

29.5

29.9

50

25.3

25.7

26.1

26.5

26.9

27.3

27.6

28.0

28.4

28.8

45

24.6

24.9

25.3

25.6

26.0

26.3

26.7

27.0

27.4

27.7

40

23.8

24.1

24.5

24.8

25.1

25.4

25.7

26.0

26.3

26.7

35

23.1

23.4

23.6

23.9

24.2

24.5

24.8

25.0

25.3

25.6

Tire Diameters for 19" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

29.930.531.131.7

32.3

32 .9

33.534.134.635.2

70

29.229.730.330.931.432.032.533.133.634.2

65

28.529.029.530.03.0531.031.532.132.633.1

60

27.7

28.2

28.7

29.2

29.6

30.1

30.6

31.0

31.5

32.0

55

27.0

27.4

27.9

28.3

28.7

29.2

29.6

30.0

30.5

30.9

50

26.3

26.7

27.1

27.5

27.9

283

28.6

29.0

29.4

29.8

45

25.6

25.9

26.3

26.6

27.0

27.3

27.7

28.0

28.4

28.7

40

24.8

25.1

25.5

25.8

26.1

26.4

26.7

27.0

27.3

27.7

35

24.1

24.4

24.6

24.9

25.2

25.5

25.8

26.0

26.3

26.6

Tire Diameters for 20" Wheels

WIDTH

185

195

205

215

225

235

245

255

265

275

ASPECT
RATIO

          

75

30.9

31.5

32.1

32.7

33.3

33.9

34.5

34.1

35.6

36.2

70

30.2

30.7

31.3

31.9

32.4

33.0

33.5

34.1

34.6

35.2

65

29.5

30.0

30.5

31.0

31.5

32.0

32.5

33.1

33.6

34.1

60

28.7

2792

29.7

30.2

30.631.131.632.032.533.0

55

28.0

28.4

28.9

29.3

29.7

30.2

30.6

31.0

31.5

31.9

50

27.327.728.128.5

28.9

29.3

29.6

30.0

30.4

30.8

45

26.6

26.9

27.3

27.6

28.0

28.3

28.7

29.0

29.4

29.7

40

25.8

26.1

26.5

26.8

27.1

27.4

27.7

28.0

28.3

28.7

35

25.1

25.4

25.6

25.9

26.2

26.5

26.8

27.0

27.3

27.6

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 would 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.

1949 to 19641965 to 19701970 to 19801980 - On
NUMERICALPHA NUMERICEUROMETRICP-METRICEUROMETRIC or P-METRIC
"90" to "80"-Series78 to 50-Series"82"-Series80-, 75-series70-series65-series60-series50-series
   145R10 165/70R10   
   145R12P145/80R12165/70R12   
 6.00-12 155R12P155/80R12    
      165/65R13  
   145R13     
   155R13P155/80R13175/70R13 195/60R13215/50R13
5.60-136.00-13A165R13P165/80R13185/70R13 205/60R13 
 6.50-13B175R13P175/80R13    
  C P185/80R13    
 7.00-13D185R13     
   195R13     
      175/65R14  
6.00-146.45-14 155R14 175/70R14185/65R14  
  B165R14P175/75R14185/70R14195/65R14205/60R14 
6.50-146.95-14C175R14P185/75R14195/70R14 215/60R14 
  D      
7.00-147.35-14E185R14P195/75R14205/70R14 225/60R14245/50R14
7.50-147.75-14F195R14P205/75R14215/70R14 235/60R14 
8.00-148.25-14G205R14P215/75R14225/70R14 245/60R14265/50R14
8.50-148.55-14H215R14P225/75R14  
9.00-14 J225R14     
9.50-14 L      
 5.60-15A155R15P155/80R15 185/65R15195/60R15 
6.00-15 B165R15P165/80R15185/70R15195/65R15205/60R15225/50R15
6.50-156.85-15C175R15 195/70R15205/65R15215/60R15 
  D      
 7.35-15E185R15P195/75R15205/70R15215/65R15225/60R15 
6.70-157.75-15F195R15P205/75R15215/70R15 235/60R15 
 8.15-15       
7.10-15 G205R15P215/75R15225/70R15235/65R15245/60R15265/50R15
 8.25-15       
 8.45-15       
7.60-15 H215R15P225/75R15235/70R15 255/60R15275/50R15
 8.55-15       
8.00-158.85-15J225R15P225/75R15235/70R15 265/60R15 
8.20-159.00-15K      
 9.15-15L235R15P235/75R15255/70R15 275/60R15295/50R15
  N     305/50R15
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.

Approx.
Tire
Diameter
P-metric and European MetricLT-metricLight TruckLight Truck
(in Inches)75-series and Higher70- and 65-series60-series and Lower85-series75-series and LowerFlotationNumeric
34 1/2     35X12.50R15LT 
33 1/2   LT255/85R16   
33    LT285/75R16  
32 1/2     33X12.50R16.5LT 
     33X12.50R15LT 
31 1/2 P275/70R16 LT235/85R16LT265/75R1632X11.50R15LT7.50R16
30 1/2P265/75R15265/70R16285/60R17LT215/85R16LT285/60R1731X10.50R16.5LT9.50R16.5LT
    LT245/75R1631X11.50R15LT 
    LT325/60R1531X10.50R15LT 
30 P255/70R16P275/60R17    
29 1/2 P245/70R16265/60R17 LT225/75R1630X9.50R15LT8.75R16.5LT
      7.00R15
29205R16255/65R16255/55R18 LT235/75R15  
205/80R16P235/70R16255/60R17    
P235/75R15P255/70R15     
28 1/2P225/75R15P225/70R16  LT225/75R1529X9.50R15LT8.00R16.5LT
 P245/70R15     
28 215/70R16P255/55R17
P275/60R15
    
27 1/2P215/75R15P225/70R15  LT215/75R15  
27P205/75R15P215/65R16255/50R17 LT205/75R15  
  P305/50R15    
  P255/60R15    
26 1/2P195/75R15P225/70R14P295/50R15 LT195/75R1527X8.50R14LT 
  P245/60R15    
26P205/75R14P215/65R15P235/60R15    
185R14P215/70R14     
25 1/2P195/75R14P205/70R14P225/60R15    

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