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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 rim and inflated to the recommended pressure. The section width varies approximately 0.2" for every 0.5" change in rim 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 rim width and inflation pressure.
  • R means Radial. I suppose there are designations for Bias Ply, but those tires are very rare.
  • 15 is the rim 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

CHEVELLESTUFF  All Rights Reserved

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 rim 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

29.9 30.5 31.1 31.7

32.3

32 .9

33.5 34.1 34.6 35.2

70

29.2 29.7 30.3 30.9 31.4 32.0 32.5 33.1 33.6 34.2

65

28.5 29.0 29.5 30.0 3.05 31.0 31.5 32.1 32.6 33.1

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

30.6 31.1 31.6 32.0 32.5 33.0

55

28.0

28.4

28.9

29.3

29.7

30.2

30.6

31.0

31.5

31.9

50

27.3 27.7 28.1 28.5

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" rim diameter tires to about 5.5" for the largest 15" rim 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 1964 1965 to 1970 1970 to 1980 1980 - On
NUMERIC ALPHA NUMERIC EUROMETRIC P-METRIC EUROMETRIC or P-METRIC
"90" to "80"-Series 78 to 50-Series "82"-Series 80-, 75-series 70-series 65-series 60-series 50-series
      145R10   165/70R10      
      145R12 P145/80R12 165/70R12      
  6.00-12   155R12 P155/80R12        
            165/65R13    
      145R13          
      155R13 P155/80R13 175/70R13   195/60R13 215/50R13
5.60-13 6.00-13 A 165R13 P165/80R13 185/70R13   205/60R13  
  6.50-13 B 175R13 P175/80R13        
    C   P185/80R13        
  7.00-13 D 185R13          
      195R13          
            175/65R14    
6.00-14 6.45-14   155R14   175/70R14 185/65R14    
    B 165R14 P175/75R14 185/70R14 195/65R14 205/60R14  
6.50-14 6.95-14 C 175R14 P185/75R14 195/70R14   215/60R14  
    D            
7.00-14 7.35-14 E 185R14 P195/75R14 205/70R14   225/60R14 245/50R14
7.50-14 7.75-14 F 195R14 P205/75R14 215/70R14   235/60R14  
8.00-14 8.25-14 G 205R14 P215/75R14 225/70R14   245/60R14 265/50R14
8.50-14 8.55-14 H 215R14 P225/75R14    
9.00-14   J 225R14          
9.50-14   L            
  5.60-15 A 155R15 P155/80R15   185/65R15 195/60R15  
6.00-15   B 165R15 P165/80R15 185/70R15 195/65R15 205/60R15 225/50R15
6.50-15 6.85-15 C 175R15   195/70R15 205/65R15 215/60R15  
    D            
  7.35-15 E 185R15 P195/75R15 205/70R15 215/65R15 225/60R15  
6.70-15 7.75-15 F 195R15 P205/75R15 215/70R15   235/60R15  
  8.15-15              
7.10-15   G 205R15 P215/75R15 225/70R15 235/65R15 245/60R15 265/50R15
  8.25-15              
  8.45-15              
7.60-15   H 215R15 P225/75R15 235/70R15   255/60R15 275/50R15
  8.55-15              
8.00-15 8.85-15 J 225R15 P225/75R15 235/70R15   265/60R15  
8.20-15 9.00-15 K            
  9.15-15 L 235R15 P235/75R15 255/70R15   275/60R15 295/50R15
    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 Metric LT-metric Light Truck Light Truck
(in Inches) 75-series and Higher 70- and 65-series 60-series and Lower 85-series 75-series and Lower Flotation Numeric
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   LT235/85R16 LT265/75R16 32X11.50R15LT 7.50R16
30 1/2 P265/75R15 265/70R16 285/60R17 LT215/85R16 LT285/60R17 31X10.50R16.5LT 9.50R16.5LT
        LT245/75R16 31X11.50R15LT  
        LT325/60R15 31X10.50R15LT  
30   P255/70R16 P275/60R17        
29 1/2   P245/70R16 265/60R17   LT225/75R16 30X9.50R15LT 8.75R16.5LT
            7.00R15
29 205R16 255/65R16 255/55R18   LT235/75R15    
205/80R16 P235/70R16 255/60R17        
P235/75R15 P255/70R15          
28 1/2 P225/75R15 P225/70R16     LT225/75R15 29X9.50R15LT 8.00R16.5LT
  P245/70R15          
28   215/70R16 P255/55R17
P275/60R15
       
27 1/2 P215/75R15 P225/70R15     LT215/75R15    
27 P205/75R15 P215/65R16 255/50R17   LT205/75R15    
    P305/50R15        
    P255/60R15        
26 1/2 P195/75R15 P225/70R14 P295/50R15   LT195/75R15 27X8.50R14LT  
    P245/60R15        
26 P205/75R14 P215/65R15 P235/60R15        
185R14 P215/70R14          
25 1/2 P195/75R14 P205/70R14 P225/60R15        

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