Conversion possibilities tractor tyres

There is a close relationship between the dimensions of front and rear tyres on four-wheel drive tractors. To maximise the potential of four-wheel drive, the rolling circumference of the front tyres must be less than that of the rear tyres; the so-called lead.
Conversion possibilities tractor tyres

Gear ratio, rolling circumference and lead

There is a close relationship between the dimensions of front and rear tyres on four-wheel drive tractors.

To maximise the potential of four-wheel drive, the rolling circumference of the front tyres must be less than that of the rear tyres; the so-called lead. Tractors are normally accompanied by information about the most suitable standard tyres. Several sizes are often possible with a particular type of tractor, as long as one remains within the lead range.

A lead of less than 0% and in excess of +6% have a negative impact on the tractor’s driving performance and steering (particularly in corners) and result in (excessive) wear in front and rear tyres and premature wear and tear in the transmission.

The ratio between the number of revolutions in the front and rear axle is very important in a mechanical four-wheel drive (this is determined by gears in the gearbox). This ratio is referred to as the gear ratio and, depending on the brand and type of tractor, is normally between 1.20 and 1.50.
 

Dynamic measurement of the gear ratio

1. Activate front-wheel drive and differential
2. Mark the front and rear tyre at the bottom or on the contact surface
3. Let rear wheel spin 10x (forwards)
4. Count number of revolutions in front wheel
5. Calculation: divide the front wheel revolutions by 10
For example: 13.5/10 = 1.35 The gear ratio!

Different width sizes on the same rim: convert from inches to mm

In some situations, you will want to use tyres with a different width size on an existing rim. This is possible, but can sometimes be difficult to convert tyre sizes from inches to mm. We are pleased to explain how this works.

An inch is equivalent to 25.4 mm. This means you can now calculate how a tyre size in inches can be converted into a tyre size in millimetres. This will not be an exact figure, but an estimate. For instance, a 12.4R20 tyres is 12.4 inches wide, which is the same as 315 mm. The tyre size that corresponds with this is the 320/85R20.

In this example, the number 320 is the width of the tyre in mm, 85 is the ratio between the height and width, and R stands for radial construction. The number 20 stands for the diameter of the rim in inches, which, of course, must be the same for both tyre sizes
 

Conversion tables

You can use the table on the following page to convert tyre sizes in inches to sizes in mm.
 

Standard tyres Wide tyres Multi-use tyres Other sizes Row crop tyres
Inch mm 70-series 65-series 65-/70-/80-series   Inch mm
6.50-16 180/95R16 260/70R16   260/70R16     180/95R16
        315/55R16      
7.50R16 250/80R16 280/70R16   250/80R16   7.50 210/95R16
        280/70R16      
10R16.5       265/70R16.5      
12R16.5       305/70R16.5      
7.50R18 250/80-18 280/70R18 320/65R18 280/70R18 11LR16 7.50 210/95R18
10.5R18 280/80R18 325/70R18 340/65R18        
12.5R18 335/80R18 365/70R18   340/80R18      
  340/80R18            
12.5/80R18       340/80R18      
16.0/70R18       405/70R18      
7.50R20 250/80R20 280/70R20     400/55-17.5 8.3 210/90R20
          280/80R18 8.3 210/90R20
          325/70R18    
          340/65R18    
9.5R20 250/85R20 300/70R20 340/65R20 300/70R20   9.5 250/85R20
10.5R20 275/80R20 320/70R20   320/70R20 365/70R18 8.3 210/90R24
  280/80R20            
11.2R20 280/85R20 320/70R20     365/70R18 8.3 210/90R24
12.4R20 320/85R20 360/70R20 420/65R20     9.5 250/85R24
12.5R20 335/80R20 375/70R20 420/65R20 340/80R20 340/75R20 9.5 250/85R24
  340/80R20            
14.5R20 375/75R20 400/70R20   360/80R20 375/75R20 11.2 280/85R24
    405/70R20   400/70R20 380/75R20    
16.0/70R20       400/70R20      
        405/70R20      
8.3R24 220/85R24       365/70R18    
  210/90R24       320/70R20    
9.5R24 250/85R24 300/70R24     320/85R20    
          335/80R20    
          340/75R20    
          360/70R20    
          420/65R20    
11.2R24 280/85R24 320/70R24   300/80R24 375/75R20 (14.9LR20) 8.3 210/95R28
          380/70R20    
          380/75R20    
          400/70R20    
          405/70R20    
12.4R24 320/85R24 360/70R24 420/65R24 340/80R24 425/75R20 9.5 230/95R28
        360/70R24 600/50-22.5 8.3 210/95R32
        420/65R24 400/70R24    
          405/70R24    
          400/60-26.5    
13.6R24 340/85R24 380/70R24 440/65R24 360/80R24   11.2 280/85R28
        380/70R24   8.3 210/95R32
        460/65R24      
14.9R24 380/85R24 420/70R24 480/65R24 400/80R24 445/70R24 12.4 320/85R28
        420/70R24 460/70R24 (17.5LR24) 9.5 230/95R32
        460/70R24 (17.5LR24)   9.5 270/80R32
        480/65R24      
16.9R24 420/85R24 480/70R24 540/65R24 440/80R24 495/70R24 11.2 270/95R32
        500/70R24 (19.5LR24) 500/70R24 (19.5LR24) 8.3 210/95R36
        540/65R24 600/55-26.5    
          700/50-26.5    
          440/65R28    
14.9R26 380/85R26     500/70R24 (19.5LR24) 500/70R24 (19.5LR24) 11.2 270/95R32
          540/65R24 8.3 210/95R36
          380/70R28    
16.9R26 420/85R26 480/70R26 540/65R26     9.5 230/95R36
            12.4 320/85R32
18.4R26 460/85R26 520/70R26   480/80R26 520/80R26 11.2 270/95R36
          580/70R26    
          480/70R28    
          540/65R28    
23.1-26 620/75R26       600/70R30 9.5 230/95R44
          710/60R30    
28LR26     750/65R26   600/70R30    
          710/60R30    
8.3R28         320/70R24 8.3 210/95R28
9.5R28 250/85R28       360/70R24    
11.2R28 280/85R28 320/70R28     380/70R24    
          405/70R24    
          440/65R24    
12.4R28 320/85R28 360/70R28 420/65R28 320/85R28 460/70R24 (17.5LR24) 9.5 230/95R32
            11.2 270/80R32
13.6R28 340/85R28 380/70R28 440/65R28 360/80R28 480/60R28 11.2 270/95R32
          480/70R24 8.3 210/95R36
          495/70R24    
          500/70R24 (19.5LR24)    
          600/55-26.5    
14.9R28 380/85R28 420/70R28 480/65R28 400/80R28 480/65R28 12.4 320/85R32
        480/65R28 520/60R28 9.5 230/95R36
            9.5 270/80R36
16.9R28 420/85R28 480/70R28 540/65R28 440/80R28 480/80R26   320/90R32
        540/65R28 580/70R26   290/95R34
          420/85R28 12.4 320/85R34
          540/65R28 11.2 270/95R36
          600/60R28    
14.9R30 380/85R30 420/70R30     580/70R26   320/90R32
          420/85R28   290/95R34
          480/70R28 12.4 320/85R34
          540/65R28 11.2 270/95R36
16.9R30 420/85R30 480/70R30 540/65R30 440/80R30 620/70R26 12.4 320/85R36
        540/65R30 600/65R28 11.2 270/95R38
          540/65R30 9.5 230/95R40
          600/60R30    
          600/60-30.5    
          710/55R30    
18.4R30 460/85R30 520/70R30 600/65R30 480/80R30 620/75R26 12.4 320/85R38
          750/65R26 9.5 230/95R42
          600/65R30 8.3 210/95R44
          600/70R28    
23.1R30   620/75R30     1000/50R25    
          650/75R30    
12.4R32 320/85R32       420/70R28 8.3 210/95R36
          480/65R28 11.2 270/80R36
24.5R32   650/75R32          
30.5LR32     800/65R32   680/75R32    
          800/65R32    
14.9R34 380/85R34       460/85R30 12.4 320/85R36
            9.5 230/95R42
16.9R34 420/85R34 480/70R34 540/65R34 440/80R34 620/75R26 13.6 340/85R38
          750/65R26 11.2 270/95R42
          600/70R30 11.2 300/85R42
          710/60R30 9.5 230/95R44
          750/55R30    
          540/65R34    
18.4R34 460/85R34 520/70R34 600/65R34 480/80R34 1000/50R25 12.4 300/95R42
          620/75R30   320/90R42
          600/65R34 11.2 270/95R44
          700/55R34    
          800/50R34    
23.1R34         650/75R32    
          620/75R34    
12.4R36 320/85R36       480/70R30 9.5 230/95R40
          540/65R30    
13.6R36 340/85R36       620/70R26 12.4 320/85R38
          600/70R28    
          520/70R30    
13.6R38 340/85R38       460/85R30 11.2 270/95R42
          600/65R30 11.2 300/85R42
          480/70R34 9.5 230/95R44
          540/65R34    
14.9R38 380/85R38       620/75R26 12.4 300/95R42
          600/70R30   320/90R42
          710/60R30 11.2 270/95R44
15.5R38 400/75R38       540/65R34 9.5 230/95R44
16.9R38 420/85R38 480/70R38 540/65R38   620/75R30 11.2 270/95R46
          600/70R34 9.5 230/95R48
          650/60R34    
          650/65R34    
          600/60R38    
18.4R38 460/85R38 520/70R38 600/65R38 480/80R38 650/75R32 (24.5R32) 12.4 300/95R46
        600/65R38 710/60R34 12.4 320/90R46
          600/65R38 13.6 340/85R46
          650/60R38 11.2 270/95R48
20.8R38 520/85R38 580/70R38 650/65R38 540/80R38 800/65R32   480/80R42
        650/65R38 1050/50R32   460/85R42
          620/75R34   320/105R46
          520/85R38 14.9 380/90R46
          620/70R38   420/80R46
          650/65R38 13.6 340/85R48
          710/60R38 12.4 320/90R50
18.4R42 460/85R42   600/65R42 540/80R38 680/75R32   320/105R46
  480/80R42     650/65R38 710/75R32 14.9 380/90R46
          800/65R32 16.9 420/80R46
            12.4 320/90R50
20.8R42 520/85R42 580/70R42 650/65R42 650/65R42 680/85R42 14.9 380/90R50
    620/70R42     800/70R32 12.4 300/95R52
          900/60R32 11.2 270/95R54
          710/75R34 12.4 320/90R54
          650/75R38    
          710/70R38    
          710/60R42    
          900/50R42    
18.4R46 460/85R46     650/65R42 680/85R32 14.9 380/90R50
  480/80R46       800/70R32 12.4 300/95R52
          650/70R38 11.2 270/95R54
          620/70R42 12.4 320/90R54
          900/50R42    
20.8R46 520/85R46 620/70R46   650/85R38 900/70R32   380/105R50
        620/80R42 650/85R38 18.4 480/80R50
        710/70R42 710/75R38 14.9 380/90R54
          800/70R38    
          900/55R38    
          900/60R38    
          580/85R42    
          650/75R42    
          710/70R42    
          710/65R46    
20.8R50 520/85R50       710/85R38   480/95R50
          650/85R42    
          680/80R42    
          710/75R42    
          800/70R42    
          900/60R42    
          750/70R44    
20.8R50 520/85R50       750/75R46    
          900/65R46    

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