English Login Home Knowledge Agricultural Conversion possibilities tractor tyres Conversion possibilities tractor tyres Agricultural 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. 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 Like to know more? 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