Understanding basic tire information
8R Series Tractor

8R Series Tractor

The tire industry has been converting to metric size designations when describing agricultural tires.  In addition, tires are placed into groups by their rolling circumference index (RCI), commonly referred to as group size.  Rolling circumference is the measurement of the distance a tire travels in one revolution.  Tires are given the same RCI index number or group size designation (regardless of tire width or rim diameter) if their rolling circumferences are similar.
 
Understanding tire group size is important for proper tractor setup when equipped with an MFWD or Independent-Link Suspension (ILS™).
Front axle marked with “5”

Front axle marked with “5”

Tractors equipped with a 1300 Series MFWD axle always have front tires that are five group sizes smaller than the rear tire group size. Tractors with Independent-Link Suspension or the 1500 Series MFWD axle can be ordered with front/rear tire combinations that are four or five group sizes different.  However, when tires are selected during the ordering process of the tractor, a “4” or “5” marking will be placed on the front axle.  All replacement tires installed on the tractor must match the group size difference (four or five steps).
 
NOTE:  The group size marking (4 or 5) is located on the front axle just to the left of the steering cylinder on the left side of the tractor.
 
Mismatched tire sizes effect electronic transmission control, which may vary tractor ground speed.  Mismatched tires may also increase wear on the tires and drivetrain components.

The metric tire rating system allows manufacturers to design tires that better meet specification requirements for diameter-to-width and rolling circumference dimensions, which are critical for MFWD tires.
 
Specific tire information is often available from the tire manufacturer, dealer, or the John Deere online configurator.  Tire terminology for both standard and metric tires is described below.

 

Standard size tire
18.4R46
18.4 = tire section width (inches)
R = radial construction
46 = rim diameter (inches) (this is not the total tire height or group size)

Metric size tire
520/85R42 158 A8 R1
520 = tire section width (mm)
85 = aspect ratio (percentage of sidewall height to section width)
R = radial construction
42 = rim diameter (inches) (this is not the total tire height or group size)
158 = load index
A8 = speed symbol
R1 = tread designation

NOTE: Detailed tire terminology definitions are described below.

Common conversions between metric and standard tire sizes:
 
Tire size conversion chart
Inch size Metric conversion (mm)
11.4 290
12.4 315
12.6 320
13.6 345
14.9 378
15.0 380
16.5 420
16.9 429
18.4 467
18.9 480
20.5 520
20.8 528
22.8 580
23.1 587
23.6 600
24.4 620
25.6 650
28.0 710
30.0 750
30.5 775
31.5 800
35.4 900
Formula for metric
to English conversion:

divide the metric number (in mm)
by 25.4 to get inches
Example:
480 mm section width / 25.4 =
18.89-in section width tire
Formula for English to
metric to conversion:

multiply English number (in inches)
by 25.4 to get millimeters
Example:
18.4-in section width X 25.4 =
467 mm section width tire

Common tire sizes by "group size"
Front Group 42 Group 43 Group 44
  290/90R38 320/80R42 540/75R34
  320/85R38 380/80R38 620/75R30
  380/80R34
(14.9R34)
420/85R34  
  420/9030
(16.9R30)
480/70R34  
  480/70R30
(18.4R30)
540/65/R34  
  600/70R28 600/70R30  
Rear Group 46 Group 47 Group 48
  420/80R46 380/90R50
(14.9R50)
380/90R54
  480/80R42
(18.4R42)
480/80R45
(18.4R46)
480/80R50
  520/85/R38
(20.8R38)
520/85R42
(20.8R42)
520/85R46
    620/70R42 620/70R46
    710/70R38 710/70R42
      800/70R38
  Group 46 Group 47 Group 48
  420/80R46 380/90R50
(14.9R50)
380/90R54
 
 
Tire terminology definitions
 
Aspect ratio - Number that provides the tire sidewall height relative to the tire width.
 
Load index (LI) - A uniform method to report the load carrying capacity of a tire. For example, a 157 LI means the tire has a maximum load-carrying capacity of  4125 kg (9100 lb) at the speed specified by the speed symbol when the tire is inflated to its rated inflation pressure. When a tire is used in single application, there would be 8250 kg (18,200 lb) carrying capacity for the axle (4125 kg x 2 tires or 9,100 lb x 2 tires). The load-carrying capacity per tire is reduced by 12 percent when the tires are used in dual application so there would be 14520 kg (32,030 lb) carrying capacity for the axle (4125 x 0.88 x 4 tires or 9,100 x 0.88 x 4 tires). When comparing tires, the higher the load index number, the higher the load capacity.
 
Example of load index based on tire size:
 
A 520/85R42 with a 157 LI and an A8 speed symbol has a maximum load of 4125 kg (9100 lb) at the required cold inflation pressure of 159 kPa (23 psi) and maximum speed of 40 km/h (25 mph).
 
A 480/80R46 with a 158 LI and an A8 speed symbol has a maximum load of 4250 kg (9350 lb) at the required cold inflation pressure of 241 kPa (35 psi) and maximum speed of 40 km/h (25 mph).
Load index chart
 
International load index numbers (per tire)
Load index lb kg Load index lb kg
135 4800 2180 163 10700 4875
136 4940 2240 164 11000 5000
137 5080 2300 165 11400 5150
138 5200 2360 166 11700 5300
139 5360 2430 167 12000 5450
140 5520 2500 168 12300 5600
141 5680 2575 169 12800 5800
142 5840 2650 170 13200 6000
143 6000 2725 171 13600 6150
144 6150 2800 172 13900 6300
145 6400 2900 173 14300 6500
146 6600 3000 174 14800 6700
147 6800 3075 175 15200 6900
148 6950 3150 176 15700 7100
149 7150 3250 177 16100 7300
150 7400 3350 178 16500 7500
151 7600 3450 179 17100 7750
152 7850 3550 180 17600 8000
153 8050 3650 181 18200 8250
154 8250 3750 182 18700 8500
155 8550 3875 183 19300 8750
156 8800 4000 184 19800 9000
157 9100 4125 185 20400 9250
158 9350 4250 186 20900 9500
159 9650 4375 187 21500 9750
160 9900 4500 188 22000 10000
161 10200 4625 189 22700 10300
162 10500 4750 190 23400 10600
 
 
Speed symbol - The top speed a tire is designed to travel.
A8 is rated for 40 km/h (25 mph).  B is rated for 50 km/h (31 mph)
 
Tread designation - The tread designation is used to describe the tread and indicate tire usage.  Designs offered are all lug- or bar-type tires and are separated into one of three specifications: R1, R1W, or R2.
 

R1 is a standard tread and is used primarily for general dry-land farming.  These tires have the shortest lug height and the narrowest spacing between lugs.
 
R1W is a wet traction tread for wet, sticky soil conditions.  This tread fills the gap between R1 and R2 tires having a deeper lug with wider spacing than R1 tires but shorter and narrower than R2. R1W is defined as having a lug height about 20 percent deeper than an equivalent R1 tire, but this could vary from 15 to 35 percent depending on the tire and manufacturer.
 
R2 is a tread type used typically with cane and rice or other crops grown in wet muck or flooded fields. Tread depth of R2 tires is approximately twice as deep as R1 tires. R2 tires also have the widest spacing between lugs to allow mud to shed easier. The wide-spaced lugs can show extra wear and cause problems with vibration when roading. R2 tires may not pull as well as R1 or R1W tires in drier soil conditions found in most row-crop applications.

Typically, tires with R2 tread should be matched on the front and rear of a tractor while R1 and R1W treads can be mixed or matched on the same tractor to meet requirements or preference.
 
Last Updated : 25-Jul-2013