Quick Answer
- 80/100-21 has a calculated overall diameter of 27.30 in (693 mm) and turns 739 times per mile. Dunlop publishes 27.95 in for its Geomax MX34 in this size.
- The size code is section width in mm, aspect ratio in percent, then rim diameter in inches. Overall diameter is the rim diameter plus two sidewalls.
- Old inch sizes such as 3.00-21 are tread-width markings. Dunlop's chart pairs each inch range with metric sizes (table below), and the Inch Sizes tab looks one up.
Front tire presets
Rear tire presets
Dirt Bike Tire Size Chart: Calculated Dimensions
Overall diameter, sidewall, circumference and revolutions per mile for common dirt bike sizes, calculated from the size code with the formulas below. A real tire can differ from these figures, so compare with the Dunlop table underneath.
| Size | Overall diameter (in) | Overall diameter (mm) | Sidewall (in) | Circumference (in) | Revs per mile |
|---|---|---|---|---|---|
| 60/100-14 | 18.72" | 475.6 | 2.36" | 58.82" | 1077 |
| 70/100-14 | 19.51" | 495.6 | 2.76" | 61.30" | 1034 |
| 70/100-17 | 22.51" | 571.8 | 2.76" | 70.72" | 896 |
| 70/100-19 | 24.51" | 622.6 | 2.76" | 77.01" | 823 |
| 80/100-12 | 18.30" | 464.8 | 3.15" | 57.49" | 1102 |
| 80/100-19 | 25.30" | 642.6 | 3.15" | 79.48" | 797 |
| 80/100-21 | 27.30" | 693.4 | 3.15" | 85.76" | 739 |
| 90/90-21 | 27.38" | 695.4 | 3.19" | 86.01" | 737 |
| 90/100-14 | 21.09" | 535.6 | 3.54" | 66.25" | 956 |
| 100/90-19 | 26.09" | 662.6 | 3.54" | 81.95" | 773 |
| 100/100-18 | 25.87" | 657.2 | 3.94" | 81.29" | 779 |
| 110/90-19 | 26.80" | 680.6 | 3.90" | 84.18" | 753 |
| 110/100-18 | 26.66" | 677.2 | 4.33" | 83.76" | 756 |
| 120/80-18 | 25.56" | 649.2 | 3.78" | 80.30" | 789 |
| 120/80-19 | 26.56" | 674.6 | 3.78" | 83.44" | 759 |
| 120/90-18 | 26.50" | 673.2 | 4.25" | 83.26" | 761 |
| 120/100-18 | 27.45" | 697.2 | 4.72" | 86.23" | 735 |
| 130/90-18 | 27.21" | 691.2 | 4.61" | 85.49" | 741 |
| 140/80-18 | 26.82" | 681.2 | 4.41" | 84.25" | 752 |
Circumference = pi x overall diameter. Revs per mile = 63,360 / circumference. For road motorcycle sizes the motorcycle tire dimensions calculator does the same arithmetic.
Dunlop Published Sizes and Rim Widths (Geomax MX34)
Overall diameter, width and applicable rim widths as published by Dunlop for selected MX34 sizes.
| Size | Position | Applicable rims (in) | Overall diameter | Overall width |
|---|---|---|---|---|
| 80/100-21 | Front | 1.60, 1.85, 2.15 | 27.95" | 3.58" |
| 90/100-14 | Rear | 1.60, 1.85 | 21.77" | 4.13" |
| 100/100-18 | Rear | 1.85, 2.50, 2.75 | 26.50" | 4.57" |
| 110/100-18 | Rear | 2.15, 2.50, 3.00 | 27.36" | 4.96" |
| 120/90-18 | Rear | 2.50, 2.75, 3.50 | 26.89" | 5.16" |
| 100/90-19 | Rear | 1.85, 2.50, 2.75 | 26.89" | 4.57" |
| 110/90-19 | Rear | 2.15, 2.50, 3.00 | 27.56" | 4.92" |
| 120/80-19 | Rear | 2.50, 2.75, 3.50 | 27.44" | 5.12" |
| 120/90-19 | Rear | 2.50, 2.75, 3.50 | 27.68" | 5.24" |
Source: Dunlop Motorcycle Tires, Geomax MX34 tire-line page (selected sizes). We have no manufacturer source for the original tire sizes of specific motorcycles, so use your owner's manual.
Dirt Bike Tire Size Conversion Chart: Inch to Metric
Dunlop's off-road size conversion chart pairs each metric width marking with other metric markings and the inch range it replaces. It changes the width marking only, so keep the same rim diameter: a 3.00-21 has a 21 inch rim, and the metric sizes in its row also end in -21.
| Tire | Metric | Other metric markings | Inch range |
|---|---|---|---|
| Front | 60/100 | 90/80 | 2.50 to 2.75 |
| Front | 70/100 | 90/80 | 2.75 to 3.00 |
| Front | 80/100 | 90/100, 100/80 | 3.25 to 3.50 |
| Rear | 80/100 | 80/90 | 2.50 to 3.60 |
| Rear | 90/100 | 110/90 | 3.60 to 4.10 |
| Rear | 100/90 | 110/80 | Not listed |
| Rear | 100/100 | 110/90, 120/80 | 4.00 to 4.10 |
| Rear | 110/90 | 120/80, 100/100 | 3.60 to 4.10 |
| Rear | 110/100 | 120/90, 130/80 | 4.00 to 4.50 |
| Rear | 120/100 | 140/80 | 5.00 to 5.10 |
| Rear | 120/90 | 110/100, 130/80 | 4.00 to 4.50 |
Source: Dunlop Motorcycle Tires, Technical Information off-road size conversion chart. Dunlop says these charts do not imply interchangeability, so consult the motorcycle manufacturer for correct replacements. The Inch Sizes tab above looks up a size such as 3.00-21.
How to Read a Dirt Bike Tire Size
The three numbers
Modern dirt bike tires use a metric code such as 110/90-19. The first number (110) is the section width in millimetres. The second (90) is the aspect ratio: the sidewall is 90% of the section width, so 110 x 0.90 = 99 mm = 3.90 inches. The third (19) is the rim diameter in inches. Overall diameter is the rim plus two sidewalls: 19 + 2 x 3.90 = 26.80 inches calculated. Dunlop publishes 27.56 inches for its Geomax MX34 in this size.
18 inch and 19 inch rears
Dunlop says that with the introduction of 19-inch motocross sizes the aspect ratio was reduced to keep the same outside diameter, for example a 250cc machine going from 110/100-18 to 110/90-19. Calculated from the size codes, a 120/80-19 has an overall diameter of about 27.4 inches and a 120/100-18 about 27.3 inches. The 18-inch tire has the taller sidewall (4.72 inches against 3.78 inches calculated). Follow your owner's manual for the rim and tire sizes your motorcycle takes.
Metric, inch and older markings
Older motocross tires were marked by tread width and newer ones by section width. Dunlop gives the example that 4.50-18 and 130/80-18 under the old markings are now sold as 110/100-18, with the same actual dimensions. Michelin warns that its old Enduro and Desert ranges are sized by width at tread level and the Starcross, AC10, Trial Light and Xlight ranges by width at sidewall level, so a 130/70-19 Cross Competition S12XC corresponds to a 110/90-19 Starcross 5. Dunlop adds that size-marking conversion charts do not imply interchangeability, so check with the motorcycle maker before changing size. The dirt bike tire guide covers choosing and fitting.
Published Dirt Bike Tire Pressures
Pressures in psi as published by Dunlop and Michelin. Convert units with the PSI to kPa converter.
| Terrain or tire | Front (psi) | Rear (psi) | Notes |
|---|---|---|---|
| Dunlop: hard conditions | 12.5 to 13.5 | 14 | Dunlop Off Road Facts |
| Dunlop: intermediate conditions | 12.5 to 13.5 | 14 | Dunlop Off Road Facts |
| Dunlop: soft conditions | 12.5 to 13.5 | 11 to 12 | Dunlop Off Road Facts |
| Dunlop: minis | 13 to 14 | 13 to 14 | Dunlop Off Road Facts |
| Michelin Starcross 6 | 13.1 | 13.1 | Michelin, minimum 11.6 |
| Michelin Enduro Medium | 14.5 | 14.5 | Michelin, minimum 11.6 front / 8.7 rear |
| Michelin Desert Race | 17.4 | 21.8 | Michelin, minimum 14.5 |
| Michelin Trial Light / Competition | 6 | 5.8 | Michelin, minimum 5.8 / 5 |
| Dual-sport off-highway | 22 minimum | 22 minimum | Dunlop; use the manufacturer's highway pressure on the road |
Check cold pressure before riding (NHTSA: not driven for at least three hours). Sources: Dunlop Motorcycle Tires Off Road Facts and Care and Maintenance; Michelin motorcycle off-road tire pressure page.
How to Use the Dirt Bike Tire Size Calculator
- 1Size Calculator tab: Enter a metric dirt bike tire size such as 80/100-21, or click a front or rear preset. You get overall diameter, sidewall height, circumference and revolutions per mile, with the working shown.
- 2Compare Two tab: Enter your current tire and the one you plan to fit. The calculator draws both to scale, gives the diameter change in inches and percent, and a speedometer correction table from 25 to 85 mph.
- 3Dunlop Specs tab: Choose a Dunlop off-road tire line and size to see its load and speed index, applicable rims, and the overall diameter and width Dunlop publishes, next to the calculated diameter.
- 4Find by Size tab: Enter a target overall diameter in inches and a rim size. The calculator lists width and aspect combinations within 3% of the target, closest first.
- 5Tire Pressure tab: Choose a published Dunlop or Michelin entry to see the maker's recommended and minimum psi for front and rear, with the source statement.
- 6Inch Sizes tab: Enter an old inch size such as 3.00-21 to see the metric sizes whose inch range includes it in Dunlop's off-road conversion chart.
- 7Rim Widths tab: Enter a rim width in inches, or click one Dunlop lists, to see which Dunlop sizes list that rim width as applicable.
Dirt Bike Tire Size Formulas
Sidewall height
Sidewall (mm) = width x aspect / 100. For 110/90-19: 110 x 0.90 = 99 mm = 3.90 in.
Overall diameter
OD = rim diameter + 2 x sidewall (in). For 110/90-19: 19 + 2 x 3.90 = 26.80 in (680.7 mm).
Circumference
C = pi x OD. For 80/100-21: 3.14159 x 27.30 = 85.74 in.
Revolutions per mile
Revs per mile = 63,360 / C (in). For 80/100-21: 63,360 / 85.74 = 739.
Speedometer correction
True speed = indicated speed x (new OD / old OD). Moving from 110/90-19 (26.80 in) to 120/80-19 (27.42 in): 60 mph indicated x (27.42 / 26.80) = 61.4 mph.
Diameter change
Change % = (new OD - old OD) / old OD x 100. The calculator labels under 1% a small change, 1 to 3% within 3%, 3 to 5% beyond it and over 5% a large change.