How do I convert an inch tractor tire size like 18.4-38 to metric?

Firestone lists 480/80R38 as the 18.4R38 size, with a 69.1 inch overall diameter and a 206 inch rolling circumference. Some makers sell the same size as 460/85R38. Likewise 16.9R38 is 420/85R38, 14.9R28 is 380/85R28 and 20.8R42 is 520/85R42. Always compare rolling circumference and rim width, not just the size name.

Why does a different tire size throw off my planter or sprayer rate?

A monitor that counts wheel turns assumes the tire size it was set up with. A taller tire covers more ground per turn, so the monitor reads a speed that is too low and meters too little product per acre. A smaller tire does the opposite. Radar and GPS speed sources are not affected, and neither are planters driven by their own ground wheels.

What is the SRI group on a tractor tire size chart?

SRI stands for Speed Radius Index, an ETRTO measure of a tire’s rolling radius. Firestone uses it to group sizes that are close enough to swap, so 18.4R38, 480/80R38 and 520/70R38 all sit in group 825. Firestone describes the table as guidance only. You still need to check lead and lag, and make sure the tire clears the fenders and steering.

What front lead should an MFWD tractor have?

Firestone says the desired lead is about 2%, anything from 0 to 5% is acceptable, and no amount of lag is. Lead means the front wheels turn faster than the rears. Over 5% calls for a smaller front tire, and any lag calls for a larger one.

Can I put a wider 70-series tire on my existing rims?

Sometimes. Firestone lists a W15 rim for 480/80R38 and a W16 rim for 520/70R38, so that swap needs a wider rim. Check the approved rim width for the replacement, the rim diameter, and clearance at full steering lock before you order.

Is a radial tractor tire better than a bias tire?

Kansas State extension figures put the gain from radial tractor tires at about 2% to 6% more productivity and 6% to 8% less fuel, because radials give better traction than bias tires with a similar tread. Keep both tires on one axle the same construction, and match front and rear circumference on an MFWD tractor.

How do I read a size like 480/80R38?

480 is the section width in millimeters, 80 is the aspect ratio, so the sidewall is 80% of the width, R means radial, and 38 is the rim diameter in inches. The rim plus two sidewalls formula gives about 68.2 inches, while Firestone measures 69.1 inches, so use the tire maker figure when accuracy matters.

Tractor Tire Size Calculator | Compare & Convert Ag Tires

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Swapping 480/80R38 (18.4R38) rear tires for 520/70R38 keeps the overall diameter at 69.1 inches, but the section grows from 18.9 to 20.7 inches and the rolling circumference goes from 206 to 209 inches, +1.5%, and both sizes sit in Firestone's SRI group 825. A wheel-speed monitor still set for the old tire would show 5.0 mph while the tractor really travels 5.07 mph, and a 32,000 seeds per acre target drops to about 31,500. On an MFWD tractor with 420/85R28 fronts and a 1.24 gear ratio, front lead falls from 2.3% to 0.9%, still inside the 0 to 5% range. At 6,000 lb per tire the wider tire needs 17 psi against 20 psi.

Current tire size

SRI group 825

Diameter69.1 in

Width18.9 in

Roll circ.206.0 in

Turns308 /mile

Replacement tire size

SRI group 825

Diameter69.1 in

Width20.7 in

Roll circ.209.0 in

Turns303 /mile

Size comparison

Firestone Ag Data Book dimensions

Current 480/80R38 (18.4R38): 69.1 in tall, 18.9 in wide, 38 in rimReplacement 520/70R38: 69.1 in tall, 20.7 in wide, 38 in rim

Overall diameter+0.0 in (+0.0%)

Rolling circumference+1.5%

Section width+1.8 in (+9.5%)

Approved rim widthW15 to W16

Firestone SRI group825 (same group)

Rolling circumference changes by +1.5%. Recalibrate wheel-speed sensors, and check MFWD lead below.

Firestone lists a W16 rim for the replacement against W15 now. Check your wheels can take it.

Wheel speed and application rate

Planters and sprayers on wheel speed

Speed on the monitor (mph)

Target rate per acre (seeds, gal or lb)

Real ground speed if you do not recalibrate

5.07 mph (+0.07 mph)

Rate actually applied

31,541 (-459, -1.4%)

To fix it, set the wheel circumference in the monitor to 209 in, or multiply a pulses-per-mile setting by 0.986. GPS and radar speed are not affected by tire size.

MFWD front lead check

Firestone: aim for about 2%, 0 to 5% is acceptable

Front tire size

Front-to-rear gear ratio

From the tractor manual. Typical MFWD ratios run from 1.2 to 1.5.

Lead with current rear tire

+2.33%

Lead with replacement

+0.86%

+0.9% lead is inside the acceptable 0 to 5% range.

Or measure it on the tractor

Front tire turns, front wheels engaged

Front tire turns, front wheels in neutral

Measured lead: +3.6%. That is in the acceptable range.

Minimum pressure at the same load

Firestone Table B, singles, 25 mph or less

Load per tire (lb)

480/80R38 (18.4R38)

20 psi

520/70R38

17 psi

The replacement carries this load at 3 psi less pressure.

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Tractor Tire Guide

Tread types, IF and VF tires, lead and lag, and wear patterns explained.

Read the Guide

How to Compare and Convert Tractor Tire Sizes

Pick the size you run now and the replacement. If you only have a code to decode, the universal tire converter splits it into width, aspect ratio and rim.
Read the comparison. A change in rolling circumference matters more than a change in diameter, because it sets wheel speed. The rolling radius calculator and the revolutions per mile calculator go deeper on that.
Check the rim warning. A wider tire often needs a wider rim, and the ideal rim width calculator shows the range a tire section suits.
Enter your monitor speed and target rate if your planter or sprayer runs on wheel speed.
On an MFWD tractor, enter the front tire and the gear ratio from the manual, or count front tire turns and type them in.
Enter the load per tire to compare minimum pressures. Once you add weight, the tractor ballast calculator works out axle loads and pressures for the whole tractor.

Standard to Metric Tractor Tire Size Chart

Overall diameter and rolling circumference are measured figures from the Firestone Ag Data Book for drive wheel tires, in inches. The SRI group is Firestone's Speed Radius Index from its size inter-changeability table: sizes in one group have a very close rolling radius.

Standard size	Metric radial	Overall diameter	Circumference	Approved rim	SRI group
12.4R24	320/85R24	45.1 in	137 in	W10	550
13.6R24	340/85R24	47.0 in	143 in	W11	575
14.9R24	380/85R24	49.0 in	148 in	W12	600
13.6R28	340/85R28	51.2 in	154 in	W12	625
14.9R28	380/85R28	53.5 in	162 in	W13	650
16.9R28	420/85R28	56.1 in	170 in	W15	675
14.9R30	380/85R30	55.8 in	167 in	W13	675
16.9R30	420/85R30	58.1 in	175 in	W15	700
14.9R34	380/85R34	59.8 in	179 in	W13	725
16.9R34	420/85R34	62.6 in	187 in	W13	750
20.8R34	520/85R34	68.2 in	202 in	W18	825
16.9R38	420/85R38	66.1 in	199 in	W15	800
18.4R38	480/80R38	69.1 in	206 in	W15	825
20.8R38	520/85R38	72.7 in	215 in	W16	875
18.4R42	480/80R42	73.2 in	218 in	W16	875
20.8R42	520/85R42	76.2 in	228 in	W18	925

Brands and tread patterns vary by a percent or two. Tire choice also changes how the tractor behaves on the road, which our rolling resistance guide covers, and load ratings depend on the construction described in the ply rating guide.

MFWD Front Lead and Lag

On a mechanical front-wheel drive tractor the front wheels are geared to turn a little faster than the rears. Changing the rear tire size changes that balance, and each 1% change in rear circumference moves lead by about one point.

Lag (below 0%)

The front wheels turn slower than the rears. Firestone says no amount of lag is acceptable.

Target (about 2%)

The fronts pull without scrubbing. Anywhere from 0 to 5% lead is considered acceptable.

Too much lead (above 5%)

The front tires turn too fast and scrub the ground. A front tire with a smaller diameter brings it back down.

If you suspect a speed or odometer error after a tire change, the speedometer error calculator shows the same effect for a road speed readout, and the tire size comparison calculator handles any two sizes. For the full tread, flexion and wear picture, see the tractor tire guide.

Tractor Tire Size Calculator

How to Compare and Convert Tractor Tire Sizes

Standard to Metric Tractor Tire Size Chart

MFWD Front Lead and Lag

Frequently Asked Questions

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