MFWD Tire Combinations and Overspeed — John Deere Row Crop Tractors

What this covers: How to pick front and rear tire combinations on an MFWD tractor so the front axle turns at the right speed relative to the rear. Covers 50/55/60 series, 7000 through 7R series, and 8000 through 8R series row crop tractors.

Why overspeed matters

On an MFWD tractor, the front axle is geared to turn slightly faster than the rears. That's intentional. It keeps the front tires pulling instead of being dragged along, which gives you better steering and better traction in the field.

That built-in difference is called overspeed, and it's given as a percentage. Two things set it: the powertrain ratio the tractor was built with, and the rolling circumference of the tires you bolt on. The ratio you can't change. The tires you can — which means every tire change moves the overspeed, whether you intended it or not.

Get it wrong and the tires tell on you. Too much overspeed and the fronts scrub off early. Too little and the fronts get dragged rather than pulling, which costs you traction and wears them unevenly. Either way you're also loading the MFWD driveline in a way it wasn't designed for.

What's in spec

Step 1 — Find the tractor's powertrain ratio

This is the front-to-rear axle speed ratio. It's set by the model, the transmission, and the axle. You need this number before you can calculate anything else.

Large frame 7000 series

Two things to watch in this table.

The transmission speed rating changes the ratio. A 7X20 PQ is 1.325 if it's a 30K or 40K machine and 1.327 if it's a 50K. Same model, different number. Don't grab the first row that matches the model name.

The 7610, 7710, and 7810 changed mid-production. Early machines are 1.319, later ones are 1.325. Read the serial plate — there's no way to tell by looking at the tractor.

8000 and 8R series

To use this table you need to know which axle is under the tractor and how many steps it has.

The easy way: ILS (Independent Link Suspension) and 1500 MFWD axles have the gear ratio stamped right on the side of the differential housing. Go read it and skip the table.

If you can't find the stamp, count teeth on the input gear:

• 8020 through MY13 8R — four step axles have a 37 tooth input gear, five step have 39.
• MY14 and later 8245R–8295R — five step axles have 38 teeth, six step have 40.
• MY14 and later 8320R–8370R — five step axles have 44 teeth, six step have 47.

Ratios for other model ranges

Powertrain ratios for the 50/55/60 series, small frame 7X00/7X10/7X20, 7130/7230/7330, and 7430/7530 tractors are published as separate reference tables. Look those up before starting the calculation if you're working on one of those machines.

Verifying the ratio when you're not sure

If there's any chance somebody has been in the driveline and swapped a part that changes the front-to-rear ratio, don't trust the chart. Measure it.

Mark a front tire and a rear tire. Turn the rear axle exactly 20 revolutions and count how many turns the front makes. Then multiply the ratio you think it has by 20 and see if the numbers line up.

Example: a tractor with a 1.3599 ratio should turn the front just over 27 times for every 20 rear revolutions. 20 × 1.3599 = 27.2.

Step 2 — Get the rolling circumference for both tires

Rolling circumference (r/c) comes out of the tire manufacturer's data book. Don't measure the tire with a tape — that's not the same number.

You need it for the rear tire and for whatever front tire you're considering. Every calculation below runs off these two figures, so if they're wrong, everything downstream is wrong.

Step 3 — Run the numbers

Three steps.

1. Find the ideal front rolling circumference. Divide the rear r/c by the powertrain ratio.
2. Pick a candidate front tire whose actual r/c is bigger than that ideal number. It has to be bigger — that's where the overspeed comes from.
3. Calculate the percentage. Subtract the ideal front r/c from the actual front r/c. Divide that answer by the ideal front r/c. Multiply by 100.

Rear tire rolling circumference is 210 inches. Tractor ratio is 1.330.

• Ideal front r/c: 210 ÷ 1.330 = 157.9 inches
• Candidate front tire has an actual r/c of 164 inches.
• 164 − 157.9 = 6.1
• 6.1 ÷ 157.9 = 0.0387
• 0.0387 × 100 = 3.9% overspeed

That's comfortably inside the 1–5% window, so the combination works.

One limitation to understand: this is a static calculation. It assumes no ballast and ignores what hitch and drawbar loads do to the tires. Real overspeed out in the field, under load, will run higher than what you calculate here.

Rules of thumb

These come from field experience. They'll keep you out of combinations that calculate fine on paper but perform badly in the dirt.

Don't mix radial and bias tires. Not if you can help it. It hurts field performance and wears the fronts faster. Switching the rears from bias to radial generally drops overspeed by 2 to 4 percent — enough that some combinations end up in slight underspeed. If the radial has the bigger rolling circumference and the MFWD gets disengaged on hard surfaces anyway, that underspeed shouldn't cause real trouble.

New fronts raise overspeed 2 to 4 percent. A worn tire has a smaller rolling circumference, so a tractor running worn fronts has been running less overspeed than the chart says. Put new tires of the same size on and it jumps back up. If the combination was already near the top of the range, fresh rubber can push it out of spec.

Use a low vibration tread design on the fronts when one is available in the size you need. Testing shows better wear.

Above 6 percent, expect accelerated front wear on firmer soils. Some R-2 cane and rice combinations do exceed 6 percent on purpose — those are built for soft, wet ground where the extra front speed isn't scrubbing against hard soil.

Inflation pressure has to match the load. Rolling circumference figures in the data books assume it does — max psi at max load, reduced psi at reduced load. Run the wrong pressure for the load and the actual rolling radius drifts off the published number. The effect is small but real, roughly 0.2 to 1 percent on overrun.

Going from single rears to duals drops overrun 3 to 6 percent. That's a big move. Rerun the math instead of assuming the existing fronts still work.

Field conditions push the number up. Draft load on the three-point or drawbar transfers weight and changes how the tires deflect, so actual in-field overspeed runs higher than the static calculation. If the tractor spends its life pulling hard, leave yourself some margin.

Green Spring Inc. is an independent supplier of agricultural equipment and parts. We are not affiliated with, authorized by, or endorsed by Deere & Company. John Deere model numbers are referenced for identification purposes only.