Free Tool
Tractor Lead-Lag Calculator
On a mechanical front-wheel-drive (MFWD) tractor, the front axle is geared to turn slightly faster than the rear so the front tires roll a little faster than the ground speed set by the rear — this is called "lead." It compensates for slip and keeps the front axle from fighting the rear one. Changing either front or rear tire size changes that ratio, which is why MFWD tractors are sensitive to tire size changes in a way two-wheel-drive tractors aren't.
This calculator compares the front/rear diameter ratio of a new tire combination against your tractor's original (factory) combination, and shows the percentage change in lead. It uses overall diameter as a stand-in for rolling circumference — accurate enough to flag a meaningful change, though a manufacturer's load-adjusted figures will differ slightly.
Most manufacturers specify a target lead range for each model, usually a few percent — check your operator's manual for the exact figure for your tractor. This tool tells you how your proposed combination compares to your original one; it can't tell you your tractor's specific target without that manual.
How to use the lead-lag calculator
\nEnter your factory front and rear sizes, then the combination you want to fit, in the lead-lag calculator above. If you do not have the overall diameters, the tire diameter calculator gives them. Our radial vs bias guide covers why you should not mix constructions on an axle.
Questions
What is a safe amount of lead for my tractor?
It depends on the tractor — manufacturers publish a target lead range (commonly a few percent) specific to each model and transfer-case ratio, in the operator's manual or a dealer's tech data sheet. This tool shows how a new tire combination compares to your original factory sizes; use your manual to know whether that change is still within spec.
What happens if the lead percentage is wrong?
Too little lead (or negative lead, where the front drags) can cause the front axle to fight the drivetrain, increasing wear and fuel use and sometimes causing the front-wheel-drive to disengage or chatter. Too much lead can cause excessive front tire scrubbing and wear. Staying close to your original factory ratio when you change sizes is the safest approach.
Can I just change the rear tires and leave the front alone?
You can, but it changes the lead ratio — enter your original front/rear sizes and your proposed new rear (with the same front) to see the resulting change before you order.