Regear calculator
Bigger tires make your gears act taller. See your highway RPM on the new tires, on your current gears and on every common ratio, and which one puts the truck back where the factory had it.
Add your tires, your axle ratio and your top gear.
Every ratio on
RPM by gear
Fuel economy by ratio
Highway, estimateWhere the mileage goes
The dash figure is exact: it reads low by the same share as the speedometer until you recalibrate. The rest is an estimate for highway driving with some hills and wind. Air drag is roughly half the load on a truck and tire rolling resistance about a quarter, so lift adds frontal area and aggressive tread and weight add rolling resistance. Gearing moves it a few percent either way. Below stock RPM the engine lugs, so an automatic downshifts and unlocks its converter on every grade and headwind, and that costs the most. Above stock RPM the engine burns a little more fighting its own friction. Back near stock RPM is usually the best mileage you'll get on the bigger tires.
Estimate. Real mileage depends on how and where you drive. Use at your own risk.
Do both axles, and buy a master install kit with them. Some ratios sit on the other side of a carrier break and need a different carrier, so check with whoever sets up your gears before you order.
How the numbers work
How is engine RPM worked out?
RPM = speed in mph × axle ratio × transmission gear × 336.13 ÷ tire diameter in inches. The 336.13 turns miles per hour into tire revolutions per minute. A 33.8 inch tire at 65 mph in a 0.716 top gear on 3.73s turns about 1,725 rpm.
What ratio gets me back to stock?
Multiply your stock ratio by the new tire diameter over the stock diameter. A truck with 3.73s going from 31.6 to 33.8 inch tires needs 3.99 to match. Nobody makes a 3.99, so the nearest ratio you can buy, 4.10, is the pick.
Should I go deeper than stock?
It depends how you drive. For daily and highway use, the first ratio at or past the stock match is the pick, and on a loaded overland rig it usually feels better than stock did. Living in the mountains, towing or carrying a heavy build, go one step deeper. A truck that mostly wheels can go two steps and trade some highway manners and mileage for crawl and low-speed control. On an FJ on 35s that works out to 4.56, 4.88 and 5.29.
How much mileage do bigger tires cost?
Two things happen. Your dash under-reports: on tires 10% taller with no speedometer fix, it shows about 10% less mileage than you're really getting. And the truck really does use more fuel: a lift adds frontal area pushing air, and heavier, more aggressive tires roll harder. On a typical build of 33s to 35s with a few inches of lift, expect something like a 10 to 20% real drop on the highway. Fix the speedometer first, then fill up twice and work it out from the pump to see where yours lands.
Do I have to do both axles?
Yes. Front and rear have to match, or the transfer case binds the moment you're in 4WD. It's the same job twice, so it's usually priced as one.
Where does my speedometer end up?
On trucks that read speed from the transmission or transfer case, a regear changes the speedometer too, and a ratio that matches stock puts it back close to right. Most newer trucks read speed from the ABS wheel sensors, and those only care about the tire. The speedometer calculator handles both.
What do the presets cover?
Factory gearing for the Toyota, Jeep and Ford trucks people build most, from spec sheets and press kits. Figures we couldn't confirm against a factory source are called out when you pick the truck. Trim packages and model years change gearing, so check your build sheet or the tag on the diff.
Use at your own risk. These calculators are here to help you plan and to inform your decisions, not to sign off on a build. Many numbers are estimates worked out from factory figures, and some factory figures are not yet confirmed. Check every number against your own truck, its manuals and a qualified shop before you buy parts, cut, weld, lift, regear or drive on it. Long Range Desert Tools is not liable for anything you do based on these calculators or their results. The full terms are in our policies.