Regear Calculator — Restore Cruise RPM
Taller tires are like a taller gear. We calculate the ratio that puts cruise RPM back. We do not tell you to buy 4.56.
What it does
new_axle = current_axle × (new_diameter / stock_diameter). That restores engine speed at the same mph. Nearby common aftermarket ratios are listed for comparison, not as a shopping order. Optional MPG field applies a first-order diameter-ratio estimate, labeled as such.
How to use it
Enter stock size, new size, and the axle ratio you have now (door sticker, glovebox, or a tag on the differential). Optional: cruise mph, transmission ratio in that gear (1.00 is direct; overdrive is often 0.70–0.85), and current MPG.
How to read the result
If you went from ~32" to 35", you need roughly 9% more axle to keep the same RPM. Which exact ring-and-pinion exists for your third member is a parts catalog, not this page. 4WD needs the same ratio front and rear or the driveline binds. Fuel economy: we only have the gearing effect. EPA’s published 1.8–2% MPG shift is for rolling resistance, not diameter — so a dollar-per-year figure with a fake coefficient is not here.
What this does not calculate
Unloaded tire diameter. Ignores converter slip, shift schedule and aero. Common ratio list is not every gear ever cut.
Does not recommend a part for a Tacoma, Jeep or F-150. Does not promise MPG.
Questions
- Why won’t you just say buy 4.56?
- Because that depends on the engine, transmission, weight and which carriers exist. Restoring RPM is geometry. “Buy 4.56” is a forum recipe.
- Will bigger tires hurt MPG?
- They usually change RPM at a given speed, and they often add weight and rolling resistance. If you enter your current MPG we show a first-order estimate from diameter alone. Treat it as an estimate.
Related tools
- RPM at Speed CalculatorHow fast the engine is turning at 65. Diameter and gears, not a guess from a forum chart.
- Odometer Correction CalculatorThe needle is a different tool. This one is about the mile counter — oil changes, warranties, resale.
- Crawl Ratio CalculatorHow much the tires are reduced in 1st and 4-low. It is just three numbers multiplied.
Estimate — not a fitment approval.
