Skip to content

Auto Mod Gear Ratio Calculator

Calculate final drive ratio, engine RPM at speed, and optimal gear spacing. Plan transmission ratios for highway cruising and acceleration goals.

Use this result well

Inputs that matter
Engine RPM, Transmission Gear Ratio, Final Drive Ratio, Tire Diameter
Output to expect
Gear Ratio & Final Drive Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
Was this tool helpful?

Related Tools

Tools you might need next

Auto Mod Engine Displacement Calculator

Calculate engine displacement from bore, stroke, and cylinder count. Convert cubic inches to liters and compare stroker or overbore combinations.

Auto Mod
Cycling Gear Ratio Calculator

Calculate gear ratio, gear inches, and development from chainring and cog. Free cycling gear ratio calculator for road cycling, bike fitment.

Sports Cycling
Reviewed Bicycle Gear Record

Reconcile one completed conventional bicycle gear-pair record from whole tooth counts, measured loaded wheel rollout, and installed crank length.

Cycling

Reference & details

How it works

RPM at Speed

Engine RPM equals vehicle speed times total gear ratio times a constant divided by tire diameter. The constant 336 applies when speed is in mph and tire diameter in inches.

RPM = (Speed × Gear ratio × Final drive × 336) / Tire diameter

Speed in Each Gear

Maximum speed in a gear occurs at redline RPM. Rearrange the RPM formula to find speed at any RPM and gear combination.

Speed = (RPM × Tire diameter) / (Gear ratio × Final drive × 336)

Optimal Final Drive Selection

Lower numerical final drive (e.g., 3.08 vs 3.73) reduces highway RPM and improves fuel economy. Higher numerical ratio improves acceleration but increases RPM at cruise.

Updated: July 2026

Example Scenarios

6th gear 0.85, final drive 3.55, 26-inch tires: RPM = (70 × 0.85 × 3.55 × 336) / 26 = 2,430 RPM. Comfortable highway cruise.

Changing from 3.27 to 3.90 final drive raises RPM at all speeds by 19%. 0-60 improves; highway RPM at 70 mph jumps from 2,100 to 2,500.

Larger 28-inch tires (vs 26-inch) reduce effective gearing by 7%. Same RPM at speed as a lower numerical final drive — recalculate after tire changes.

Common Mistakes to Avoid

Confusing "lower gearing" with a smaller number

Lower gearing means a higher numerical ratio (3.90 is lower gearing than 3.08). Lower numerical ratio = taller gearing = less acceleration, lower highway RPM.

Calculating RPM without including final drive ratio

Total gear ratio is transmission gear times final drive (and transfer case if 4WD). Omitting final drive produces RPM values that are off by 3–4×.

FAQ

RPM = (MPH × Transmission ratio × Final drive × 336) / Tire diameter (inches). Example: 70 mph, 0.75 overdrive, 3.42 final, 26" tires = 2,310 RPM.

Most modern engines cruise efficiently at 1,800–2,500 RPM. Below 1,500 RPM may lug the engine; above 3,000 RPM at highway speed wastes fuel and increases wear.

Higher numerical ratio (3.73 vs 3.08) means more acceleration but higher RPM at any speed. "Lower gearing" means higher numerical ratio — the terms are counterintuitive.

Larger diameter tires lower effective gearing (fewer RPM at same speed) like a smaller final drive number. Recalculate RPM after any tire size change.

Drag builds maximize 1st gear acceleration and ensure the car crosses the finish at redline in top gear. Calculate trap speed in top gear at redline to verify gearing matches quarter-mile speed.

About Auto Mod Gear Ratio Calculator

Gear ratios and final drive determine engine RPM at highway speed, acceleration in each gear, and fuel economy. This calculator computes RPM at any speed, speed in each gear, and helps select final drive ratios for your tire size and performance goals.