# Gear and Speed Calculator

Gear and speed calculator. Convert RPM, gear ratio, axle ratio, and tyre diameter into road speed (MPH and km/h), back-solve engine RPM at any speed, or find the gear ratio that matches a target cruise.

## What this calculates

This gear and speed calculator connects engine RPM, gear ratio, final drive, and tyre diameter to road speed in one sweep. Use it as a speed by gear calculator to see MPH per gear at any RPM, flip to RPM-from-speed to plan a cruise, or solve for the gear ratio that will give you a target top speed at redline. SI and imperial units are both supported.

## Inputs

- **Solve For** — options: Speed from RPM (speed by gear calculator), RPM from speed, Gear ratio for a target speed — Pick the unknown. Speed-by-gear mode returns MPH and km/h at the input RPM in every gear.
- **Engine RPM** (RPM) — min 0 — Engine speed for the calculation. Used in Speed-from-RPM and ratio-for-speed modes.
- **Transmission Gear Ratio** — min 0 — Ratio of the gear you are computing. 1.00 = direct drive, 0.78 = typical 5th overdrive.
- **Final Drive Ratio** — min 0 — Differential or rear-end ratio.
- **Tyre Diameter** (in) — min 0 — Overall tyre diameter. A 225/70R16 is about 28.4 in. A 265/70R17 is about 31.6 in.
- **Target Speed** (mph) — min 0 — Used in RPM-from-speed mode (road speed input) and ratio-for-speed mode (desired cruise).

## Outputs

- **Road Speed** (mph) — Vehicle speed at the specified RPM and ratios.
- **Road Speed** (km/h) — Vehicle speed in kilometres per hour.
- **Overall Drive Ratio** — Transmission ratio x final drive ratio.
- **Engine RPM** (RPM) — Engine RPM for the given road speed (RPM-from-speed mode).
- **Gear Ratio Needed** — Transmission gear ratio that gives the target speed at input RPM (ratio-for-speed mode).

## Details

The gear and speed formula

Road speed from engine RPM is one equation with three inputs. The gear ratio (transmission) tells you how many engine turns per driveshaft turn. The final drive ratio tells you how many driveshaft turns per axle turn. Multiplying gives the overall drive ratio = gear ratio x final drive. Combine with tyre diameter:

MPH = (RPM x tyre diameter x pi) / (overall ratio x 1056)

The 1056 constant is 12 in/ft x 5280 ft/mi / 60 min/hr, so the units all cancel to miles per hour when tyre diameter is in inches. Divide the result by 0.621371 to get km/h. Invert the equation to back-solve RPM from road speed: RPM = (MPH x overall x 1056) / (tyre x pi). Invert again to solve for the required gear ratio: gear ratio = (RPM x tyre x pi) / (MPH x 1056 x final drive).

Speed by gear calculator reference values

  
    GearTrans ratioFinal driveRPMTyre inMPH
  
  
    1st3.583.7365002637.8
    2nd2.023.7365002667.0
    3rd1.353.73650026100.2
    4th1.003.73650026135.3
    5th overdrive0.783.7330002680.0
    Highway cruise0.783.7322502660.0
  

You can read this as a speed by gear calculator: pick the row that matches your gear and RPM, check the MPH. The calculator below does the same math with exact precision for any combination you enter.

Picking the right gear and final drive

The gear and speed calculator becomes most useful when you are comparing builds. A common trade is a shorter final drive (numerically higher, say 4.10 instead of 3.73) to improve acceleration at the cost of highway cruise RPM. With a 4.10 instead of 3.73 at 70 MPH in top gear on 26 inch tyres, engine RPM jumps from roughly 2626 to 2887 RPM - about 10 percent higher. That hurts fuel economy on long drives but drops 0 to 60 by half a second or more.

The mirror trade is a taller tyre. Up-size from 26 in to 31 in and effective gearing drops by 31 / 26 = 1.19x. Acceleration softens, top speed grows, and highway cruise RPM drops. This is why off-road trucks often regear to 4.88 or 5.13 after a tyre upsize: the axle change restores the original effective ratio.

How this relates to the bigger transmission picture

A speed by gear calculator is really a single-gear slice of a full gear-ratio chart. If you want to see all six gears at once (e.g. comparing a TR-6060 manual to a ZF 8HP automatic), use the transmission gear ratios calculator. If you just need one answer (am I cruising at 2200 RPM or 2800 RPM at 70 MPH?), this gear and speed calculator is faster.

## Frequently Asked Questions

**Q: How does a gear and speed calculator work?**

A: It plugs engine RPM, transmission gear ratio, final drive, and tyre diameter into MPH = (RPM x tyre x pi) / (overall x 1056). Multiply gear ratio by final drive to get the overall ratio. At 3000 RPM with a 0.78 overdrive, 3.73 axle, and 26 in tyre: overall = 2.91, MPH = (3000 x 26 x pi) / (2.91 x 1056) = 80.0 MPH.

**Q: What is a speed by gear calculator?**

A: A speed by gear calculator is the same math applied to multiple gears at once. Enter engine RPM and tyre diameter, then for each transmission ratio the tool returns the road speed. Useful for shift-point planning on the drag strip or choosing which gear to cruise in on the highway. This calculator does one gear at a time; the transmission gear ratios calculator handles all six at once.

**Q: How do I find engine RPM from road speed?**

A: Invert the speed equation: RPM = (MPH x overall x 1056) / (tyre x pi). At 70 MPH in 5th (0.78) with a 3.73 axle and 26 in tyre: overall = 2.91, RPM = (70 x 2.91 x 1056) / (26 x pi) = 2626 RPM. Switch the calculator mode to RPM from speed and it does the math automatically.

**Q: What gear ratio do I need for a given top speed?**

A: If you know the engine RPM you plan to redline at (say 6500) and the top speed you want (say 155 MPH) on 26 in tyres with a 3.73 final, solve the gear equation for gear ratio: gear = (RPM x tyre x pi) / (MPH x 1056 x final) = (6500 x 26 x pi) / (155 x 1056 x 3.73) = 0.87. A 0.87 overdrive in top gear delivers 155 MPH at 6500 RPM with those tyres and axle.

**Q: How does tyre diameter affect speed?**

A: Directly. The formula is MPH proportional to tyre diameter, so a 10 percent bigger tyre raises road speed by 10 percent at any given RPM and gear. Going from 26 in to 31 in (19 percent larger) raises MPH at 3000 RPM from 80.0 to 95.4 in the example setup. It also drops engine RPM at a given cruise speed by the same fraction.

**Q: Does this gear and speed calculator work in metric?**

A: Yes. Tyre diameter accepts mm, speed accepts km/h, and the calculator returns both MPH and km/h in every mode. Internally the math is in inches because the 1056 constant builds in 12 in/ft and 5280 ft/mi, but the input and output toggles handle unit conversion automatically.

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Source: https://vastcalc.com/calculators/physics/gear-and-speed
Category: Physics
Last updated: 2026-04-08