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Automotive Calculators

Tire Size and Gear Ratio Calculator

2026-09-27 6 min

Match gears to bigger tires: corrected ratio to restore stock feel plus speedometer error.

Corrected Gear Ratio
4.14 :1
Restores stock overall
Speedometer Error
11.1 %
Reads low by this much

Calculator

Stock Tire Diameter(in)
in
New Tire Diameter(in)
in
Stock Gear Ratio(:1)
:1
Results update live as you type

Live statistics

Corrected Gear Ratio

4.14 :1

Restores stock overall

Speedometer Error

11.1 %

Reads low by this much

Results

Optimal

Corrected Gear Ratio

4.14:1

Restores stock overall

Speedometer Error

11.1%

Reads low by this much

Planning estimate from standard automotive formulas. Validate with dyno, timeslip or datalog data when precision matters.

Visualizations

Results overview

4.1Corrected Gear Ratio
11.1Speedometer Error

Corrected Gear Ratio vs. Stock Tire Diameter

Corrected Gear Ratio

4.1:1
26

Formula

Taller tires lengthen effective gearing; numerically higher gears in the same proportion restore the original overall.

Corrected = stock ratio × new tire ÷ stock tire

Variables

SymbolNameUnitDescription
oldStock diameterinFactory tire
newNew diameterinPlus-size or lift tire
ratioStock gears:1Factory axle ratio

Explanation

Plus speedometer truth: uncorrected, the speedo reads low by the diameter percentage.

Step-by-Step Calculation

Follow each step to understand how the result is calculated from your inputs.

Step 1

Record the inputs.

Stock Tire Diameter:27 in
New Tire Diameter:30 in
Stock Gear Ratio:3.73 :1
Result: 3 inputs ready
Step 2

Apply the formula.

Corrected = stock ratio × new tire ÷ stock tire
Result: Primary result = 4.14 :1
Step 3

Read every derived output.

Corrected Gear Ratio:4.14 :1
Speedometer Error:11.1 %
Result: Corrected Gear Ratio = 4.14 :1
Final Answer
4.14

Worked Examples

See how the calculator works with different scenarios.

Scenario

Stock 27 in, new 30 in, 3.73 gears.

Inputs

Stock:27 in
New:30 in
Ratio:3.73

Calculation

corrected = 3.73 × 30 ÷ 27 = 4.14; speedo error +11.1%

Result

4.14 gears (≈4.10s) · speedo reads 11% low.

Interpretation

4.10s restore stock feel; recalibrate the speedometer either way.

Understanding Your Result

Your Result: Corrected Gear Ratio: 4.14 :1

What This Means

Gears that cancel the taller-tire penalty, and the speedo lie if you skip them.

Why It Matters

The mandatory calculation behind every lift and plus-size fitment.

Professional Interpretation

Treat the primary result as a planning estimate, then validate on a dyno, at the track, or with logged sensor data. If measured numbers disagree by more than ~10%, re-check inputs (units are the usual culprit) before changing parts.

Recommendations

  • •Double-check units on every input before trusting the result
  • •Compare against the worked example below to sanity-check magnitude
  • •Validate with measured data (dyno, timeslip, datalog) when it matters

Learn — fuel economy, deeply

Bigger tires are taller gears in disguise. Regear by the same proportion to get your truck back.

1 · Where the gallon goes (energy path)

Engine heat

60–70% lost

Coolant + exhaust carry most energy away. Hybrids recover some via Atkinson cycle + regen.

Driveline

5–15% lost

Transmission, differential, accessories. CVTs and 8–10 speed autos lose less.

Aero drag

∝ v²

Doubles from 55→78 mph. Roof boxes hurt more than trunks.

Rolling + brakes

10–20%

Tires, weight, stops. Regen braking (hybrid/EV) reclaims city braking energy.

4 · Measure like a pro (the 3-tank protocol)

  1. Fill to first click at the same pump, record the odometer (Tank 0).
  2. Drive normally for 2–3 tanks; always refill to first click at the same pump.
  3. MPG = total miles ÷ total gallons across all tanks — averaging kills pump-click noise.
  4. Cross-check with the trip computer; a persistent >10% gap means the computer needs calibration — or the car needs service.
  5. Log with an OBD-II dongle for per-trip MPG if you want to isolate commute vs. highway behavior.

Practical Tips

  • •Start from the default values — they describe a typical combination
  • •Use the sensitivity chart to see which input moves the result most
  • •Keep units consistent (all inches or all mm, never mixed)

Common Mistakes

Mixing units (inches with mm, psi with kPa)
Convert every input to the unit shown on its field before calculating.
Trusting a single output blindly
Cross-check with the worked example and, when it matters, a dyno or datalog.

Assumptions

This calculator makes the following assumptions to provide estimates:

Steady-state model: Formulas assume stabilized conditions; transients, heat soak and traction limits are excluded.
Standard factors: Empirical constants (BSFC, VE, drivetrain loss) use typical values — your combination may differ.

Limitations

Be aware of these limitations when using this calculator:

Estimate, not measurement: Predictions carry real-world scatter of roughly ±5–15% depending on the model.
No transient effects: Shift time, wheelspin, spool lag and heat soak are not modeled here.

Frequently Asked Questions

Answers to common questions about this calculator.

Do I really need to regear?
Over ~10% taller tires, yes for towing/performance; daily drivers often accept the loss with a speedo recalibration.
Speedo reads high or low?
Taller tires: reads LOW (going faster than indicated). Shorter: reads high.

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Related Guides

Deepen your understanding with these educational resources.

Glossary

Technical terms used in mortgage calculations explained in plain language.

MPG
Miles per gallon. Distance travelled per gallon of fuel; higher is better. US gallon = 3.785 L.
L/100 km
Litres consumed per 100 kilometres; lower is better. Standard metric efficiency unit.
MPGe
Miles per gallon-equivalent. EV metric: miles travelled on 33.7 kWh (energy of 1 gal of gasoline).
CAFE
Corporate Average Fuel Economy — US fleet-average efficiency standard enforced by NHTSA.
Rolling resistance
Energy lost deforming tires as they roll. Low-pressure or wide tires raise it.
Aerodynamic drag
Air resistance growing with the square of speed — why highway MPG falls fast above 60 mph.
Hypermiling
Driving techniques (coasting, steady speed, anticipation) that maximise real-world MPG.
OBD-II
On-board diagnostics port. Scanners and dongles read live MPG, fuel trim and fault codes.

Authoritative References

Trusted sources and standards referenced by this calculator.

Applicable Standards

EPA Fuel Economy Testing (40 CFR Part 600)CAFE — Corporate Average Fuel Economy Standards (NHTSA)WLTP — Worldwide Harmonised Light Vehicle Test ProcedureSAE J1321 / J1711 — Fuel consumption test proceduresISO 2575 — Road vehicles symbols & efficiency markings

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