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Off-Road Tire Size Calculator

2026-09-27 6 min

True tire diameter, circumference and revs-per-mile from metric size (width, aspect, rim).

Tire Diameter
31.61 in
Circumference
99.3 in
Revs Per Mile
638 /mi

Calculator

Section Width(mm)
mm
Aspect Ratio(%) Help
%

Sidewall height as % of width

Rim Diameter(in)
in
Results update live as you type

Live statistics

Tire Diameter

31.61 in

Circumference

99.3 in

Revs Per Mile

638 /mi

Results

Optimal

Tire Diameter

31.61in

Circumference

99.3in

Revs Per Mile

638/mi

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

Visualizations

Results overview

31.6Tire Diameter
99.3Circumference
638.1Revs Per Mile

Tire Diameter vs. Section Width

Tire Diameter

31.6in
2542

Formula

Converts metric sidewall height to inches, doubles it (top + bottom), and adds the rim.

Diameter = rim + 2 x (width x aspect / 2540)

Variables

SymbolNameUnitDescription
SSidewall heightinwidth x aspect / 2540

Explanation

A 265/70R17 works out to ~31.6 in tall and ~638 revs/mile — the baseline for gearing and speedometer corrections.

Step-by-Step Calculation

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

Step 1

Record the inputs.

Section Width:265 mm
Aspect Ratio:70 %
Rim Diameter:17 in
Result: 3 inputs ready
Step 2

Apply the formula.

Diameter = rim + 2 x (width x aspect / 2540)
Result: Primary result = 31.61 in
Step 3

Read every derived output.

Tire Diameter:31.61 in
Circumference:99.3 in
Revs Per Mile:638 /mi
Result: Tire Diameter = 31.61 in
Final Answer
31.61

Worked Examples

See how the calculator works with different scenarios.

Scenario

Common mid-size truck fitment.

Inputs

Width:265 mm
Aspect:70%
Rim:17 in

Calculation

Sidewall 7.30 in; dia 31.61 in; circ 99.29 in; 63,360/99.29 = 638

Result

31.61 in, 638 revs/mile.

Interpretation

Use 31.6 in for effective-gear and speedo-error math.

Understanding Your Result

Your Result: Tire Diameter: 31.61 in

Typical range: Stock 29-32 in; built rigs 33-40 in.

What This Means

True rolling diameter of the metric tire size.

Why It Matters

Diameter drives clearance, gearing, speedometer error and crawl-speed math.

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

Metric tire codes hide the true diameter — width and aspect ratio must be converted to inches first.

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

  • •Measure worn tires — they run ~0.5 in short
  • •Use loaded radius for speedo work
  • •Re-gear when diameter grows 10%+

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.

Why 2540 in the formula?
Sidewall (mm) converts to inches by dividing by 25.4, and aspect is a percent (divide by 100): 25.4 x 100 = 2540.
Do revs-per-mile match the tire sticker?
Approximately — manufacturers publish loaded revs/mile, which run 1-3% higher than this unloaded geometric value.

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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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