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Tire Comparison Calculator

2026-09-27 6 min

Diameter change, speedometer error and clearance gain between two tire sizes.

Diameter Difference
22.2 %
Speedometer Error
22.2 %
positive = speedo reads low
Clearance Gain
3.00 in
radius gain

Calculator

Diameter — Tire A(in)
in
Diameter — Tire B(in)
in
Results update live as you type

Live statistics

Diameter Difference

22.2 %

Speedometer Error

22.2 %

positive = speedo reads low

Clearance Gain

3.00 in

radius gain

Results

Optimal

Diameter Difference

22.2%

Speedometer Error

22.2%

positive = speedo reads low

Clearance Gain

3.00in

radius gain

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

Visualizations

Results overview

22.2Diameter Difference
22.2Speedometer Error
3Clearance Gain

Diameter Difference vs. Diameter — Tire A

Diameter Difference

22.2%
030

Formula

Rolling-diameter math linking size change to speedometer bias and ground clearance.

Diff% = (B - A)/A x 100; speedo error = (B/A - 1) x 100; clearance = (B - A)/2

Alternative Form

True speed = indicated x (B/A); gearing effectively tallens by B/A

Variables

SymbolNameUnitDescription
DDiameterinOverall tire diameter
ESpeedo error%Percent the speedometer under-reads

Explanation

Only radius lifts the axle — half the diameter gain becomes clearance, all of it biases gearing and speedo.

Step-by-Step Calculation

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

Step 1

Record the inputs.

Diameter — Tire A:27 in
Diameter — Tire B:33 in
Result: 2 inputs ready
Step 2

Apply the formula.

Diff% = (B - A)/A x 100; speedo error = (B/A - 1) x 100; clearance = (B - A)/2
Result: Primary result = 22.2 %
Step 3

Read every derived output.

Diameter Difference:22.2 %
Speedometer Error:22.2 %
Clearance Gain:3.00 in
Result: Diameter Difference = 22.2 %
Final Answer
22.2

Worked Examples

See how the calculator works with different scenarios.

Scenario

Stock versus oversize off-road fitment.

Inputs

A:27 in
B:33 in

Calculation

Diff 22.2%; speedo 22.2% low; clearance +3.0 in

Result

22.2% taller, +3 in clearance.

Interpretation

Regear and recalibrate — gearing and speedo shift over a fifth.

Understanding Your Result

Your Result: Diameter Difference: 22.2 %

Typical range: Plus-size street 1-3%; off-road oversize 10-25%.

What This Means

How much taller the new tire is and what it costs in calibration.

Why It Matters

Big tires lift and grip but corrupt speed, gearing and shift points.

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

Tire comparison is rolling-diameter change and its side effects.

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

  • •Stay under ~3% without recalibration
  • •Recheck clearance at full lock and stuff
  • •Regear to restore stock effective ratio

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.

Will bigger tires throw off my speedometer?
Yes — taller tires under-report speed by the diameter ratio. Multiply indicated speed by B/A for true speed.
How much clearance do bigger tires add?
Half the diameter gain — a 6-inch taller tire lifts axles about 3 inches.

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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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