Tire Size Calculator
Compare original and new tire sizes, calculate overall diameter, speedometer error, and plus-minus sizing compatibility.
Did our AI summary help? Let us know.
Stay within 3% overall diameter to avoid speedometer and safety system issues. Plus sizing = bigger rim, lower sidewall; minus = smaller rim, taller sidewall. Every 1% diameter increase makes speedo read ~0.6 mph slow at 60 mph.
Ready to run the numbers?
Why: Changing tire size affects speedometer accuracy, ABS, and traction control. Stay within 3% diameter to avoid issues.
How: Sidewall = Width × Aspect Ratio / 100. Diameter = (Sidewall × 2) + Rim. Speedo error = (New Circ / Old Circ − 1) × 100%.
Run the calculator when you are ready.
Tire Size Calculator — Compare, Convert & Check Compatibility
Compare original and new tire sizes. Calculate overall diameter, speedometer correction, and check if your new tires are compatible.
🛞 Tire Scenarios — Click to Load
Original Tire Size
New Tire Size
Side-by-Side Comparison
Calculation Breakdown
For educational purposes only — not financial advice. Consult a qualified advisor before making decisions.
💡 Money Facts
265/70R17 = 265mm wide, 70% aspect, 17" rim.
±3% diameter keeps speedo within ~2 mph at 60.
Plus sizing improves handling; minus improves ride comfort.
Wider tires typically reduce MPG by 1–3%.
Key Takeaways
- A tire code like 265/70R17 means: 265mm wide, 70% aspect ratio (sidewall height), Radial, 17" rim
- Stay within 3% overall diameter of original to avoid speedometer, ABS, and traction control issues
- Plus sizing (bigger rim, lower sidewall) improves handling; minus sizing (smaller rim, taller sidewall) improves ride comfort
- Every 1% diameter increase makes your speedometer read ~0.6 mph slow at 60 mph
How to Read a Tire Size Code
Frequently Asked Questions
How much bigger can I go?
Stay within 3% of the original overall diameter. This keeps your speedometer accurate (within ~2 mph at highway speeds) and avoids triggering ABS/traction control errors. Most vehicles can handle 1" diameter increase without issues.
What is plus sizing?
Plus sizing means increasing rim diameter while decreasing sidewall height to maintain the same overall diameter. +1 sizing = +1" rim, lower profile tire. This improves handling and looks but makes the ride firmer. The key is keeping overall diameter within 3%.
How does tire size affect speedometer?
Larger tires make your speedometer read SLOWER than actual speed (you're going faster than shown). Smaller tires make it read FASTER. A 3% diameter increase means when your speedo says 60, you're actually doing ~61.8 mph.
Will bigger tires affect gas mileage?
Wider and heavier tires increase rolling resistance, typically reducing MPG by 1-3%. Taller tires (larger diameter) change the effective gear ratio, which can improve highway MPG slightly but hurt city MPG. The net effect is usually a small decrease.
Can I put different width tires on my car?
You can usually go 10-20mm wider than stock. Check your wheel width recommendation (stamped on the wheel or in the owner's manual). Too wide and the tire won't seat properly; too narrow and handling suffers. Also check for fender clearance.
Official Data Sources
Disclaimer: Tire fitment depends on vehicle-specific factors (wheel well clearance, suspension, load rating). Always consult your vehicle manual and a tire professional before changing sizes. This calculator provides dimensional comparisons only — it does not guarantee fitment for your specific vehicle.
Related Calculators
Accelerated Auto Loan Payoff Calculator
Calculate how much time and money you can save by accelerating your auto loan payoff. Compare strategies including extra payments, lump sums, and bi-weekly...
AutoAuto Payment Calculator
Calculate your monthly car payment, total interest costs, and amortization schedule with comprehensive auto payment calculator. Includes trade-in values...
AutoAuto Refinance Calculator
Calculate potential savings from refinancing your auto loan. Compare current vs. new loan terms, determine break-even point, and analyze total cost savings...
AutoAuto Refinance - Lower Monthly Payment Calculator
Calculate the loan term needed to achieve your desired lower monthly payment when refinancing your auto loan. Compare total interest costs, break-even...
Auto0% Dealer Financing vs Bank Loan Calculator
Compare dealer financing offers with bank loans including rebates, opportunity costs, and break-even analysis. Make informed decisions about your auto...
AutoElectric vs Gas Vehicle Calculator
Compare total cost of ownership between electric and gas vehicles. Calculate fuel savings, break-even points, tax credit impact, and environmental benefits....
Auto