Plus sizing fits a larger wheel while keeping overall diameter almost unchanged: plus zero (same rim, wider tire), plus one (+1″ rim, lower aspect ratio, e.g. 205/55R16 → 225/45R17 = +0.38% diameter), plus two (+2″). Aim for a diameter change as close to 0% as possible so the speedometer stays accurate.
The tire industry's rule of thumb: stay within ±3% of the original overall diameter. Beyond that, speedometer/odometer error, gearing, ABS/ESC calibration and clearance all become real concerns — and on AWD vehicles mismatched diameters can damage the drivetrain.
Frequently asked questions
How do I read a tire size like 205/55R16?
205 is the tread width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of the width), R means radial construction, and 16 is the rim diameter in inches. So 205/55R16 has a 112.75 mm sidewall (205 × 0.55) and fits a 16-inch wheel.
How do I calculate speedometer error after changing tire size?
Speedometer error equals the percentage change in overall tire diameter. If your new tires are 3% larger in diameter, your actual speed is 3% higher than the speedometer reads — e.g. the speedo shows 100 km/h while you are really doing 103 km/h. Enter your two sizes above and the calculator shows the exact error at any speed.
Will a bigger tire size fit my car?
The industry rule of thumb is to stay within ±3% of the original overall diameter. Within 3% is generally acceptable; 3–5% needs caution (speedometer, ABS/ESC and gearing are affected); beyond 5% is not recommended. You must also check fender clearance, rim-width compatibility and load rating. The will-it-fit verdict above applies this logic automatically, but it is general guidance — not a fitment guarantee.
What is plus sizing (plus zero, plus one)?
Plus sizing keeps the overall tire diameter nearly identical while fitting a larger wheel: plus zero keeps the same rim diameter with a wider tire, plus one moves up one inch in rim diameter with a lower aspect ratio (e.g. 205/55R16 → 225/45R17), plus two moves up two inches. The goal is a diameter change as close to 0% as possible so the speedometer stays accurate.
What does LT mean in a tire size like LT265/70R17?
LT stands for Light Truck. LT tires have stronger sidewalls and higher load capacity than P (passenger) tires of the same size, designed for pickups, SUVs and vans that carry heavy loads or tow. The size math is identical — LT265/70R17 has the same dimensions as 265/70R17.
What is the difference between metric and flotation tire sizes?
Metric sizes (e.g. 265/70R17) give width in millimeters, aspect ratio and rim diameter. Flotation sizes (e.g. 33x12.50R17) give overall diameter in inches, width in inches and rim diameter — common on trucks, Jeeps and off-road vehicles. This calculator accepts both formats and converts between them.
What is a good load index for my tires?
The load index is a number (e.g. 91) that maps to a maximum weight per tire — 91 means 615 kg. Your replacement tires must meet or exceed the load index specified on your vehicle's door placard. Use the load index chart above to look up any value.
What do speed ratings (H, V, W, Y) mean?
The speed rating letter indicates the maximum sustained speed the tire is certified for: H = 210 km/h, V = 240 km/h, W = 270 km/h, Y = 300 km/h. Like load index, replacement tires should meet or exceed the rating your vehicle manufacturer specifies.
How do bigger tires affect gear ratio?
Larger-diameter tires act like a taller gear: effective axle ratio = original ratio × (old diameter ÷ new diameter). For example, 3.73 gears with tires 3.5% larger behave like ~3.60 gears — slightly slower acceleration but lower cruising RPM. Enter your axle ratio above to see the exact effect.
What is the legal tire size increase in Australia (NSW/QLD)?
Under Australia's NCOP (VSB 14, Section LS), a change in tyre diameter of up to 50 mm generally does not require certification, while Victorian guidance limits MA passenger cars to 15 mm and allows MC 4WDs and NA–NB utes up to 50 mm. Rules are adopted per state, so always verify with your state authority (e.g. NSW, QLD) — this tool shows your diameter change against these commonly cited limits for reference only, not as legal advice.
Can I use different tire sizes on an AWD vehicle?
No — all four tires on an AWD/4WD vehicle should be the same size (and similar tread depth). Mismatched rolling diameters force the center differential to work constantly and can cause expensive drivetrain damage. Replace AWD tires as a set of four.
How do bigger tires affect fuel economy?
Bigger, wider tires typically reduce fuel economy: more rolling resistance, more aerodynamic drag and extra rotating weight. A few percent larger diameter alone has a small effect, but aggressive all-terrain tires 2+ inches larger can cost 5–15% in real-world economy. The comparison above shows the exact diameter change driving these effects.
What are revolutions per mile and why do they matter?
Revolutions per mile is how many times the tire rotates over one mile — it follows directly from the circumference. It matters for speedometer/odometer calibration, gearing calculations and comparing rolling behavior between sizes. The dimensions table above lists it for both tires.
Can this calculator look up tire size by VIN or registration plate?
No — this tool does not have a vehicle database and cannot decode VINs or registration plates. For your factory size, check the driver's door jamb placard, the owner's manual, or the tire sidewall itself. Everything here runs 100% in your browser with no data lookup.
Do motorcycle and bicycle tire sizes work here?
Yes. Metric motorcycle sizes (e.g. 120/70ZR17), scooter sizes (e.g. 90/90-10) and bicycle sizes (700x25C, 29x2.3, 26x2.10) are all supported. The same diameter math applies, so speedometer error and comparison work the same way.
Is my data sent anywhere when I use this calculator?
No. This tool runs 100% client-side in your browser — your tire sizes never leave your device, there are no accounts, no tracking of your inputs, and no external requests.