About the Tire Size Comparator
“Will these fit?” is one of the most-asked questions in car ownership, and the answer is mostly arithmetic nobody wants to do on a phone in a tire shop. This tool parses both sizes — metric and flotation — and computes what actually changes: overall diameter, sidewall, ride height, revolutions per mile, and the one that surprises people, speedometer error: at an indicated 70 mph on a +3% tire you are really doing 72.1.
Plus the decoder ring for everything else moulded into the sidewall: load index to pounds from the standard table, speed symbol to mph, and the DOT date code to the week the tire was made. The geometry engine was validated in Node against published manufacturer diameters and an independent computation path before shipping.
225/45R17, LT265/70R17 121/118S, 33x12.50R15…The closest-diameter sizes to your current tire at each rim — every option within 3%, closest first. The classic way to go up a rim size without changing anything else.
Both sizes with the full geometry, the diameter delta and your load/speed requirements — printable, so the conversation at the counter starts from numbers.
How to Use the Tire Size Comparator
Type both sizes exactly as the sidewall states them and compare. The tool parses metric sizes (with or without P/LT prefixes, ZR markings, and load/speed suffixes) and flotation sizes, computes the geometry of each, and leads with the two numbers that decide most swaps: the percentage diameter change and what your speedometer will be wrong by.
Reading a Tire Size in Ten Seconds
225/45R17 91V: 225 is the section width in millimetres; 45 is the aspect ratio — the
sidewall as a percentage of the width; R is radial construction; 17 is the rim diameter in inches; 91 is
the load index (1,356 lb per tire from the standard table); V is the speed rating (149 mph sustained).
Off-road sizes skip the ratios: 33x12.50R15 is simply 33 inches tall and 12.5 wide.
The Swap Maths That Actually Matters
Change the diameter and three things move together: the speedometer (counts revolutions, assumes the old size — taller tire, dash under-reads), the odometer (same error, compounding into your service intervals, warranty mileage and resale figure), and the effective gearing (taller tires are a longer final drive: lazier acceleration, lower cruise RPM). The classic “plus one” swap — bigger rim, shorter sidewall, near-identical diameter — exists precisely to get the look without moving those three, and the ladder panel finds those sizes for any rim.
Related tools: the Tire Pressure & Temperature tool for what the seasons do to PSI, the True Cost of Ownership Calculator for what the upsize does to the budget, and the VIN Decoder & Recall Watch for the rest of the car. Browse every Auto & Vehicle tool for more.
Geometry from the stated dimensions; real tires vary slightly by model and inflation. Fitment is finally about your car’s wheel well, offset and suspension travel — within ±1.5% diameter is usually uneventful, but nothing here inspects your fender clearance. Load index and speed rating requirements are on the driver’s-door placard; never fit below them.
Frequently Asked Questions
How is tire diameter calculated from the size?
A metric size reads width / aspect R rim: 225/45R17 is 225 mm wide, the sidewall is 45% of that (101.25 mm), and the rim is 17 inches. Overall diameter = rim + two sidewalls = 17" + 2 × 3.99" ≈ 25.0". Flotation sizes state it directly: 33x12.50R15 is simply 33" tall. Everything else on this page — circumference, revolutions per mile, speedometer error — is geometry on that number.
How much diameter change is acceptable?
The industry rule of thumb for replacement sizing is to stay within about ±3% of the original diameter, and plus-size swaps are usually designed to land within 1–2%. Inside ±1.5% almost nothing changes; between 1.5% and 3% your speedometer and odometer drift noticeably and gearing shifts slightly; past 3% you are into re-calibration territory and, on many cars, rubbing risk at full lock or full compression. The verdict badge uses those bands — but clearance is ultimately about your particular car's wheel well, which no calculator can see.
Which way does the speedometer error go?
The speedometer counts wheel revolutions and assumes the factory diameter. A taller tire covers more road per revolution, so the dash under-reads: with a +3% diameter, an indicated 70 mph is really about 72.1. A shorter tire does the opposite — you are going slower than indicated, and your odometer racks up miles you never drove. The table shows indicated versus actual at every common speed, and subscribers get the year-of-driving odometer drift.
What do the load index and speed letter mean?
The number is a row in the standardized load table: 91 means 1,356 lb per tire, 94 means 1,477 lb. Fit tires at or above your original index — the placard inside the driver's door states the requirement. The letter is the sustained-speed rating: T = 118 mph, H = 130, V = 149, W = 168, Y = 186; a ZR marking alone means "over 149" with W or Y refining it. Neither is a suggestion to try; both assume correct inflation and load.
How do I read the DOT date code?
The last four digits of the DOT string are week and year: 4321 means week 43 of 2021. It matters because rubber ages regardless of tread: NHTSA and most manufacturers advise inspection from six years and many say replace by ten, even on a tire that "looks fine" — and a three-digit code means the tire predates 2000 and belongs in a museum, not on a car. Check it when buying "new" tires too; stock that sat in a warehouse for years is legally new but materially older.
