Reading a Tire Sidewall and Choosing a New Size
Every tire carries a block of numbers that tells you its size, load rating, speed rating and age. This guide decodes them, shows the arithmetic behind overall diameter and speedometer error, and explains how to move to a different size without rubbing, throwing off your gearing or overloading the tire.
Decoding the size
Take a common size, 245/45R18 100W. The 245 is the section width in millimeters, measured from sidewall to sidewall on the tire when mounted on a specific wheel width. The 45 is the aspect ratio, meaning the sidewall height is 45 percent of the width. The R means radial construction, and 18 is the wheel diameter in inches. The 100 is the load index and the W is the speed rating.
Because width is metric and wheel diameter is imperial, the arithmetic mixes units. Sidewall height is width times the aspect ratio: 245 mm × 0.45 = 110.25 mm. Overall diameter is the wheel diameter plus two sidewalls: 18 in × 25.4 = 457.2 mm, plus 2 × 110.25 = 220.5 mm, for 677.7 mm, or 26.7 inches. The Tire Size Calculator does this for two sizes side by side so you can see the difference at a glance.
Load index and speed rating
The load index is a code, not a weight. An index of 100 corresponds to 1,764 pounds per tire at the tire's maximum inflation pressure, and each step up adds a few percent more. Never go below the load index the vehicle manufacturer lists on the door-jamb placard, and remember that pickups and vans often need Load Range or XL/Extra Load tires. The speed rating is a letter that shows the highest sustained speed the tire was tested for, for example S for 112 mph, T for 118, H for 130, V for 149, W for 168 and Y for 186. A lower rating than stock can affect handling and warranty, so match or exceed the original unless you have a good reason.
The date code and age
Find the letters DOT followed by a group of characters. The last four digits are the week and year of manufacture: 2824 means the 28th week of 2024. Rubber ages even in storage, and many tire and vehicle makers advise replacing tires at around six years regardless of tread, with ten years as an absolute limit. When buying, check the date rather than assuming a tire on the rack is new.
Overall diameter and why it matters
Diameter controls three things: the speedometer reading, the effective final-drive ratio and the physical clearance in the wheel well. A tire that is 3 percent taller than stock makes the speedometer read 3 percent low, so at an indicated 60 mph you would really be going about 61.8. It also lowers the effective gearing by the same amount, which dulls acceleration slightly and lowers engine rpm at cruise. Many shops use 3 percent as a rule of thumb for how far you can stray from the original diameter without causing problems, and the Speedometer Gear Calculator helps if you need to recalibrate.
| Change from stock diameter | Speedometer effect at 60 mph indicated | Typical consequence |
|---|---|---|
| -3% | About 58.2 mph true | Quicker acceleration, higher rpm |
| 0% | 60 mph true | As the maker intended |
| +3% | About 61.8 mph true | Slight loss of acceleration, check clearance |
| +6% | About 63.6 mph true | Noticeable gearing change, rubbing risk |
Plus sizing
Plus sizing means fitting a larger wheel and a lower-profile tire so overall diameter stays about the same. Going from a 225/50R17 to a 235/40R19 keeps the diameter close but gives a shorter sidewall, sharper steering response and a firmer ride. The trade-offs are a harsher ride, more risk of wheel damage from potholes and often higher tire cost. The Plus-Size Tire Calculator finds tire sizes for a new wheel diameter that stay within a few percent of your current diameter.
Width and wheel fit
Every tire size has an approved range of wheel widths, listed by the tire maker. A 245 tire is usually happy on wheels about 8 to 9.5 inches wide. Put it on a wheel that is too narrow and the sidewalls pinch in, which hurts handling and wear. Too wide and the tire is stretched, exposing the rim edge. Wider tires also need more room inside the fender and in front of the suspension, so offset and backspacing matter as much as the tire itself; read Choosing Wheel Offset and Backspacing before you buy.
Checking a replacement size: a short routine
- Write down the original size and the load index from the door placard.
- Calculate original and new overall diameter and note the percent difference.
- Confirm the new load index and speed rating meet or beat the original.
- Check the approved rim width range against your wheel.
- Measure clearance to the fender, spring, strut and inner liner at full lock and full compression.
- Recheck the speedometer against GPS after the change.
Tire pressure and the placard
The pressure printed on the sidewall is the maximum the tire can safely hold, not the pressure to run. Use the figure on the door-jamb placard, set when the tires are cold, and adjust for loads if the manual says to. Changing to a different size often calls for a different pressure, especially a different load index, so check the tire maker's load and inflation table.
Common mistakes
- Choosing by width alone and ignoring diameter.
- Mixing different tire sizes on the same axle or, for all-wheel-drive cars, different diameters front and rear, which can damage the drivetrain.
- Forgetting that the traction and tread-life ratings (UTQG) are the maker's own numbers and are only comparable within one brand.
- Buying old stock without reading the date code.
Frequently asked questions
What does the 100W on a tire mean?
The number is the load index, a code that corresponds to a maximum load per tire, and the letter is the speed rating. A load index of 100 is about 1,764 pounds and W means the tire was tested to 168 mph. Always match or exceed the original ratings.
How much can I change tire diameter before it causes problems?
A change of up to about 3 percent from the original overall diameter is the usual guide. Beyond that the speedometer error, gearing change and clearance problems become more noticeable. All-wheel-drive vehicles are more sensitive to diameter differences between axles.
How old is too old for a tire?
Many manufacturers advise having tires inspected annually after five years and replacing them at about six years, with ten years as the outer limit regardless of how much tread is left. The four-digit date code after DOT gives the week and year it was made.
Tools used in this guide
- Tire Size Calculator
- Plus-Size Tire Calculator
- Speedometer Gear Calculator
- Wheel Offset & Backspacing Calculator
More guides
- Choosing Wheel Offset and Backspacing
- Gear Ratios Explained: Axle, Transmission and Tire Size
- Sizing an Air Compressor for Your Shop
- Wiring a Garage: Subpanels, Circuits and Wire Gauge
- All GarageCalcs guides
Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.