How to Choose a Plus Size: A Chart for Matching the Outer Diameter

Plus sizing means keeping the outer diameter the same

Plus sizing means using a larger wheel rim diameter and making the tire's sidewall thinner (a lower aspect ratio) to compensate. If the outer diameter stays close to the original, you can change the look and handling with little effect on the speedometer, ride height or fender clearance.

Minus sizing is the reverse: a smaller rim diameter with a thicker sidewall. It's common for winter tires, since tires and wheels cost less and the ride is softer. The idea is the same as plus sizing, matching the original outer diameter.

Chart of candidates from common original sizes

For common original sizes, these are combinations with a small diameter difference that are often on sale. The differences are calculated values.

Enter your original size in the tool and it lists sizes within ±3% by rim diameter, closest diameter first. Tap one to put it in the "new" slot and you can immediately check the speedometer error and ride height change. The candidates are calculated, though, so some may not actually be on sale. When a size looks promising, check the tire maker's size list to see whether it's sold.

Four things to check besides the outer diameter

A matching outer diameter doesn't guarantee a fit. These are the four points where plus sizing most often goes wrong.

1. 1

   Is the load index (LI) at least the original?

   A lower aspect ratio means less air in the tire, so it supports less weight at the same pressure. Going from 205/55R16 91V to 225/45R17, for example, gives an LI of around 91 in the standard version and around 94 in XL. Choose one that doesn't fall below the original LI.

2. 2

   If it's XL, reset the air pressure

   To get the same load capacity as the original from an XL tire, you may need a higher pressure than the car's specified value. Set it from the tire maker's pressure table (which maps load index to pressure).

3. 3

   Does the rim width suit the tire?

   Each tire has a set range of rim widths it can be mounted on. For 225/45R17, around 7–8J is typical. With the wrong width, the tire gets stretched or bulges out.

4. 4

   Do the inset, PCD and hub bore suit the car?

   A different rim width also shifts the wheel's outer and inner edges. You can check whether it pokes past the fender or hits the suspension in How to Calculate Flush Fitment. Also choose a bolt spacing (PCD), bolt count and hub bore that suit the car.

Going down a size on winter tires

Many people choose winter tires one inch below the original size. Tires and wheels cost less, and the thicker sidewall copes better with rough snowy roads and curbs. On cars with large brakes, however, a smaller wheel may hit the brake caliper and not fit.

• First check the smallest rim diameter that fits your car in the owner's manual or a car parts store's fitment table
• On the same model, a size fitted to a lower trim can often be used as it is
• Going 10 mm narrower raises ground pressure on snow and lowers the price, but check the LI doesn't fall below the original
• If you're buying new wheels, choose a PCD, bolt count, hub bore and inset that suit the car

As long as the outer diameter matches the original, a smaller rim has almost no effect on speedometer error or vehicle inspection. You can see how a diameter difference shows up on the speedometer in Speedometer Error and Inspection Standards.

Steps to settle on a size

1. 1

   Check the original size and LI

   Note the original size, LI and speed rating from the driver's door label or your current tire's marking (How to Read Tire Sizes).

2. 2

   Narrow the candidates by outer diameter

   From the tool's plus-size table, pick sizes with a diameter difference within ±1–3%.

3. 3

   Check availability and LI

   In the tire maker's size list, check whether the size is sold, whether its LI is at least the original, and whether it's XL.

4. 4

   Decide the wheel width and inset

   Choose a J width within the tire's range, and calculate an inset that doesn't poke out or interfere on the inside.

5. 5

   Do a final check at a shop

   If the car is lowered, or you're going much wider than original, have a shop test-fit it on the car.

FAQ

What size should I use to take 205/55R16 to 17 inches?

225/45R17 is the standard choice, with an outer diameter of 634.3 mm, only about 0.38% larger than 205/55R16 (631.9 mm). To keep the width, there's 205/50R17 (+0.78%), and for something slightly smaller, 215/45R17 (−1.04%).

How much should the aspect ratio drop when plus sizing?

The rule of thumb is 5–10 lower for each inch you go up. Together with a width 10–20 mm greater, choose a combination with a diameter difference within ±3% (ideally ±1%).

Is it OK if the load index drops when plus sizing?

Avoid tires below the original LI. They may not support the car's weight properly and it may fail vehicle inspection. Choose an XL tire with a higher LI at the same size, and reset the pressure from the tire maker's table.

How many inches can winter tires go down?

Most cars can go down one inch, but the smallest rim diameter that clears the brake caliper varies by car. Check the original size on lower trims or a car parts store's fitment table.

Does plus sizing make fuel economy worse?

With a matched outer diameter, engine revs hardly change, but wider tires and heavier wheels tend to make fuel economy slightly worse. Lightweight wheels reduce the effect.