What plus sizing means
Plus 1 means a rim 1 inch bigger in diameter, plus 2 means 2 inches bigger, and so on. To keep the speedometer and the gearing close to where they were, the tire's sidewall gets shorter by about as much as the rim grows, so the overall diameter stays nearly the same. Minus sizing goes the other way: a smaller rim with a taller sidewall, often chosen for winter tires.
- Current size: the size on your tire or door placard, like 205/55R16.
- Diameter within: how far the new overall diameter may drift. A common rule of thumb is about 3%.
- Width change: keeps the list to sizes close to your current width. Wider tires need a wider rim.
The formula
sidewall (in) = width (mm) × aspect ÷ 100 ÷ 25.4
diameter (in) = rim + 2 × sidewall
difference % = (new diameter ÷ current diameter − 1) × 100
real speed = speedometer reading × new diameter ÷ current diameterWorked example
A 205/55R16 is 16 + 2 × 4.44 = 24.88 in tall. On a 17 in rim, 205/50R17 is 25.07 in (+0.78%), 225/45R17 is 24.97 in (+0.38%) and 215/45R17 is 24.62 in (−1.04%). On an 18 in rim, 215/40R18 is 24.77 in (−0.43%) and 225/40R18 is 25.09 in (+0.84%).
With 225/40R18, the speedometer shows 60 mph when you are doing 60.5 mph. The sidewall shrinks from 4.44 in to 3.54 in. For a minus 1 winter set, 205/60R15 is 24.69 in (−0.78%), and 195/65R15 is 24.98 in (+0.41%).
What to check before buying
- Load capacity: a lower profile tire of the same width often carries less. Compare the load index with the one for your placard size on the load index chart.
- Rim width: every tire has a range of rim widths its maker approves. A much wider tire may need a wider rim than you have now.
- Clearance: a wider tire moves closer to the suspension and the fender.
- Availability: the list is computed, so some sizes are rare or not made at all.
To put your current size next to one candidate, use the tire size comparison.
Questions
What is plus 1 tire sizing?
A rim 1 inch bigger with a shorter sidewall, so the overall diameter stays close. From 205/55R16 (24.88 in), 205/50R17 is 25.07 in, 0.78% taller.
What size is plus 2 for 205/55R16?
On an 18 in rim, 215/40R18 is 24.77 in (−0.43%) and 225/40R18 is 25.09 in (+0.84%). Both stay well within 3% of the original 24.88 in.
Does plus sizing change my speedometer?
Only by the diameter difference. With 225/40R18 instead of 205/55R16, the dial shows 60 mph at a real 60.5 mph.
What is minus sizing for winter tires?
A rim 1 or 2 inches smaller with a taller sidewall. For 205/55R16, 205/60R15 is 24.69 in (−0.78%) and 185/75R14 is 24.93 in (+0.19%).