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TireMath HQ

Wheel upgrades

Plus Sizing Calculator

Plus sizing is the trade of sidewall for wheel: a bigger rim, a shorter tire, and the same overall diameter so nothing the car measures has to change. Enter what you run now and this lists the sizes that hold your rolling diameter, closest match first, with the sidewall you give up at each step.

The size the wheel is running now.

Diameter tolerance
Common sizes

Sizes that hold 205/55R16's 24.88″ diameter within ±3%

Plus size candidates for 205/55R16, closest diameter first
StepSizeDiameterDifferenceSidewall
Plus One225/45R1724.97″+0.38%101.3 mmCompare
Plus One245/40R1724.72″−0.65%98.0 mm
Plus One205/50R1725.07″+0.78%102.5 mmCompare
Plus One215/45R1724.62″−1.04%96.8 mm
Plus One235/45R1725.33″+1.80%105.8 mmCompare
Plus One235/40R1724.40″−1.91%94.0 mm
Plus One215/50R1725.46″+2.36%107.5 mmCompare
Plus One205/45R1724.26″−2.47%92.3 mm
Plus Two215/40R1824.77″−0.43%86.0 mm
Plus Two245/35R1824.75″−0.51%85.8 mm
Plus Two225/40R1825.09″+0.84%90.0 mm
Plus Two205/45R1825.26″+1.55%92.3 mm
Plus Two235/35R1824.48″−1.61%82.3 mm
Plus Two205/40R1824.46″−1.69%82.0 mm
Plus Two235/40R1825.40″+2.10%94.0 mm
Plus Two225/35R1824.20″−2.72%78.8 mm
Plus Two215/45R1825.62″+2.98%96.8 mm
Plus Three215/35R1924.93″+0.19%75.2 mm

Closest match

Within 3%

225/45R17 measures 24.97″ against the original 24.88″ — a difference of +0.38%. The sidewall drops from 112.8 mm to 101.3 mm, which is the trade: sharper turn-in, less protection from potholes.

Plus sizing holds the rolling diameter and therefore the speedometer. It does not tell you whether a wider tire clears the strut or the fender liner — measure, or ask a fitter.

How does plus sizing work?

An inch of extra wheel diameter adds 25.4 mm to the assembly, half of it above the hub and half below. To cancel it, the tire has to lose 12.7 mm of sidewall — and since sidewall is a percentage of section width, that usually means both a wider tire and a lower aspect ratio at the same time.

That is where the plus-one shorthand comes from: +1 inch of wheel, +10 mm of width, −10 points of aspect ratio. It is a rule that happens to land close, not a derivation. On a 205/55R16 it lands within 0.4%; on other sizes it can miss by more than the 3% rule allows, which is why the calculator above searches the actual manufactured size ladder instead of applying the shorthand blindly.

The plus-size ladder, worked through

Starting from 205/55R16 at 24.88″, here is what each step actually costs and preserves.

Plus size ladder starting from 205/55R16
StepSizeDiametervs baseSidewallSidewall lost
Base205/55R1624.88″—112.8 mm—
Plus 1225/45R1724.97″+0.38%101.3 mm−11.5 mm
Plus 2225/40R1825.09″+0.84%90.0 mm−22.7 mm
Plus 3245/30R1924.79″−0.36%73.5 mm−39.3 mm

By the top of the ladder — 3 steps up, from 205/55R16 to 245/30R19 — the rolling diameter has barely moved, but the sidewall has lost 39.3 mm, over a third of it. That is the entire trade, in one number.

What plus sizing does not fix

It does not change ground clearance, gearing or speedometer calibration, which is the point. It also does not tell you whether the wider tire clears the strut at full lock, because that depends on the wheel's offset and the suspension geometry — neither of which is encoded anywhere in a tire size.

If you want a taller tire rather than a bigger wheel, that is a different calculation with different consequences: the speedometer error calculator covers it.

Common questions about plus sizing

What is plus sizing?
Fitting a larger-diameter wheel while reducing the tire's aspect ratio so the overall rolling diameter stays the same. Plus one means one inch more wheel, plus two means two. The speedometer, gearing and ABS calibration are all preserved because the rolling circumference does not change.
What is the rule for plus one sizing?
The classic shorthand is: add one inch of wheel, add 10 mm of section width, and drop the aspect ratio by 10 points. 205/55R16 becomes 225/45R17, which is 24.97 inches against 24.88 — a difference of 0.38%.
Does plus sizing hurt ride quality?
Yes, and the amount is measurable. Going 205/55R16 to 225/45R17 cuts sidewall height from 112.8 mm to 101.3 mm — an 11.5 mm reduction in the only part of the wheel assembly with any compliance in it. Steering response improves for exactly the same reason.
How far can you plus size?
Diameter is the easy constraint: stay within 3% and the instruments stay honest. The real limits are that low-profile tires below about 35 series get scarce and expensive, and that each step adds unsprung weight, which works against both ride and acceleration.

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