Free tool
Going to bigger wheels? Check the size change first
Plus sizing means a larger wheel with a lower-profile tire, keeping the overall diameter close to the original. Enter your current size and the new one to compare overall diameter, sidewall height and revolutions per mile, and see what your speedometer would read at 30, 45, 60 and 70 mph. The common 3% figure is a guideline, not a fitment approval.
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Plus-size & speedometer
Going bigger on the wheels? See how the overall size changes and what your speedometer will read.
Metric (225/45R17), light truck (LT265/70R17 121/118S) and flotation (31x10.50R15) sizes all work. Start from the size on your door placard.
The new size is 0.1% taller overall: 26.3 in → 26.3 in (668 → 669 mm). Within the common 3% guideline. At an indicated 60 mph you'd actually be going about 60.0 mph. A taller tire makes the speedometer read low.
+0.1% overall diameter
The new size is 0.1% taller overall: 26.3 in → 26.3 in (668 → 669 mm).
Within the common 3% guideline
The overall diameter changes by 3% or less, the range fitment guides commonly work to.
| Current | New | Change | |
|---|---|---|---|
| Overall diameter | 26.3 in | 26.3 in | +0.1% |
| Sidewall height | 118 mm (4.7 in) | 106 mm (4.2 in) | −12 mm |
| Revs per mileunloaded | 767 | 766 | −0.1% |
| Section width | 215 mm | 235 mm | +20 mm |
| Wheel diameter | 17 in | 18 in | +1 in |
Nominal figures worked out from the sizes. Real dimensions vary a little by tire model and wheel width, and a loaded tire turns slightly more times per mile.
Dashed outline: current size. Solid: new size.
| Speedometer says | You're actually going |
|---|---|
| 30 mph | 30.0 mph |
| 45 mph | 45.0 mph |
| 60 mph | 60.0 mph |
| 70 mph | 70.0 mph |
At an indicated 60 mph you'd actually be going about 60.0 mph. A taller tire makes the speedometer read low.
Load index
Add the load index to both sizes, such as 225/40R18 92W, to compare it. The new tire's load index should match or exceed your vehicle maker's.
- The new size mounts on an 18-inch wheel instead of a 17-inch one, so it needs different wheels.
3% is a guideline, not a fitment approval. Load index, clearance and wheel width all matter. Have fitment confirmed before you order.
Sources: Michelin, Changing Tire Sizes (opens in a new tab); Discount Tire, Speedometer Accuracy (opens in a new tab); Michelin, Load Rating & Speed Rating (opens in a new tab)
Text version of this demo
Compares a current and a new tire size by overall diameter, sidewall height and revolutions per mile, and shows what the speedometer reads at 30, 45, 60 and 70 mph. The common 3% diameter figure is a guideline, not a fitment approval, so have fitment confirmed before you order.
Step by step
How to use it
- Enter the size on your door placard as the current size. The tires on the car now may not match it.
- Enter the size you're considering. Metric, LT and flotation sizes all work, or tap an example.
- Check the overall diameter change, the drawing and the speedometer table.
- Add the load index to both sizes, such as 225/40R18 92W, to compare it too.
- Have fitment confirmed before you order. Clearance, wheel width and load index all matter.
Questions
The things people ask next
What is plus sizing?
Fitting a larger-diameter wheel with a lower-profile tire, such as going from 205/55R16 to 225/40R18, so the overall diameter stays close to the original. One inch more on the wheel is plus one; two inches is plus two.
How much can the overall diameter change?
Fitment guides commonly work to 3% or less. That's a guideline, not an approval: load index, clearance to the body and suspension, and wheel width all matter, so have fitment confirmed before you order.
Why does my speedometer read differently with bigger tires?
The speedometer counts how often the wheels turn. A taller tire covers more ground each turn, so the car is going faster than the speedometer says, and a shorter tire does the opposite. The odometer is off by the same amount.
Do I need new wheels?
If the wheel diameter in the size changes, such as R17 to R18, yes, and the tool says so. A much wider tire can also need a wider wheel, so check the rim widths the tire maker lists for that size.