Free wheel and tyre tools
Tyre plus-size calculator
Find nominal tyre sizes within 2% or 3% of your current diameter. Compare speedometer changes and estimated rim widths for your target wheel size.
How it works
We check widths from 155 to 355 mm in 10 mm steps and profiles from 25 to 80 in steps of 5 on your chosen rim. Diameter is rim inches × 25.4 + 2 × width × profile ÷ 100. Results are ranked by absolute diameter difference, then by width difference. Every matching combination is shown, including sizes that may not be manufactured. Rim-width bands use our existing measuring-rim estimate with a half-inch allowance each side, not an approved fitment table.
References
A worked example
A 225/45 R17 has a nominal diameter of 634.3 mm. On an 18 inch rim, 225/40 R18 measures 637.2 mm: 2.9 mm larger, or 0.46%. With an originally accurate speedometer, an indicated 60 mph becomes about 60.27 mph. The estimated rim-width starting band is 7 to 8 inches; check the exact tyre model's approval.
Quick reference
Plus-one and plus-two sizes for common tyres
| Current | Plus one | Plus two |
|---|---|---|
| 195/65 R15 | 195/60 R16 (+0.93%) | 195/50 R17 (-1.21%) |
| 205/55 R16 | 205/50 R17 (+0.78%) | 205/45 R18 (+1.55%) |
| 225/45 R17 | 225/40 R18 (+0.46%) | 225/35 R19 (+0.91%) |
| 225/40 R18 | 225/35 R19 (+0.46%) | 225/30 R20 (+0.91%) |
| 245/40 R18 | 245/35 R19 (+0.14%) | 245/30 R20 (+0.28%) |
The narrowest size, up to 20 mm wider, that keeps the diameter within 2%. A size can work on paper and still not be made, so check availability.
Explained in our guides
- Plus-sizing wheels: tyre diameter and speedometer changesWork through a 17 to 18 inch wheel change, compare tyre diameters and speedometer effects, and understand why a 3% rule cannot approve fitment.
- Brake caliper clearance: checking wheels with a templateCheck spoke and barrel clearance for standard or upgraded brakes, print a brake template at the correct scale, and understand what offset cannot prove.
- How to choose wheel width for a tyre: use the approved rangeChoose a rim width using the exact tyre manufacturer's data, distinguish measuring rims from approved ranges, and check the effect on inner and outer clearance.
Questions
Should I choose the closest diameter?
Diameter is a useful first filter, but the closest result might be too wide, unavailable or unsuitable for your car. Check approved sizes, load and speed ratings, rim width, brakes and body clearance before choosing.
Does a 2% or 3% result mean it is safe?
No. These are search limits, not approval thresholds. Your car maker may require a much closer match, especially between axles on an all-wheel-drive car. Keep to the vehicle and tyre manufacturers' requirements.
Can I use this to go down a wheel size?
Yes. Enter a smaller target rim. The same geometry finds taller-profile tyres with a similar diameter, but a smaller wheel may not clear your brakes.
Keep checking
Use these results to plan a setup. Confirm fitment, tyre approval and clearances with a qualified professional before buying or fitting parts.