Advanced Wheel & Rim Size Calculator
Compare two complete wheel-and-tire setups — section width, aspect ratio, rim size, offset, backspacing. Reports overall diameter, sidewall height, revs/mile, speedometer error at multiple speeds, track-width change, poke / tuck, ride-height delta, and effective gear-ratio change.
How to Use
- Enter your current (stock) tire + wheel in Setup A — or paste a code like "225/45R17" into the Tire Code field to auto-fill.
- Enter the new setup in Setup B — offset and rim width change the fender clearance and track width.
- All outputs (overall diameter, speedometer error, revs/mile, track change, poke/tuck) calculate live.
- Check the two visualizers — the <em>Side view</em> overlays both tires so diameter differences are immediately visible; the <em>Top view</em> shows wheel position relative to the fender.
| Measurement | Setup A | Setup B | Δ | Δ % |
|---|
| Speedometer reads | Actual speed | Error |
|---|
Formulas
Tire code reference
225 — Section width in mm.
45 — Aspect ratio (sidewall height as % of width).
R — Radial construction (D = diagonal/bias, B = bias-belted).
17 — Rim diameter in inches.
91 — Load index (91 = 615 kg per tire). See table below.
W — Speed rating (W = 270 km/h / 168 mph). See table below.
M+S — Mud & Snow rating (three-peak snowflake = severe snow cert).
Speed rating chart
Load index (selected — see tire for full range 0-150)
| LI | kg | lb | LI | kg | lb |
|---|---|---|---|---|---|
| 75 | 387 | 853 | 95 | 690 | 1521 |
| 80 | 450 | 992 | 100 | 800 | 1764 |
| 85 | 515 | 1135 | 105 | 925 | 2039 |
| 87 | 545 | 1202 | 110 | 1060 | 2337 |
| 89 | 580 | 1279 | 115 | 1215 | 2679 |
| 91 | 615 | 1356 | 120 | 1400 | 3086 |
| 93 | 650 | 1433 | 125 | 1650 | 3638 |
| 94 | 670 | 1477 | 130 | 1900 | 4189 |
Rim-width ↔ tire-width fitment
Every tire has a recommended, minimum, and maximum rim width. Running too narrow pinches the bead and rounds the contact patch; too wide stretches the sidewall and makes the tire profile pointy (sometimes called "stretch").
Rule of thumb: minimum ≈ section × 0.0224, ideal ≈ × 0.0236, max ≈ × 0.0276 (convert mm to inches via /25.4). The calculator above shows if your current rim is in range.
About this calculator
Modelled after the classic wheel-size.com comparison tool but with added outputs for track-width change, poke / tuck (how much the wheel sits outside the fender relative to stock), backspacing auto-conversion, effective gearing impact, and speedometer error at three reference speeds. The side-view visualizer overlays the two tire outlines so you can see at a glance how much ride height changes; the top-view visualizer shows the rim relative to a reference fender line so you can check poke before ordering wheels.
All calculations are client-side — the tool doesn't send your build data anywhere, doesn't store cookies, and works offline once the page is cached. Input values are locale-independent (the Units toggle only changes the display, not the internal math).
Frequently Asked Questions
How much diameter change is safe?
Stay within ±3% overall diameter to avoid significant speedometer / ABS / stability-control error. Beyond 3% you should recalibrate the speedometer (OBD tuner or instrument-cluster service) or adjust final drive. Anti-lock braking uses wheel-speed sensors that reference ABS stored calibrations — large mismatch can trigger traction-control intervention at inappropriate times.
What is offset, really?
Offset (mm) is the distance from the wheel's mounting hub-face to its centerline. <strong>Positive offset</strong> = hub face is OUTBOARD of centerline → wheel tucks into the fender. <strong>Negative offset</strong> = hub face is INBOARD → wheel pokes out. Stock passenger cars usually have +35 to +55 mm. Trucks and off-road rigs go to 0 and beyond negative (−12 to −44) for a stanced/deep-dish look. Drifting/track wheels often run +15 to +25 to get them closer to fender.
Offset vs backspacing?
They describe the same geometry two ways. <strong>Backspacing</strong> = distance from mounting face to the <em>inboard</em> lip of the rim. Formula: backspacing (in) = (rim_width/2) + (offset/25.4). A 9-inch wheel with +20 mm offset has backspacing = 4.5 + 0.787 = 5.29 in. Offset is international / metric; backspacing is the older US/truck convention.
Rim width limits for a given tire width?
Each tire has a recommended rim-width range. Rule of thumb: min rim width = section width × 0.0224, ideal = × 0.0236, max = × 0.0276 (convert mm to in via /25.4). A 245-wide tire fits rims roughly 8.0 - 9.5 inch, ideal 8.5. Running narrower pinches the contact patch; running wider stretches the sidewall and changes handling.
Will bigger wheels hurt acceleration?
Slightly. Overall tire diameter changes final-drive effective ratio proportionally: new_ratio = old_ratio × (new_Ø / old_Ø). Going 3% larger = 3% taller gearing = slower 0-60 but higher top speed. Rotational mass increase also hurts — a 28-lb 20-inch wheel vs a 22-lb 17-inch wheel costs measurable 0-60 time from unsprung rotational inertia.
What does plus-sizing mean?
Going to a larger-diameter rim while keeping the overall tire diameter constant so speedometer and gearing stay correct. Plus-1 = one inch larger rim + shorter sidewall (lower aspect ratio). Example: 225/60R16 → 225/55R17 → 235/45R18. Handling sharpens (shorter sidewall flex) but ride gets harsher (less sidewall air).
Common Use Cases
Plus-size wheels
Upsize rim and compensate with a thinner sidewall to keep overall diameter within 3% of stock — sharper turn-in without speedometer error.
Off-road oversize
Step up from 31" to 33" tires on a 4x4 — this tool shows you the new circumference, speedo error, and gearing penalty.
Stance / fitment
Calculate poke or tuck for a new offset / wider rim before buying. See exactly how far outboard the new wheel sits relative to the fender.
Performance swap
Compare 225/45R17 vs 245/40R18 — track width change, contact-patch increase, rotational mass change.
Speedometer calibration
After a tire size change, use the Speedometer error table to see how much off your indicated speed is at 30/60/90 mph.
Last updated: