- Calculator
- Tire Code
- Size Charts
- Offset Guide
- FAQ
The Wheel & Tire Calculator is a free online tire size calculator that lets you compare two different wheel and tire combinations side by side. Enter your current setup as Wheel 1 (base) and your proposed new size as Wheel 2, and instantly see the full fitment data — no button click needed.
It calculates diameter, circumference, poke, inset, speedometer error, ride height change, arch gap change, and ideal rim range — everything you need to check before ordering new wheels or tires.
Checks if wheels fit inside your arches
Shows speedometer impact of size change
Side view & front view update live
Calculate suspension & arch gap change
Three critical fitment measurements you need to understand before changing wheel size or offset. Our wheel offset calculator shows all three instantly.
Poke (outer clearance) is how far your tire sticks out beyond the fender/arch lip. A small positive poke (flush or slightly proud) is the target look for most builds. Too much poke risks rubbing the arch liner. Our tire fitment calculator shows poke in mm for each wheel setup.
Inset (inner clearance) is the gap between the inner edge of your tire and the nearest suspension or brake component. Too little inset and your tires will foul the strut, caliper, or inner arch. Insufficient inset is a safety concern and cannot be fixed with spacers alone.
Rubbing happens when poke is too high (outer rub at arch) or inset is too low (inner rub at strut). Always check both values before buying new wheels. Low-profile tires rub less on bumps; wider tires are more susceptible. Our wheel and tire calculator flags these issues automatically.
P215/ 65R15 95H
P
P = Passenger car. LT = Light Truck. ST = Special Trailer. T = Temporary (spare).
215
The tire's width from sidewall to sidewall in millimeters. This is the first number you enter in the tire size calculator.
65
The sidewall height as a percentage of the section width. 65 means the sidewall is 65% of 215mm = 139.75mm tall.
R15
R = Radial construction. 15 = the wheel/rim diameter in inches this tire fits.
95H
95 = load index (690 kg max per tire). H = speed rating (up to 130 mph / 210 km/h).
Changing your tire size directly affects your speedometer accuracy. Here’s how to calculate speedo error and what it means for your vehicle.
Your speedometer works by counting the number of wheel revolutions per second and multiplying by the tire’s circumference to calculate speed. When you fit a larger-diameter tire, each revolution covers more distance — your actual speed is higher than displayed. With a smaller tire, the opposite occurs.
A tire size difference of just 3% from OEM spec can cause your speedo to read 2–3 mph off at 60 mph. This also affects your odometer, trip computer, and — critically — ABS and traction control calibration.
General rule: Taller tire = speedo reads LOW (you’re going faster than shown). Shorter tire = speedo reads HIGH (you’re going slower than shown).
Speedo Error % = ((New Circumference − Original Circumference) ÷ Original Circumference) × 100 Actual Speed = Displayed Speed × (New Circ ÷ Original Circ)
| Size Change | Error % | At 60mph actual | Speedo reads |
|---|---|---|---|
|
205/55R16 → 225/45R17 |
+0.5% |
60 mph |
59.7 mph |
|
205/55R16 → 215/55R16 |
+1.2% |
60 mph |
59.3 mph |
|
205/55R16 → 235/40R18 |
-1.8% |
60 mph |
61.1 mph |
|
225/45R17 → 245/35R19 |
-2.3% |
60 mph |
61.4 mph |
|
265/70R17 → 285/75R17 |
+3.9% |
60 mph |
57.8 mph |