Results
Original tire diameter
25.72 in
New tire diameter
26.58 in
Speedometer error
3.35 %
Actual speed at indicated 60
62.0 mph
Formula / model
Sidewall height = width x aspect ratio / 100 / 25.4, overall diameter = wheel diameter + 2 x sidewall height, speed error % = (new diameter - original diameter) / original diameter x 100
Use the tire size and speedometer error calculator to compare old and new tire specs before a wheel change creates gearing or accuracy surprises.
Enter your current numbers or target values below, then use the live results to review original tire diameter, new tire diameter, speedometer error, and actual speed at indicated 60 before you commit to the next parts or setup change.
The speedometer reads vehicle speed based on the number of driveshaft or wheel-speed sensor rotations per unit of time. When tire diameter changes, the distance covered per rotation changes — but the sensor still counts the same rotations. A larger tire covers more ground per revolution, causing the speedometer to read lower than actual speed.
A tire's overall diameter is calculated from 3 specifications printed on the sidewall: section width (mm), aspect ratio (%), and wheel diameter (inches). The sidewall height equals width × aspect ratio ÷ 100. The overall diameter equals wheel diameter + 2 × sidewall height. Changing any of these 3 values alters the effective gearing and speedometer calibration.
The formula decodes the metric tire size notation into an overall diameter in inches:
A 245/40R18 tire has a sidewall height of 245 × 0.40 ÷ 25.4 = 3.86 inches. The overall diameter is 18 + 2 × 3.86 = 25.72 inches. Switching to a 275/35R19 gives a 25.58" diameter — 0.14" smaller despite the larger wheel and wider tread.
Speedometer error is proportional to the diameter change: Error% = (New Dia − Old Dia) / Old Dia × 100. A 3% increase in diameter means the speedometer reads 58.2 mph when actual speed is 60.0 mph. Federal guidelines allow ±5 mph at 55 mph. A 1-inch diameter change on a 26" tire produces roughly 3.8% error — enough to affect GPS-verified speed, odometer accuracy, and cruise RPM calculations.
Interactive — linked to form inputs above
The table below compares popular tire sizes, their overall diameters, and the speedometer error relative to a baseline 245/40R18 (25.72" diameter).
| Tire Size | Diameter (in) | vs. 245/40R18 | Actual at 60 indicated |
|---|---|---|---|
| 225/45R17 | 24.97 | −2.9% | 58.3 mph |
| 245/40R18 (baseline) | 25.72 | 0.0% | 60.0 mph |
| 275/35R19 | 25.58 | −0.5% | 59.7 mph |
| 275/40R20 | 28.65 | +11.4% | 66.8 mph |
| 315/35R17 | 25.68 | −0.2% | 59.9 mph |
| 295/45R18 | 28.44 | +10.6% | 66.4 mph |
A taller tire acts as a numerically lower rear gear. Going from a 26" to a 28" tire with 3.73 gears produces the same cruise RPM as running 3.46 gears on the original tire. This reduces launch performance but improves highway fuel economy and lowers cruising noise.
A taller tire under-reports mileage — the odometer records fewer miles than actually driven. A 4% diameter increase means the odometer reads 9,600 miles when the vehicle has actually traveled 10,000 miles. This affects maintenance schedules, resale value documentation, and warranty mileage verification.
Modern vehicles compare wheel speed sensor readings across all 4 corners. Mismatched tire diameters between axles can trigger ABS, stability control, or AWD system faults. Most systems tolerate up to 3% diameter difference before intervention logic is affected.
These are the next calculator pages most likely to be useful once you have this result in hand.
Drivetrain
01Speed and rpm relationships through tire, gear, and transmission ratio.
Drivetrain
02Converter slip from mph, trap rpm, tire diameter, and final drive ratios.
Chassis Dynamics
03Static axle loads and acceleration weight transfer.
How tire size and weight interact with displacement to determine acceleration feel.
Unit conversions that matter when comparing engine specs across different measurement systems.
Motorcycle tire sizing context for cc-class displacement comparisons.
It estimates original tire diameter, new tire diameter, speedometer error, and actual speed at indicated 60 from values such as original width (mm), original aspect ratio, and original wheel diameter (in).
Start with original width (mm), original aspect ratio, and original wheel diameter (in) because those are the core values that move original tire diameter the most. Then refine the secondary inputs to match the exact combination.
It is a solid planning tool built around the stated formula and assumptions, but final results still depend on real measurements, hardware tolerances, tuning, and operating conditions.
Yes. Change the inputs to reflect your exact parts, operating target, or comparison scenario, then review how the outputs respond before you make the next decision.
A useful next step is to compare the result with Gear Ratio & RPM Calculator, Torque Converter Slip Percentage Calculator, and Vehicle Mass Center & Weight Transfer Calculator so the rest of the combination stays aligned.
