Tire Size Comparison & Speedometer Error Speed Offset Calculator
Swapping tire sizes changes the rolling diameter, which throws off both your speedometer and odometer since they're calibrated to the stock tire's revolutions per mile. This calculator converts your stock and new tire specs into exact diameters and shows the true speed behind every indicated reading.
Your new tires are 7.0% taller, so your speedometer under-reports — true speed is 74.9 mph when it shows 70 mph.
| Speedometer shows | Actual true speed |
|---|---|
| 60 mph | 64.2 mph |
| 70 mph | 74.9 mph |
| 80 mph | 85.6 mph |
- 1.
Diameter(in) = (Width × (Aspect/100) × 2 / 25.4) + RimDiameter - 2.
RevsPerMile = 20168 / Diameter - 3.
TrueSpeed = IndicatedSpeed × (NewDiameter / StockDiameter)
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- Width
- Tire section width in mm
- Aspect
- Sidewall height as % of width
- RimDiameter
- Wheel diameter in inches
Why tire diameter drives speedometer accuracy
Your speedometer doesn't measure speed directly — it counts wheel/driveshaft revolutions and converts them using the factory tire diameter. Fit a taller tire and each revolution covers more ground than the computer assumes, so the gauge under-reports your real speed; a shorter tire does the opposite.
Reading a tire size code
In 265/70R17, 265 is section width in millimeters, 70 is the aspect ratio (sidewall height as a percent of width), and 17 is rim diameter in inches. Overall diameter combines sidewall height (doubled, for top and bottom) with rim diameter, converted to inches.
Real-world consequences beyond the speedometer
Odometer mileage, cruise control accuracy, fuel economy readouts, and even ABS/traction control calibration all rely on the same revolution count, so a meaningfully different tire diameter can cause inaccurate trip data and, in some vehicles, drivetrain fault codes.
How much diameter change is acceptable
Most enthusiasts consider up to about 3% diameter change negligible for daily driving, while anything beyond 3-5% is worth correcting with a speedometer recalibrator, a gear ratio change, or an ECU tune, especially on vehicles with electronically controlled transmissions.
Common tire diameter reference
| Tire size | Diameter (approx.) |
|---|---|
| 225/65R17 | 28.7 in |
| 245/70R17 | 30.6 in |
| 265/70R17 | 31.6 in |
| 285/75R17 | 33.6 in |
| 315/75R16 | 34.6 in |
| 35x12.5R17 | 35.0 in |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: stock tire width 206.7 mm
On the lower / more conservative end. Your new tires are 19.2% taller, so your speedometer under-reports — true speed is 83.4 mph when it shows 70 mph.
Example 2: stock tire width 265 mm
A typical middle-of-the-road setup. Your new tires are 7.0% taller, so your speedometer under-reports — true speed is 74.9 mph when it shows 70 mph.
Example 3: stock tire width 344.5 mm
On the higher / more demanding end. Your new tires are 6.0% smaller, so your speedometer over-reports — true speed is only 65.8 mph when it shows 70 mph.
Quick answers about the Tire Size & Speedometer Error
What exactly does the Tire Size & Speedometer Error work out?
Swapping tire sizes changes the rolling diameter, which throws off both your speedometer and odometer since they're calibrated to the stock tire's revolutions per mile. You enter stock tire width, stock aspect ratio, stock rim diameter and new tire width (plus 3 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 7 fields: stock tire width, stock aspect ratio, stock rim diameter, new tire width, new aspect ratio, new rim diameter and cruising speed (indicated). Nothing else is needed and nothing is stored.
How is it calculated — why tire diameter drives speedometer accuracy?
Your speedometer doesn't measure speed directly — it counts wheel/driveshaft revolutions and converts them using the factory tire diameter. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for tire Size & Speedometer Error?
In 265/70R17, 265 is section width in millimeters, 70 is the aspect ratio (sidewall height as a percent of width), and 17 is rim diameter in inches. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Common tire diameter reference" table on this page tell me?
It is the reference range this tool works against — 6 rows from "225/65R17" (28.7 in) up to "35x12.5R17" (35.0 in). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Do I have to press a button or reload the page to see the result?
No. Tire Size & Speedometer Error runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Tire Size & Speedometer Error leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
Yes. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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