Ford Tire Size Calculator: Compare Sizes and Check Speedometer Error

Fitting a different tire size changes the rolling diameter, and that changes what your speedometer reads, how the gearing feels and whether the wheel still clears the arch. This calculator compares the size your Ford left the factory with against the size you are considering, and shows the difference in plain numbers.

The original size is printed on the label in the driver’s door opening. If you are not sure where to look, see the Ford data plate decoder.

What the numbers mean

The figure that matters most is the change in overall diameter. Everything else follows from it. A larger diameter makes the wheel turn more slowly for the same road speed, so the speedometer under reads and you are actually going faster than the dial says. A smaller diameter does the opposite.

Ground clearance changes by half the diameter difference, not the whole of it, because only the bottom half sits between the axle and the road.

The three percent rule

The usual guidance is to stay within about three percent of the original diameter. That is not a legal limit, it is a practical one. Inside that band the speedometer error stays small, the anti lock brake and stability systems keep working with the wheel speed values they expect, and the tire normally still clears the suspension and bodywork at full lock.

Beyond that band the systems that read wheel speed start working from a wrong assumption, and on some vehicles that shows up as false traction control activity rather than an obvious fault.

Methodology

Every figure here comes from the nominal geometry encoded in the size itself. Nothing is looked up and nothing is estimated.

For a size written as 235/70R16 the sidewall height is the section width multiplied by the aspect ratio, so 235 by 0.70, giving 164.5 mm. The overall diameter is two sidewalls plus the wheel, so 164.5 doubled plus 16 inches converted at 25.4 mm to the inch, giving 735.4 mm. Circumference is that diameter multiplied by pi. Revolutions per mile is 1 609 344 mm divided by the circumference.

The speedometer figure is the ratio of the two diameters. If the new tire is one percent larger, the true speed is one percent higher than the dial shows, because the instrument counts wheel rotations and assumes the factory rolling distance for each one.

Where these numbers stop being exact

This is nominal geometry, and real tires deviate from it. Two tires marked 235/70R16 from different manufacturers can differ by several millimetres, because the marked size is a designation rather than a measurement.

The calculation also uses the free diameter of an unloaded tire. Under the weight of the car the contact patch flattens and the effective rolling radius is smaller, and it changes with inflation pressure and load. That is why a speedometer check against a satellite reading will not match the calculated figure exactly.

Diameter is also only one of the things that has to match. A replacement has to carry at least the original load index and speed rating, has to suit the rim width, and has to physically clear the suspension and bodywork through the full range of steering and travel. None of that is visible in the size alone, and none of it is checked here.

The inflation pressure on your door label was specified for the original size. If you change size, that figure no longer transfers directly and the correct pressure should come from the tire manufacturer’s load tables.