FORScan Tire Size Calculator

1. Enter the new tire size in three parts: width, aspect ratio and wheel diameter. For 285/70R17, that is 285, 70 and 17.
2. Enter the stock size from the tire placard on the door jamb. Add the published revs per mile from the tire maker spec sheet if you have it.
3. Copy the revolutions per mile or the circumference in the unit your configuration software asks for. Check the speedometer error to see what the change will correct.

Here is what each result means:

The tire size value is a number stored in a vehicle module that tells it how far the vehicle travels per wheel turn. The speedometer, odometer and often the transmission use it to turn wheel speed into road speed.

FORScan is a third-party software scanner for Ford, Mazda, Lincoln and Mercury vehicles, according to its developer. Owners use tools like it to change this setting after fitting a different tire size. Depending on the vehicle and module, the setting may ask for revolutions per mile or for a tire circumference, so this page gives both.

This calculator does not list module names, addresses or stored codes. Those differ by vehicle, model year and software version. Check the documentation for your own software and vehicle before you change anything, and save a backup of the original settings.

MultiCalculators is not affiliated with or endorsed by FORScan, Ford Motor Company or any vehicle maker. Product names are used only to describe what the calculator helps with.

It builds the tire diameter from the size code, turns it into a circumference, and counts how many circumferences fit in a mile.

1. Multiply width by the aspect ratio for the sidewall height, then add two sidewalls to the wheel diameter in mm.
2. Multiply by pi for the circumference in mm, then divide by 10 for cm or by 25.4 for inches.
3. Divide one mile, which NIST gives as exactly 1,609.344 m, by the circumference.
4. With a published figure entered, use it as revs per mile and work the circumference back from it.

For metric units, the km to miles converter and the mm to inches converter show each step on its own.

A pickup came with 265/70R17 tires and now runs 285/70R17.

New tire: diameter = 17 x 25.4 + 2 x 285 x 0.70 = 431.8 + 399 = 830.8 mm (32.71 in). Circumference = pi x 830.8 = 2,610 mm, or 261.0 cm, or 102.76 in. Revs per mile = 1,609,344 / 2,610 = 617.

Stock tire: diameter 802.8 mm, circumference 2,522 mm, 638 revs per mile.

Speedometer before recalibrating: the new tire is 3.49% taller, so the speedometer reads 3.37% low. At 60 mph shown, the truck is really doing about 62.1 mph.

What it means: enter 617 revs per mile, or 2,610 mm or 261 cm of circumference if your setting asks for that, then confirm with a GPS speed check.

The best value is the one that matches how far the tire really rolls. These factors move it away from the code.

Loaded vs Unloaded Size

A tire squashes under the weight of the vehicle, so it rolls a little shorter than its unloaded size. Tire makers often publish a loaded revs per mile figure, which is usually a little higher than the size math.

Tread Depth

Deep off-road tread adds height when new and wears down over time. The best setting for a new tire drifts slightly as it wears.

Brand Differences

Two tires with the same code can differ in real height. The spec sheet for the exact model beats the code.

Setting Units and Steps

Some settings accept revs per mile, others a circumference, and some only fixed steps. Pick the nearest value your software allows.

Both describe the same tire; your software decides which one it wants.

Calculated vs Published Figure

After a Lift or Larger Tires

Taller tires make the speedometer read low and the odometer under-count. Work out the new value before you connect to the vehicle.

Switching Wheel Sizes

Moving from 18 inch to 17 inch wheels with a taller sidewall can keep the diameter close to stock. Compare both sizes here to see whether a setting change is needed at all.

Fine-Tuning After a GPS Check

A small leftover error is normal. The percent error calculator turns a GPS and speedometer pair into a percentage you can apply to the setting. Convert km/h readings with the km/h to mph converter.

1. Entering Diameter When the Setting Wants Circumference

A 32.71 in diameter is a 102.76 in circumference. Entering the wrong one makes the speedometer wildly wrong.

2. Moving the Number the Wrong Way

Taller tires need a lower revs per mile but a higher circumference. Check the direction column above before you save.

3. Mixing Cm and Mm

2,610 mm is 261.0 cm. A setting in cm given a mm value is off by a factor of ten.

4. Skipping a Backup

Save the original module settings before any change, so you can restore them.

5. Trusting Unsourced Codes

Hex or byte values copied from forums differ by vehicle and year. Work from the tire math and your own software documentation.

The math is exact for the size you enter; a published loaded figure or a GPS check gets closer to the real rolling distance.

About 576 for a tire that is truly 35.0 inches tall: 63,360 / (pi x 35) = 576. Real 35s are often a little shorter, so a published loaded figure is better when you have it.

Enter whichever your setting asks for. This calculator gives revs per mile, revs per km and circumference in mm, cm and inches for the same tire.

The vehicle still counts turns of the old, smaller tire. Each turn of the new tire covers more road, so the true speed is higher than the reading.

Our rough guide is within about 1% of a GPS reading. A leftover error that size can come from load, pressure and tread wear, so small further tweaks are normal.

Often, yes, because the odometer also counts wheel turns. Confirm it on your vehicle by driving a known distance, such as a measured mile.

On the tire maker spec sheet for the exact model and size. It is usually listed as revs per mile or revolutions per km.

No. The setting depends on the whole tire diameter. A 275/65R18 at 814.7 mm and a 265/70R17 at 802.8 mm differ by 1.5%, whatever the wheel sizes.

No. It only does the math. Nothing is read from or written to any vehicle, and you enter the result yourself in your own software.

Yes. Revs per mile and circumference are the same for any car or truck tire. Only the place you enter the value differs.

No. The sizes you type never leave your device. Only a result you choose to Save is kept, and it stays in this browser.

• Tires and the tire label: what each part of the size means (NHTSA; blocks some bots, verified 200 in a browser session).
• 49 CFR 571.139, FMVSS 139: tire definitions for light vehicles (eCFR, NHTSA).
• NIST Guide to the SI, Appendix B.8 (mile = 1.609344 km, inch = 2.54 cm) (NIST SP 811).
• What is FORScan? (developer description) (FORScan developer site; cited only for the product description).