See the size change before you buy it
Type two tire sizes and Fitment Ledger draws them on one sheet, to the same scale, with the dimensions marked. Below the drawing it shows what the change does to your diameter, your revolutions per mile and your speedometer. Every figure is geometry on the size code, and the formula sits next to the number.
That pairing lifts the axle 1.55 inches, adds 40 mm of section width, and drops 69 revolutions out of every mile. The drawing is where the 39 mm of extra sidewall actually goes.
Two tools, one piece of arithmetic each
-
Tire size calculator
Parses metric and flotation sizes, gives section width, sidewall, overall diameter, circumference and revolutions per mile, compares two sizes with the percentage diameter difference, and prints the speedometer error at 30, 45, 60 and 75 mph. CSV and print included.
-
Wheel offset calculator
Converts offset to backspacing and back for any wheel width, with an editable flange allowance, and reports the inner and outer edge movement separately, because a wheel can be safe on one and not the other.
-
Bolt pattern guide
How to measure a 4-, 5-, 6- or 8-lug bolt circle, why 5x114.3 and 5x115 are different patterns, and what the 0.35 mm radial error does to a tapered lug seat.
-
Every formula, written out
The methodology page lists each equation the site uses, the constants, the one editorial assumption, and the sources for the load index and speed symbol tables with their retrieval date.
What this site does not have
There is no vehicle lookup here. Fitment Ledger will not tell you the bolt pattern, centre bore or factory tire size of a 2018 anything, because publishing a vehicle fitment database means licensing one, and the licence terms set specific limits on how that data may be redistributed and stored. Inventing the numbers instead is worse than not having them: a bolt pattern that is wrong by 0.7 mm looks like it fits and is not safe.
So the site does the part that needs no licence and no database. Tire and wheel geometry is arithmetic you can check by hand, and the measurement methods for a bolt circle work on the car in front of you. For the numbers specific to your vehicle, the Tire and Loading Information label on the driver's door edge and the owner's manual are the authorities. The methodology page states what would have to change before any vehicle page appeared here.
Guides
-
Fitment notes, October 17, 2026
5x114.3 vs 5x115: Why 0.7 mm Makes Them Different
5x114.3 vs 5x115 compared from geometry: the 0.35 mm error at every stud, what it does to a tapered lug seat, and how to tell which pattern you have.
-
Guides, October 17, 2026
Bolt Pattern Guide: How to Measure 4, 5, 6 and 8 Lug
A bolt pattern guide from first principles: what PCD means, how to measure 4-, 5-, 6- and 8-lug hubs, and why a near-match like 5x114.3 is unsafe.
-
Guides, October 17, 2026
Tire Size Meaning: Every Number and Letter Explained
Tire size meaning, field by field: service type, width, aspect ratio, construction, rim, load index and speed rating, with cited reference tables.
-
Guides, October 17, 2026
Wheel Offset vs Backspacing: One Measurement, Two Ends
Wheel offset vs backspacing explained with the conversion formula, why a width is always required, and how each one changes inner and outer clearance.