Methodology: Where Every Number Comes From

Tiremason's rule: every displayed number is produced by a tested formula, and every formula and threshold on the site is listed here with its source. Nothing on the site is hand-typed into a page.

Formula ledger

Overall tire diameterdiameter_mm = rim × 25.4 + 2 × width × aspect ÷ 100Standard tire geometry; identical to tiresize.com and crawlpedia.com calculators
Revs per milerevs = 1,609,344 ÷ circumference_mm1 mile = 1,609,344 mm exactly
RPM at speedrpm = ratio × mph × 336 ÷ tire diameter (in)Hot-rod constant 336; cross-checked against crawlpedia.com to the integer
Effective ratio after tire changeeffective = old_ratio × new_diameter ÷ old_diameterRestores stock wheel-turns-per-mile
Fuel costcost = distance ÷ mpg × priceDirect unit algebra
MPG from fill-upmpg = (odometer₂ − odometer₁) ÷ gallonsStandard fill-up method, same pump-stop convention
US mpg → L/100kmL/100km = 235.214583 ÷ mpg1 US gal = 3.785411784 L; exact
US mpg → UK mpgUK mpg = US mpg × 1.200951 US gal = 0.83267384 UK gal; exact

Threshold ledger

Green tire-difference threshold< 3% diameterConservative community consensus; common shop policy line (see guides/tire-size-difference-safety)
Grey zone3–5%Documented disagreement between the 3% and 5% schools; both quoted in full on the site
Red tire-difference threshold> 5%Rub/clearance and gearing risk; unanimous across sources
AWD caution> 0.5 in circumference differenceCommunity-documented shudder-under-acceleration reports
AWD critical> 0.75 in circumference differenceCommunity-documented differential/transfer-case damage; 6.7% owner case destroyed a transfer case
LSD / G80 caution> 1% circumference differenceCommunity reports of lockup intervention on mismatched axles
4x4 front/rear match≤ 2% ratio differenceManufacturer and off-road community consensus; wind-up failures above
Dashboard MPG optimism flag> 3% over hand calculationRecurring owner-community measurement range 3–8%
Engine input guard bandsrim 10–30, width 105–445 mm, aspect 25–85, distance 1–20,000 mi, price $0.50–15/galSanity limits; out-of-range inputs are rejected with an explanation

How the content is built

  1. Community evidence is collected from owner forums (43 threads sampled across r/4x4, r/Jeep, r/Justrolledintotheshop, r/roadtrip and others), tagged for sizing rules, failure reports, and tool feedback.
  2. Competing formulas and thresholds are cross-checked against published calculators (tiresize.com, crawlpedia.com) and documented disagreements are kept as disagreements, not averaged away.
  3. Every formula is implemented once as a pure function with an assertion net (17 pinned cases), used by both the server-rendered examples and the live calculators.
  4. Each calculator lists its assumptions on the page, with this page as the ledger.