Tire Size and Effective Gear Ratio
rev/mi for old vs new tires, effective ratio, cruise speed, recommended axle ratio.
Example
You enter
- Original tire size P265/70R17
- New tire size 33x12.50R17
- Axle ratio 3.73
- Top gear ratio 0.84
- Target cruise RPM 1800
You get
- Diameter orig (in) 31.61
- Diameter new (in) 33
- Rev per mi orig 638.1
- Rev per mi new 611.15
- Cruise speed at target RPM 56.4 mph
Checked against published worked examples from: PowerNation.
Details, formula, and sources
Formula or tableEffective tire diameter from metric size (e.g., 285/75R17): D_in = (W_mm × ratio/100 × 2 + R_in × 25.4) / 25.4. Revs/mile = 63 360 / (π × D). Effective ratio = axle_ratio × (rev/mi_new / rev/mi_stock) x top gear -- a taller tire turns fewer revs per mile and LOWERS the effective ratio (3.73 to a 33 in tire reads about 3.57).
Source and editionTire & Rim Association (TRA) Yearbook by name; ETRTO Standards Manual by name; SAE J267 metric-tire size convention.
Free accessTRA / ETRTO licensed; principles free at most tire-manufacturer technical pages (Goodyear, Michelin).
What governsEstimate. AHJ and licensed professional govern.
API field namesoriginal_size, new_size, axle_ratio, top_gear_ratio, target_rpm, diameter_orig_in, diameter_new_in, rev_per_mi_orig, rev_per_mi_new, cruise_mph
- Axle-ratio candidate set 3.73 / 4.10 / 4.56 / 4.88 / 5.13 / 5.38 (common live-axle ratios)OEM ring-and-pinion catalogs