Tire Size, Speedometer and Gearing Calculator

Tire geometry and indication

For section width $W$, aspect-ratio number $A$ and bead-seat diameter $d$, $h=WA/100$ and $D=d+2h$, with explicit inch-to-mm conversion. An entered overall diameter replaces the code-derived value. Geometric circumference is $C=\pi D$. Tire Rack's dimension article supplies the method; its 225/45R17 example rounds intermediate values, so this tool retains additional precision.

Let $q=D_n/D_o$. With unchanged original wheel-based calibration, $v_{actual}=qv_{indicated}$. Actual-speed increase relative to indication is $100(q-1)$; indication and odometer bias relative to truth is $100(1/q-1)$. Discount Tire's worked example checks the correction direction and magnitude.

Engine speed and regear targets

Convert true road speed to mm/min, divide by circumference in mm for wheel rpm, then multiply by axle and transmission ratios: $n_e=(v/C)f g$. The effective original axle ratio on new tires is $f_o/q$; the axle ratio restoring original RPM is $f_o q$. To reach a requested cruise RPM, $f_{target}=n_{target}/(n_{wheel,new}g)$. This uses exact unit conversions rather than the rounded US shortcut constant 336. TREMEC Performance Catalogue, attached page 2 (printed pages 2–3), provides independent direct-gear and overdrive examples; attached page 7 gives the editable default Magnum ratios. Torque-band boundaries and redline are user inputs; no torque curve is inferred.

Offset and static clearances

With positive offset $ET$ toward the outer wheel face, effective new offset is $ET_n-s$ for spacer thickness $s$. Inner tire extent is $I=ET+W/2$; outer extent is $O=W/2-ET$. Subtract the increase of each extent from its original measured gap. Radial arch gap loses $(D_n-D_o)/2$. Wheel bead-seat movements use wheel width instead of tire section width. Track change for a symmetric pair is $2(ET_o-ET_{eff,n})$. This convention follows Tire Rack's offset definition.

• Nominal unloaded dimensions and ideal no-slip rolling; actual loaded rolling circumference may differ from $\pi D$. An entered measured diameter is still a geometric diameter, not a loaded rolling-circumference measurement.
• The original speedometer is accurate, senses wheel rotation and remains unrecalibrated. An axle change can also alter a transmission-driven speedometer; that sensor effect is not included.
• Fixed gear ratios, fully engaged clutch or locked torque converter, no tire slip and the same transmission before and after. Include a fixed transfer reduction in both axle inputs if required.
• The useful torque band is a user-supplied RPM window. Default 2000–5500 rpm and 6500 rpm redline are illustrative, not specifications for the default gearbox or any engine.
• Tire width stays equal to the entered section width. No empirical correction for measuring-rim width is applied. Wheel width is bead-seat width; flanges, spokes and brake envelopes are not modelled.
• Clearance gaps are perpendicular distances to a static rectangular envelope at a fixed hub position; no camber, steering, suspension sweep or body movement is solved.

Use for first-pass dimensional and drivetrain comparisons of conventional round road tires with one to six forward ratios. The input ranges are calculation limits, not approved sizes or operating limits. A redline road speed is a kinematic limit only, not an attainable or permitted speed. Ratios shown as targets may not exist as hardware.

Positive nominal gaps do not certify fitment. Use the exact tire manufacturer's dimensions and verify full steering and suspension travel, brake and wheel-flange clearance, bearing and spacer suitability, and vehicle-system calibration. No universal percentage tire-size allowance is assumed. Load/speed index tables, tire/rim approval ranges, structural capacity, traction, acceleration, fuel consumption and regulations are not assessed. Tire, wheel, spacer and axle installation belongs to a qualified person.

Gillespie is cited as the requested vehicle-dynamics background reference by DOI. Its paid full text was not used or attached; the implemented equations and numerical benchmarks are supported by the open sources below.