Tire size, gearbox ratios and final drive in; road speed in every gear, engine RPM at cruise, wheel torque and how the differential splits wheel speeds in a turn out.
Tire size
mm
%
R"
Gears
Gearbox ratios from the manual or the maker's data. Overdrive gears are below 1.00.
Final drive (ring and pinion)
: 1 ratio
ring / pinion teeth
RPM
N·m
km/h
m
m
Turn radius to the centre of the axle. 0 means driving straight.
Open differential, seen from above
Every gear
How it is calculated
Tire diameter from the ISO size code (width / aspect ratio R rim). Rolling radius under load is about 2–3% smaller, so real speeds are a little lower and the speedometer usually reads a little high.
Tire diameter D = rim·25.4 + 2 · width · aspect/100 (mm)
Circumference C = π · D
Overall ratio i = gear ratio · final drive, final drive = ring teeth / pinion teeth
Road speed v = RPM / i · C · 60 / 1 000 000 (km/h, C in mm)
Engine RPM RPM = v · i · 1 000 000 / (60 · C)
Wheel torque T_wheel = T_engine · i · η, η ≈ 0.90 (both wheels together)
Open differential n_inner = n_carrier · (R − t/2)/R, n_outer = n_carrier · (R + t/2)/R
Spider gears turn at (n_outer − n_inner)/2 · side-gear teeth / spider teeth
An open differential splits torque equally between the two wheels, so the wheel with less grip limits both. Common ring and pinion sets: 3.08 (40/13), 3.23 (42/13), 3.42 (41/12), 3.73 (41/11), 3.91 (43/11), 4.10 (41/10), 4.44 (40/9), 4.56 (41/9).