toolbox · 02
gear ratio & speed
speed per gear, rpm at cruise, and the diff ratio to restore gearing after a tire change.
Gear ratio & speed
| Gear | Ratio | Top mph @ 6500 | RPM @ 70 mph |
|---|---|---|---|
| 1 | 3.20 | 49 | 9,223 |
| 2 | 2.10 | 75 | 6,052 |
| 3 | 1.40 | 113 | 4,035 |
| 4 | 1.00 | 158 | 2,882 |
| 5 | 0.80 | 197 | 2,306 |
Restore gearing after a tire change
how the math works
road speed comes from four numbers: engine rpm, the gear you're in, the final drive, and tire diameter. the transmission divides engine speed by the gear ratio, the differential divides it again by the final drive, and whatever is left spins the tire.
speed (mph) = rpm × tire dia (in) × π × 60 / (gear × final drive × 63,360)
cruise rpm = speed × gear × final drive × 63,360 / (tire dia × π × 60)
a 3.73 rear with a 0.85 overdrive and a 28 inch tire turns about 2,410 rpm at 75 mph. swap to a 4.10 and the same cruise climbs to roughly 2,650. that difference is the whole trade: acceleration against highway buzz and fuel.
restoring gearing after a tire change
taller tires numerically lower your effective final drive: the engine turns fewer times per mile, so the car feels lazier off the line and the speedometer under-reads. the fix is a ring-and-pinion swap that puts the effective ratio back where it was.
new final drive = old final drive × new tire dia / old tire dia
going from a 30 inch tire to a 33 on a 3.73 means a 4.10 gets you back to stock feel (3.73 × 33 / 30 = 4.10). the calculator above does this in the last section.