Gear Ratio and Vehicle Speed
Vehicle speed depends on engine RPM, the combined effect of transmission and final drive gear ratios, and tire size. The overall gear ratio multiplied by the engine speed determines how fast the wheels rotate, and tire circumference converts that rotation to linear speed. Understanding this relationship is essential for drivetrain design and performance optimization.
Lower gears have higher ratios for strong acceleration, while higher gears have lower ratios for fuel-efficient cruising. The final drive ratio (differential) affects all gears equally and is chosen to balance acceleration performance with top speed and fuel economy.
Speed Formula
Typical Gear Ratios by Gear
| Gear | Ratio Range | Speed at 3000 RPM* |
|---|---|---|
| 1st | 3.0-4.5 | 15-22 mph |
| 2nd | 1.8-2.5 | 25-38 mph |
| 3rd | 1.2-1.6 | 40-55 mph |
| 4th | 0.9-1.1 | 55-75 mph |
| 5th | 0.7-0.85 | 75-95 mph |
*Approximate, with 3.73 final drive and 26" tires.
Frequently Asked Questions
How does tire size affect speed?
Larger tires increase speed at any given RPM because each revolution covers more distance. Going from 25" to 27" tires increases speed by 8% at the same RPM. However, larger tires also reduce effective gear ratio, which can decrease acceleration and put more strain on the drivetrain.
What is the best final drive ratio?
It depends on the application. Lower numerical ratios (like 3.08) favor fuel economy and highway cruising. Higher ratios (like 4.10) provide better acceleration and towing capability. Most street cars use 3.23-3.73, balancing performance and efficiency.