Part of the Automotive collection — 4 tools available
Speed & RPM Calculator
Calculate vehicle speed from engine RPM, gear ratio, and tire size. Find your speed at any RPM in any gear for performance tuning and gear ratio planning.
About This Calculator
Understanding the relationship between engine RPM and vehicle speed is essential for gear ratio selection, tire size changes, and performance tuning. If you swap to taller gears for better highway fuel economy or install larger tires, your engine RPM at cruising speed changes — sometimes dramatically. A 200 RPM change at 70 mph affects both fuel economy and engine wear over thousands of miles. Our speed and RPM calculator shows exactly how fast you're going at any RPM in any gear, accounting for gear ratio, final drive, and tire diameter.
The Formula Behind This Calculator
RPM = (Speed * Gear ratio * 336) / Tire diameter Optimal cruise: 1,800-2,500 RPM Redline: 5,500-7,000 RPM.
Understanding the math helps you verify results and make better decisions for your project.
Related Tools
How to Use
- 1Enter the engine RPM you want to check.
- 2Input the gear ratio for the specific gear you're calculating.
- 3Set your vehicle's final drive (differential) ratio.
- 4Enter your tire diameter in inches.
When to Use
- →Checking highway cruising RPM after changing tire sizes or gear ratios.
- →Planning a gear ratio swap for better acceleration or fuel economy.
- →Verifying speedometer accuracy after tire or gear changes.
Tips
- ✓Ideal highway cruising RPM is 2000-2500 for most engines — higher wastes fuel, lower lugs the engine.
- ✓Every 1 inch increase in tire diameter reduces effective gear ratio by about 3.5%.
- ✓When changing gear ratios, remember to recalibrate your speedometer or GPS-check your speed.
FAQ
How do I find my gear ratios?
Check your vehicle's owner manual or search online for your transmission model. Typical gear ratios: 1st 3.5:1, 2nd 2.1:1, 3rd 1.4:1, 4th 1:1, 5th 0.8:1, 6th 0.65:1.
What is the 336 constant in the formula?
The constant 336 converts between wheel RPM, tire circumference (inches), and miles per hour. It equals 12 inches/ft × 5280 ft/mi ÷ 60 min/hr.
Will bigger tires make my speedometer read high or low?
Bigger tires cover more ground per revolution, so you're going faster than the speedometer shows. A 10% larger tire means your actual speed is 10% higher than displayed.
Related Calculators
Tire Size Calculator
Compare tire sizes and see the difference in diameter, width, sidewall height, and revolutions per mile. Essential before switching tire sizes on any vehicle.
Fuel Cost Calculator — Calculate Gas Cost for Any Trip
Calculate fuel cost for any trip. Enter distance, fuel efficiency (MPG), and gas price to estimate total fuel cost and gallons needed for your road trip.
Car Loan Calculator — Monthly Payment Calculator
Calculate your monthly car payment and total interest. Enter loan amount, interest rate, and term to see the full cost of financing your next vehicle.