A car's real-world top speed is dangerous to test on the road and difficult to calculate using theoretical formulations. While most manufacturers publish top speed figures, these tend to be conservative, and many cars are capable of exceeding such published numbers. Numerous formulas exist where you can plug in parameters about your vehicle to arrive at a theoretical top speed, but the input required for these formulas is not always easy to access or estimate. These formulas, combined with the published speed rating for your car, however, will give you a rough idea about the capability of your car.
- Skill level:
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Things you need
- Wheel size
- Tire width
- Tire height
- Engine rpm redline
- Differential ratios
- Car's weight
- Drag coefficient
- Frontal area
- Horsepower rating
Calculate the redline theoretical top speed. Shift the vehicle into the highest gear, drive at a constant 1,000rpm and record your speed. Divide this speed by 1,000 to arrive at speed per rpm at top gear. Read your vehicle's redline, or maximum, rpm from the dashboard, and multiply that number by the speed per rpm you calculated. This will give you the top speed you can attain if your vehicle actually can rev up to redline in top gear.
Check your manufacturer's speed restrictions. Many modern vehicles feature software limiting the maximum speed, often due to safety concerns. Check the vehicle's user manual where such statements as "...electronically limited to a top speed of..." can be found. Auto magazines and websites often test top speeds in controlled settings and will provide further clues about the limitations of the car, whether imposed by the manufacturer or vehicle design. If you cannot find any such data, you can roughly assume that your vehicle will reach the theoretical top speed calculated in the previous step.
Enter the data into an online formula. If you have access to key technical figures about your car, you can use a formula to estimate top speed. Naturally, the more you know about your car, the more accurate the estimate will be. Try to gather all relevant data such as weight, tire dimensions, horsepower and torque ratings, drag coefficient and gearing about your car and enter them into a formula. The resulting top speed will be the best estimate you can get without actually testing the car on a racetrack.
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