In the mid 1980s my oldest Son 16 y/o declared that the Kawasaki Ninga 1000's top speed was 200mph. We were in a motorcycle dealership. the top speed on the speedometer of the ninja was 200 mph. I thought this was excessive. I checked a road test in a motocycle magazine, the top speed was 155mph. Crazy fast, but not even close to the speedometer. Back in my day it was the opposite. I owened a 1963 Galaxie 406 cu inch 405 hp the speedometer went to 120 mph but the tested top speed was 150mph. My recent automobles have been: 1991 Mazda Rx-7 150 mph speedometer. Top speed 135 mph. 2002 Mitsubishi Eclipse. !40 mph Speedometer. Top speed 133 mph. Current automobile 2010 Kia Forte SX. 150 mph speedometer. Top speed 132mph. Nowdays the maximum speed on the speedometer does not nessecarialy match the actual maximum speed.
Speedometers always go past the actual speed which a car, bike or truck can reach. Top speed depends on engine power (measured in brake horsepower), gearing at the transmission and final gear ratio, wheel diameter and eventually with tire slippage and mainly - the aerodynamic drag.
At high speeds, wind ressistance even with no wind is enormous and it requires alot of available torque to push through and accelerate to a higher speed.
However, this torque is reduced by many power losses that are very poorly calculated by the manufacturer. There is NO car that reaches the actual horsepower it is supposed to have. The manufactuer measures power as a mathematical calculation of the torque and rev information recieved over the engine's flywheel.
However, friction in the clutch and the transmission gearing, syncros and lubricant - mainly in automatics that have torque converters and planetary gearsets - are very dramatic. Also, every additional 10-15 kilograms effect the car's power to-weight ratio so even one adult passenger can effect a car's acceleration and to speed.
Any addition or change to the car's outer firm, including a small crack up one of the windows will increase aero drag dramatically at high speeds, and any activation of electric power utilities inside the car would cost in further horsepower: High beams, loud-volume stereo and mainly the air-conditioning are serious power users that increase ressistance on the alternator dramatically.
Hot air also reduces horsepower and if it's moist, salty or dusty - that all reduces oxygen to the engine and reduces the combustability of the fuel-air mixture. Fuel quality also makes a difference, and either new engines or engines that are worn just enough to cause a little bit of the compression to slip into the sump - will all reduce horsepower. Even thicker or older motor-oil and even the oil in the transmission and the differential all have an impact.
Soot in the motor, dirt in the fuel, smoke in the air, an old catalytic converter, air filter that isn't perfectly new, sludge in the throttle housing and intake manifold - they all have an impact.
Overall, we are talking about at least 20-30km/h less on the top speed and a reduction of 20-40% of the horsepower in everyday conditions. Of course, full speed tests should be done in a controlled environment.
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