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SSI Insert revised (no index tag!): 22-Feb-2016

Theoretical, based on power and drag co-efficient

The power necessary for a given top speed is measured at the wheel. But if the gearing isn't right, the rpm at which max power is made will not coincide with the cars max speed.

The equation is...

Hp = Cd * A * v^3 / 146000

Where A = frontal area in sq. ft. V = velocity in mph, Cd = Co-efficient of drag

A std 1071S makes about 70hp @ 6k rpm at the flywheel so assuming 18% losses that's about 57.5hp at the wheel. Its got a top speed of 93mph and we can assume its gearing gives around 16mph per 1krpm. So at 95mph it will be revving at 5,800rpm which is close to its max power rpm.

57.5 = 93^3 * Cd*A / 146000

Cd*A = 10.44

So if we have 77hp at the wheel, then the max speed should be about 102.5mph

Max speed = CUBEDROOT [(BHPwheels * 146000) / 10.44 ]

Or take a 850 mini - about 34hp at the crank or around 27.5hp at the wheel. Plug the numbers in and you get a max speed of around 73mph which, according to road tests of the day, is it's max speed.

It's not perfect, for example losses in the gearbox aren't a simply percentage and the CdA isn't the same for all models of Mini. But it's not bad.

For comparison purposes here are some other Cd.A data for other cars:

Lotus 7 |
9.7 |

MGB-GT |
8.09 |

MG Midget |
8.7 |

D type |
6.78 |

Mk2 Jag |
14.02 |

The mini may be small, but it's Cd is crap.

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SSI Insert revised: 23-Feb-2016