h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Nope, my petrol usage is most efficient at 55mph, according to the manual.

However the engine is happiest at 95!

EeeeeeHaaaaa!

I have a friend who has a 750cc motorbike. He claims that a full tank, £10 worth, takes him about 90 miles. My 1.8 fuel injected Cavalier does about the same and drags around about a ton of steel, plastic, people and glass. What's all that about. (It isn't even a proper motorbike, you know, one that drips oil from the crank case and has to be overhauled every week!)

PP

I have a friend who has a 750cc motorbike. He claims that a full tank, £10 worth, takes him about 90 miles. My 1.8 fuel injected Cavalier does about the same and drags around about a ton of steel, plastic, people and glass. What's all that about. (It isn't even a proper motorbike, you know, one that drips oil from the crank case and has to be overhauled every week!)

PP

B*gg*r

PP

From my rough knowledge of physics travelling at a greater speed increase the force excerted by the car on the road, as force equals mass times acceleration. This would increase the resistance require from the bridge to balance the downward force of the car.
As for the bridge swaying that would be down to resonnance, which would only occur if the wavelength caused by the motion of your car matched that of the brides natural vibration, causing the bride to join in the vibration and sway as happened with the millenium ray of light(bridge).
Travelling at 30mph will not improve fuel efficincy unless it is the speed at which optimum fuel efficiency is achieved, which from memory in normally nearer to 50mph.

I have an 1100cc 1970 Morris Minor. It takes about £16 to fill, on which it will go about 150 miles, complete with the ton of steel and people (but not much plastic - I think it was seen as exotic in those days). Which is comparable. Only it probably takes me longer. Much longer.

I'm not sure what any of that proves.

Oops, simulpost.

Force equals mass times acceleration, but accelerating in a car does not increase the downward force - the acceleration isn't in that direction. And in any case, the car could be travelling at a steady speed in which case acceleration is not relevant. It could be argued that going faster actually decreases the downforce, because the aerodynamics of most road-going cars tends to make them lift at speed - that's what spoilers are supposed to counteract.

The speed for optimum fuel efficiency varies from car to car - it IS often around 50 to 55, but it is rarely EeeeeHaaaaa!

Speed restrictions on weak bridges are not imposed to prevent imminent catastrophic structural failure but to slow down or halt continued gradual damage. While increasing speed doesn't necessarily increase downward force on all bridges, it often can. As Jim Bowen ("super, smashing, great!") mentioned this often applies to hump-backed bridges: while a car is trying to go forwards, the bridge is trying to push it up over the hump, and the faster the car goes, the harder the bridge has to work to overcome that forward momentum.