Hi Daniel,

I'd say that often in practical life both, time and distance are fixed and we need to fnd the (minimal) speed in which we need to travel to meet these contraints. For such pruposes it is more practical to give the speed (which is s/t). But using v=s/t instead of "1/v" = t/s likey just has historical reasons.

Daniel -

A calculation such as "I can divide 45 by 30 to get 1.5 hours..." has often been my experience, but I had never questioned the use of mph instead of hpm (or hours per km).  Shame on me, as your question suddenly made it clear that blind acceptance of mph meant I just wasn't thinking. (But since it isn't likely to change ...) 

Running a race, the distance of the race is fixed, and the time is important. Hmm.

I guess if you are going anywhere, by any means, a map may show you distance, and you want to know how long it will take.  So hpm makes good sense. Why not use hpm to multiply by the miles?

I did not know that "...volume/distance for gas mileage..." was used anywhere, but if you want to know the range you will have from a given volume of fuel, here distance per volume would be easier.

It would be interesting to know the history ["historical reasons" noted by Jochen] as to who decided what ratios to use.   Not likely from one person, such as comparing Newton's calculus notation to that of Leibniz, where one person's way of doing things might be chosen over another, but perhaps a history that may be a little more convoluted.  But it would be interesting to know how this came about.  (I like learning about that kind of thing, such as the fact that nylon got its name from being invented in New York and LONdon, or that railroads brought about time zones that did not exist before the late 1800s.)  If anyone knows about the history of mph as a 'standard,' I'd like to know. 

Cheers - Jim

Thank you everyone for the answers. Measurement is a fun topic!

In fact, we have two ways to express measurements in our daily work. In colloquial language, we can speak for example in Portuguese the words speed and velocity. Both words are understandable by everybody as synonyms. By contrast, in the scientific language, the use of a term has to be formal and this formality will have consequences on the results from the issue one researcher has been studying. Looking for uniformity, the SI defined the quantities by using letters. Thus, velocity is expressed by L/T from which researchers can deduce formulae and verify their veracity. In a car race the diameter of the wheel needs to have a diameter that supports the energy from the engine and the physical forces acting on the vehicles during their levels of velocity because every wheel rotation has L = 3.14*D. L is the length from every rotation of the wheel without forget the width. In the airplane the movement obeys a straight line and we can calculate the velocity and the acceleration as before. By contrast, I deduced that the seeds have a percentage of normal seedlings at the end of the germination test (non-dimensional) and a rate of seed protrusion along the evaluation period with dimension 1/T, and everybody can deduce the same using the dimension symbols of the quantities involved in the process using a probability distribution where each outcome can be 0 or 1. In a pipe line, the flux of oil or water will depends on the fluid viscosity, distance, and diameter of the pipe. At the end we have the forces acting on the wall of the pipe. In summary, I think we have to distinguish both situations during our work. MPH or Km/h do not make any difference.

I hope this response can help.

Thanks,

Walter.

José,

I think the question was not about mph vs. km/h but about mph vs. hpm (or km/h vs, h/km). Or to take your seedlings example: why do we (generally) express the germination rate as "number of protrusions per time" instead of the (mathematically equaly valid) "time needed to get a protrusion".