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21-Topaz II

## EXTERNAL: Units

Funny you should use tire sizes. While the wheel size is in fact in
inches, the section width is in millimeters. Since the side wall height
is a percentage of the section width, determining a tire's height is
difficult as you have to convert units to do the math. Knowing how tall
a tire is is important if you're changing wheel sizes and you want your
speedometer accurate.

Doug Schaefer
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Doug Schaefer | Experienced Mechanical Design Engineer
2 REPLIES 2
1-Newbie
(To:dgschaefer)

All,

Way back in the 60’s, most of my courses used metric measures (though you were not so concerned with hardware types}. Some companies put inch inside the box and metric on the outside (!?).

Most of the military is in SI. Many multinational companies with products that are export require that the interface hardware be metric. Finding American screws in most countries is harder than finding metric screws here.

One company was changing from inch/fractional to inch/decimal and sure enough 10 inches= 1 foot! To mitigate the errors, we got into the habit of designing with decimal fractions, .312 not .30 or .35.

Bill

Funny you should use tire sizes. While the wheel size is in fact in
inches, the section width is in millimeters. Since the side wall height
is a percentage of the section width, determining a tire's height is
difficult as you have to convert units to do the math. Knowing how tall
a tire is is important if you're changing wheel sizes and you want your
speedometer accurate.

Doug Schaefer

1-Newbie
(To:dgschaefer)
Whilst we're on the analogy of automotive, this must be one of the most
mixed up measuring system of all - despite Mr Ford.
Some drive on the left, some on the right, someplaces both at the same
time.
We all think we know what a horsepower is but can you describe it?
The gallons are different depending where you are, making MPG very
difficult to compare.
Tire wear is totally different - where does all that rubber that comes