I'm a little confused, which isn't all that uncommon.

I just installed a Cateye wireless computer on one of our bikes, and couldn't find any mention in the documentation about placement of the magnet and sensor. Seems to me they should both be as close to the rim as possible, but this model has a sensor mount that puts it about 3 inches below the rim. Seems to me the closer both are to the hub, the more it's going to distort your speed, mileage, etc.

Is there a rule of thumb on the best place to mount these parts, like xx inches from the rim? And if it's in the wrong spot, will your numbers be appreciably affected?

Every wired one I've ever used puts the sensor about three inches from the hub.

the speed is calculated by 1 rev x the programed wheel/tire size. placement doesn't matter.

Yeah, lechat is right. I didn't even realize the full meaning (lack of understanding) of your question. The sensor doesn't count distance, it counts revolutions. Because every modern cyclocomputer has a place to input the circumference of your wheel, all it needs to do is count the number of revolutions and multiply by the known circumference to get the total distance traveled.
Since every part of the wheel undergoes the same number of revolutions the sensor can theoretically go anyhwere, even though the radial distance traveled by the sensor will vary depending how far out from the hub it is. It will still only cross the magnetic pickup once per revolution, giving the same set distance every time.

The circumference you program into the cyclocomputer is sometimes measured in inches, sometimes centimeters and sometimes millimeters. So even though your computer is displaying your distance in miles, behind the scenes it's actually counting total millimeters (or cms or ins) traveled and doing the appropriate math conversions.

The best thing to do to have your computer give you accurate readings is to make sure you get the circumference programmed in correctly. There are general numbers for different kinds of wheels/tires but the best way is to measure the actual wheel on your bike. For instance, put a mark on the ground and line it up with your valve stem. Roll it forward one entire revolution until the valve stem is again in the exact same position and mark it. Measure the distance between the marks. That's the number that goes into the computer. For even more accuracy, sit on the bicycle while you roll it so it takes into account your weight pressing down on the tire (which will change the effective circumference slightly).

That makes sense, now. Very clever, even if I'm not.

I've heard closer to the hub is SLIGHTLY better, in that it's not moving quite as fast as it passes the sensor, therefore, it has less change of error.

:shrug:

You would think it closer to the rim would help avoid mis-communication between sensor and receiver. At least with a wireless.

I've always put it as close to the hub as possible, because that way you can get the sensor closer to the magnet.

The farther from the hub you get, the more flex you get and the sensor can get bumped by the magnet when climbing or sprinting. Close to the hub these deflections are very minor, so you can put it a lot closer.

Also, don't forget to put the sensor in FRONT of the fork leg, not behind. If you put it behind, and it gets pulled into the spokes, you can break things. In front it just gets bumped back out again if it hits the spokes, no harm, no foul.

Placement on the fork is really preferred over where on the wheel. Furthest from the ground, meaning less danger from twigs etc.

This one specifies that no more than 70 centimeters should separate the sensor and the computer. It also needs a gap of 5mm or less between the magnet and the sensor. The sensor swings on a horizontal axis, so you can make that adjustment pretty easily from just about anywhere on the wheel. I put it as high as possible because it's an mtb--less chance of crap building up on it.

I thought the same about sensor placement, but on my 7.2 FX the only way to get correct readings was to place the magnet close to the outer edge of the wheel away from the hub. The accuracy went from not even close to radar accurate. Check with customer support of your computer, they should know proper placement.