question about track width and backspacing
 

4crawler (Roger) has a lot of great information on his website; I've learned a lot there (and had good experiences buying products from him). I read his webpage (below) on calculating track width but have a question.

http://www.4crawler.com/Diesel/Tires...ningTrackWidth

Here Roger describes an example with a 2" increase in tire width, 2" increase in wheel width, and 1" decrease in backspacing. He states that the inside plane would remain the same as a 2" wider tire (1" inside, 1" outside) was compensated by a 1" decrease in backspacing. However, doesn't this exclude the contribution of the increased wheel width?

With the same backspacing, would not a 2" wider tire and 2" wider wheel not have the same inside plane as all the tire's extra width is on the outside? (i.e increases track width)

So shouldn't this statement:

'For each 2" wider tire, the outside edge of the tire will be 1" farther out from the vehicle, as well, the inner edge of the tire will be 1" closer to the vehicle'

Be better stated as:

'For each 2" wider tire on the same wheel, the outside ...'

So for his example:

2" wider wheel => 2" increase in track
(2" wider tire - 2" wider wheel)/2 sides => 0" increase in track
1" less backspacing => 1" increase in track
------
total of 3" increase in track (per side)

And would not the inside plane not be:
1" less backspacing => 1" increase in inside distance
(2" wider tire - 2" wider wheel)/2 sides => 0" increase in inside distance
------
total of 1" increase in internal plane distance

So to calculate backspacing for wheels in order to keep the same internal distance as stock:

new backspacing = stock backspacing - (increase in tire width - increase in wheel width)/2

So for going from stock 15x7 (4.625 backspacing), 225/75/15 (28.3x8.9) to 16x8, 315/75R16 (34.6x12.4):

increase in wheel width = 1"
increase in tire width = 3.5"

4.625 - (3.5-1)/2 = 3.375" backspacing

This way, all of the extra tire width goes on the outside, increasing the track. So track width is simply increased by the increase in tire width, or 3.5" in this example.