Stock Rim Material Thickness Estimate

[ct_racer]

I am trying to estimate the width of my rims at the bead to determine what size tires to buy. I'd like to be as close as possible without actually removing the tires so I can still have a rolling chassis while working on the bike this winter. If I measure the outside width of the rims, what is a good amount to subtract from the measurement to get the inside width?

They are spoked wheels (from a 79 cb750k, fwiw)...

Also the tires that came on the bike are the wrong size I believe.

 

[dualitymike]

If they're the stock steel wheels that came on the bike, they're 1.85 wide front and 2.15 wide rear.

 

[SONICJK]

It will also be stamped into the rim somewhere.