Is the use of the standard size tire safe on a 1/2" narrower rim? If so, does anybody see a reason why going to a slightly less wide (but much lighter) rear wheel might be bad?

Kevin

 

You should look on the tirerack site to find the detailed
specs for the tire. It will quote the range of rim sizes
that a given tire will be OK with. In you case, a half-inch
narrower would very likely be OK.
Depending on what the tire's ideal rim size is, it might
hurt or help ultimate traction *a little*. If you ran lower
or higher tire pressure than recommended it would make
much more difference. The lightness of the narrower wheel
would help rough-road handling and acceleration *a little*.
Joe

 

A wheel that is .5 of a inch narrower on the rear should be fine, some aftermarket fitments only offer that.

 

Yes it will work I have mounted stock rear tires on several aftermarket wheels for customers that where 10.5 as long as the .5 is off the inside not the outside of the wheel.