rear wheel 19 to 18 inch

Just wondering how much this effects the rpm's?

Or speedo drive-instrument accuracy.

Slightly depends on the size of the tire profiles fitted but my 19 and 18 tires are within 1/2 inch OD of each other. IIRC I measure the circumference of each to find the 19 maybe 1.5" longer per roll. Not a significant difference that would cause one to get a ticket nor feel much if at all in gearing change of thrust. I switched between these ride to ride at times d/t flats and mainly felt the extra mass of 18x120 tire over the 110x19.