I'm starting to confuse myself thinking about it more and more trying to make sense of it and how to explain it.
The thing that helped me get it straight in my head is to think in terms of torque/force rather than "effective gear ratio." The math looks like this (starting with an arbitrary input torque t0 at the driveshaft):

ta=t0*gr: ta is torque on axleshaft, gr is gear ratio
F=ta/r: r is tire radius (=1/2 diameter) F is Force between wheel and ground (this is what makes you go).
combined: F=t0*gr/r
solve for t0: t0=F*r/gr
for comparing when something changes:
t0=F1*r1/gr1=F2*r2/gr2 (remember that t0 doesn't change for our purposes)
if we assume 1 is the "stock" configuration, then to compare 2 to 1
F2/F1=(r1/gr1)/(r2/gr2)=(r1/gr1)*(gr2/r2)
Example: stock gear ratio 3.55 with a 30 inch diameter tire
new is 3.73 with 35 inch diameter tire
F2/F1=(15/3.55)*(3.73/17.5)=.90 so the new configuration will put 10% less torque (F2=90%*F1) to the ground and will thus feel more sluggish than stock.