[cupidstoy]

i think tire pressure may contribute most to the smoothness and bump absorbing quality of the ride. if your tire sidewall is stiff enough, try airing down in 5 psi increments front separate from back to see if you can get the balance you are looking for. be careful, of course, that you don't go too low where a tire can roll off the rim (usually you are safe to go down to 1/2 the max pressure posted on the sidewall or possibly lower). your gas mpg and cornering will suffer a bit, but the ride will definitely be much more forgiving on the straight highway stuff - especially in the fronts where you feel it the most.

i know it sounds contradictory, but i actually get a smooter ride from shorter sidewall tires with good build quality and running lower psi. you'll have to invest in a tire specifically sized for this goal next time you purchase new rubber. you can't air down a regular 65 series tire without getting a lot of roll in corners. but moving to 55 or 50 series with a good sidewal and possibly inching up your rim size plus running lower pressure gives a nice combination of forgiveness on bumps plus decent handling on corners.