Originally Posted by AlanMintaka

Thanks once again to all for these informative replies. There's a lot of great advice here that I can use when comparing tires for my Trek.

Also, I got refresher course in tire sizes. So I need to do more homework now to see how the newer size designations match up with the capacities of the Trek rims. Those are Matrix 750's, 622 X 19.

There is a wealth of info on that Sheldon Brown tire size page to help me with that. For the 19mm rims, the current tire width of 38mm is slightly larger than the average recommendation. The max recommended tire width is 44mm. In inch land, that translates as 1.5" to slightly less than 1.75". That's not a lot of play.

That being said, what about a different set of rims for the Trek? I can use Sheldon Brown's chart to match a wider tire size with a wider rim easily enough. The front wheel also looks like a no-brainer. Just buy a new wheel with a wider rim and tire, install it, adjust the brake pad clearance, and go.

That back wheel...

Have a good one,