Originally Posted by
sdimattia
To answer your original question, there's no reason you can't throw gravel tires on the Trek, with respect to tire clearance. Trek aluminum will certainly be lighter but only by a little.
Why do you believe 27.5" wheels can't keep up with 700c, regardless of terrain? Your companion is going to have to work harder because the ratio of her weight to the bike's weight is greater. Certainly, you can alleviate some of this by buying a lighter bike. But smaller diameter wheels do not increase or decrease rolling resistance compared to a larger diameter wheel (unless you're riding at 25mph or more).
i have a limited number of posts i can do so i'm going to try to be as complete with this answer. I believe the tire size makes a difference for 2 reasons, one is anecdotal experience and the other is pi (3.14) multiplier....the difference between 26 inch and 29 inch, even though its about 3 inches is really more than 9 inches in the amount of road covered (circumference) per full revolution of tire on road....can you imagine how many miles more the other person has to work the longer the ride even if the tires seem close in diameter? I don't want to be rude to a riding partner and make them work harder with an unequal exercise all the while giving them the illusion that we are doing the same workout and that they need to learn to keep up.....we already reached 40 miles round trip and are moving up to 70 miles round trip per every other day.