Our tandem has 26 (MTB) size wheels. The tires are 1.9" wide, and it's tricky to get the rear wheel in just right so it doesn't scrape the chain stays. The tires are heavy and have a bit too much tread, so I'm going to try some lighter, smoother, and narrower tires. With these tires so wide, I figured we wouldn't need more than 40 or 50 psi, but I was wrong. We need about 80 psi to feel stable. The bike is very whippy so having tires so hard doesn't make the ride harsh. We have gotten used to the whippiness, and it doesn't bother us.

I just bought some Michelin Wild Run'R 26x1.4 tires which purport to turn a MTB into a road bike. This might be a big change for us, and we will see how we like it. I'll experiment with pressures below and above 80 psi.