Bicycle Mechanics - How do I tell the actual tire size?

I bought and installed a Continental Contact 700x42 tire but it ended up being smaller than the 700x42 Continental TownRide that came original on the bike. The Contact tire is about 36mm wide at the widest point; the TownRide tire is 42mm at the widest. The diameter also appears to be smaller.

I am a bit disappointed because I like the bigger tires because they flatten the dilapidated pavement here. I think I might end up returning the Contact tire because they're about 15% smaller than what I thought they'd be.

How do I tell if a tire will actually be actually close to 42mm when installed? I'm half-tempted to get the 47mm size, but I cannot seem to find them anywhere and if those are 15% smaller, the tire would still be less than 40mm. But I'm thinking if I go that size, I should maybe consider the the Schwalbe Big Apple 50mm.

The bike is a rigid hybrid Marin Muirwoods 29er, and at 30lbs for just the bike, speed isn't a huge issue for me. But since I'm looking at 42mm and maybe even 50mm tires, that was probably obvious.

622 is diameter at the rim - brake track center, bead seat, second number is with ,
Knobbys and different rim withs change things some what ..

so You want fat 29er tires.. The mold is metal .. but the tires coming out of that mold vary
by small differences in the materials someone puts into that mold,,
hundreds of times a week.

comparing different tires by the same company.. you noted a difference ..

How do I tell if a tire will actually be actually close to 42mm when installed?

Install it and measure is reliable..

Very few manufacturers stick to the measured size. Try Schwalbe's for the width shown on the sidewall. These are 40's. http://www.schwalbetires.com/product_search?field_etrto_value_many_to_one=622-42

It's impossible for a manufacturer to ensure that a tire will inflate to it's nominal size, because he can't control the rim width which is part of the total cross-section circumference. That said, most manufacturers try to be somewhat close, based on an assumption of the intended rim width.

Manufacturers vary in their assumptions, and some tend to always be low, others tend to be high, but I'm surprised at the 15% difference in 2 tires from the same maker.

Unfortunately there's no 100% reliable to predict exact width but you can approximate it with a dollar bill. Wrap the bill around an uninflated tire bead to bead, and mark the distance, then add the inside rim width, and measure that distance. Divide by Pi (or 3 is close enough) to approximate inflated width.

This isn't a calibrated, precise way to get the width, but will give a fairly accurate prediction of the difference between two tire. Once you get used to it, and start compensating for tread, and where on the bead to start and finish, you'll get pretty decent predictions of how wide any tire will inflate to.

Another problem is that manufacturers will often make tires smaller than the specified size in order to make the weight look better against competitors.

One of the most useful tools I've ever bought:

http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17953.jpg (http://www.harborfreight.com/6-inch-utility-caliper-7914.html)
http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17953.jpg (http://www.harborfreight.com/6-inch-utility-caliper-7914.html)

One of the most useful tools I've ever bought:....

I agree and am a big fan of low end vernier calipers for all sorts of applications where I don't need super precision (where I use a better vernier caliper). But I think the OP meant how could he predict the tire width before buying and mounting them.

One of the most useful tools I've ever bought:

http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17953.jpg (http://www.harborfreight.com/6-inch-utility-caliper-7914.html)
http://www.harborfreight.com/media/catalog/product/cache/1/image/9df78eab33525d08d6e5fb8d27136e95/i/m/image_17953.jpg (http://www.harborfreight.com/6-inch-utility-caliper-7914.html)

Uh...what the heck do you think I used measure the tire width. Still that doesn't help me predict the size before I mount the tire or that my new Continental tire would be 15% smaller even though they are both sold as the same size.