Tough one. It's not a problem if the axle goes all the way through teh collar, but doesn't extend past it more than a hair. If you have a collar inside of the wheel, you might be able to remove that one and just use a washer or a spacer or nothing at all, depending on the gear. If you shop around, you might be able to find wheel thickness measurements, I know different brand wheels vary in thickness (since I've had the same kind of thing happen when changing wheels), but I haven't a clue as to what brands are narrower in various sizes.

Is it wire gear with a single strut that bends in to the axle? If you want to get extreme, you can cut off the bend, and put on bolt-on axles that are often used with retracts. That can give you a longer axle to work with (too much trouble in my book, but possible). If it's flat metal gear, you can use a longer bolt instead of an axle.