When we're looking at the impact on the enviroment, think about the amount of energy and raw materials that go into producing a new car/bike. We're riding bikes that were 30 years old, not just following the consumer culture and buying new.
Dave
Firstly, that's a very important point. The amount of energy used up in the production of a new vehicle is of course extremely hard to measure, and something that no manufacturer is going to want to admit to anyway. It's not comparable to the little energy involved in keeping an old bike on the road. The environmental importance of extended use rather than re-cycle and re-manufacture are rarely taken seriously enough by the green movements and/or govts.
Secondly, fuel efficiency. That's a very detailed link and shows some large variation between models, interesting indeed. It's worth remembering that if we took every single vehicle off the roads then we would still produce over 80% of the CO2 we produce now. We also produce considerably less CO2 as a nation than we did around 1960 (these statistics apply to the UK).
Thirdly, local environmental impacts; all activity should be considered by it's affect on the local environment too. For instance, if you could convince all the people in your area to take up biking, you'd have no more traffic jams (impacting upon the urban environment especially). Indeed, every time you use a bike instead of a car, you're reducing congestion and helping improve all the other road-users miles per gallon.
Fourthly, you feel guilty about going for a ride just for the sake of it? Everything we do uses energy. Sit around watching TV & you're burning up electricity, heating/aircon , etc - get a few thousand of you doing that & you need a power station to keep you going. 50000 people go to a football match, how do they get there- by using energy in their cars, trains and buses. I say keep riding.
