You'll save a lot more gas/petrol simply by choosing a car that is designed to be efficient. You don't have to go to the extremes of buying an electric/hybrid, merely staying away from SUVs and sports cars does a lot. Saves you money too. It is worth spending at least $1000 or so more on a new vehicle that uses 10% less fuel (very rough order of magnitude estimate at typical ranges of fuel consumption).

I believe it's better to buy the vehicle that's appropriate to your needs - not too big and not too small. If all you do is drive by yourself to and from work than any small car, motorcycle, or even a bicycle is often the best choice. On the other hand, if you need to carry large numbers of people on a regular basis and/or haul heavy stuff (construction equipment) than a big pickup or SUV may be the best choice.

I believe it's better to have one vehicle that meets most or all of your needs than to have two vehicles. The energy and resources consumed to build that second vehicle may well be far greater than what you'd save by having a big vehicle for some needs and a small one for daily driving. The other alternative is to rent the bigger vehicle only when you need its capabilities and stick to the most efficient smaller vehicle that can handle your local climate.

Living in Colorado, I need a vehicle that can handle bad weather (like the icy roads we're experiencing this morning or the snow we had last Sunday). An all wheel drive vehicle like my Honda CR/V does quite well in bad weather, gets over 25 MPG in my daily driving, and is big enough to carry my grandkids or other stuff in the back. It seems to be the best overall vehicle to meet my needs. Now, if the Chevy Volt comes out in a few years for a reasonable price and can handle bad weather, I'll be very tempted to buy one. My 7 year old CR/V might be up for replacement about then.