It's not just about fuel additives (though that is a factor). In winter your car's engine takes slightly longer to warm up to optimal - so stays in "cold start enrichment" mode slightly longer. For old farts like me, it means the automatic choke stays on longer, uses more fuel.
Originally Posted by dany2010
Also in winter, you are putting more load onto the electrical system with your headlamps, heated screens, heated seats, stereo and goodness-knows-what that the alternator has to accommodate, putting more load on the engine to charge the battery back up and using fuel it doesn't use in the summer months.