Today I noticed that my tyres looked a little flat. I realised that I had not taken the time to check them for ages.... Like many people, I probably rely upon the garage to do this when the car is serviced, but I suspect that they didn't do it last time. My only excuse is that my tyres are low profile jobs, so it's actually a little harder to detect visually if they are low on pressure.
Anyhow, checked them at the service station and was shocked to find that they were way down. Closer to 20psi (140kPa) rather than 32psi (220kPa). Now, fuel efficiency is reduced by one percent for every 20 kPa of under-inflation (http://www.maic.qld.gov.au/forms-publications-stats/pdfs/tyre-pressure-…). Mythbusters found that tyres underinflated by 15% used 1.2% more fuel (http://www.discovery.com/tv-shows/mythbusters/mythbusters-database/unde…)
I did some rough calculations, and worked out that my under inflated tyres would have been making my car use about 4% more petrol than it should which in my case works out to roughly $64 per annum for fuel. So. Worth checking your tyre pressures! It only took me a couple of minutes to do at the local service station. Although the fuel savings may not be huge, proper tyre pressures will also ensure your tyres will last longer (saving money) as well as make your car handle better and (frankly my biggest concern) in turn be safer.