One of the areas of research before I bought the car was fuel economy. This is especially true since the GTI is so similar to the 40-50MPG TDI diesel. In fact, the most common question I get about this car is whether I considered a TDI instead. Wow, did I. It was far too spartan and uninteresting for the price. Now the GTD diesel from Europe, with the same GTI trim would have been a contender.
The GTI gets good gas mileage, but it takes premium fuel. Before I bought the car, I did the math comparing my Mazda 6 to the GTI and figured I was saving about $40/month (offset by $26/month in higher insurance). When I got the car, this savings got a bit muddled. It wasn't that I wasn't getting good mileage, in fact my highway mileage is better than expected, it's that it takes Top Tier gasoline.
Top Tier gas is a standard for detergent enhanced gas. So rather than just any premium fuel, I have to use premium fuel from an approved Top Tier gas station. There are 22 Top Tier manufacturers, but only a couple are local: Shell and Chevron. Shell has a bad reputation with its nitrogen enriched gas, so that leaves Chevron. I can find a local Chevron station at roughly the same price as my local cheap gas place, but it's one more hassle I wasn't expecting. It's probably as cumbersome as hunting for diesel fuel.
Now you're probably thinking Top Tier is bunk, which might be true. Having worked for Chevron petroleum engineers in the past, I can tell you they're true believers in the quality of their gasoline. If Top Tier was significantly more expensive, I might argue the point, but since it's roughly the same, why take chances?