Nice choice of car.
I've got a 1990 gti 8v and also commute 50+ miles each way to work and back. I get around 420miles to a tank (98ron unleaded). I find that the low down torque makes my journey more relaxed when in traffic and on the odd occasion lots of fun when darting down back roads.
However, i would also love to try a 16v. They are undoubtedly a better car compared to the 8v as they are more powerful and have up-rated suspension and brakes. Fuel consumption and insurance are always made an issue of but ultimately these cars are not supposed to be economical a-b cars. And in comparison with other cars of its day with similar performance it had pretty good fuel consumption.
That's my rant over with! Sorry to go on. I would just like to add that i am extremely happy with my GTI and can't think of a better car for the money.
