Travel fees have been computed at 3.25 a gallon for quite some time. Gas has been higher and it's been lower around here. When it consistently stays above that mark, then we'll adjust accordingly. We're not worrying about the fluctuations.
If an event (DJ only) is over 50 miles away, then we tack on a modest travel fee that the client is well aware of before signing the contract. If they book our largest photography and dj package we'll travel up to 100 miles without charging anything. When it's photography only, we have no travel charges unless they exceed the 100 mile barrier.
Bottom line, everyone has to do what is working for them. Not everyone drives the same vehicle, hauls the same amount of gear, has the same driving conditions, etc. which are some of the factors that affect your driving costs.