Yes, taxis will always swindle you and that is the ever-persistent behavior of Chinese drivers.
In Xi'an, the meter drops at 5 Yuan and you can go 3 km before it switches to 0.7 every 1/2 km after that.
Always offer the first price. KNOW your Chinese numbers!! This is essential anywhere in China. If someone will take you somewhere for more than 20 Yuan you know it's a trick, so use the meter. Trust me, for 20 Yuan, he could bring you to Mexico for christ's sake.
WARNINGS FOR XI'AN
Yes, I wrote that in capital letters for a reason. Read the following extremely carefully, even if you only plan to spend a few hours in Xi'an. The second you leave the train / bus station, there will be pick-pockets around you. There will be people walking on your heels. People in shops will tend to stand 5 cm from your face. They have no concept of 'personal space', like westerners do, so don't take it as an insult if this happens to you. What always works best for me is to keep a fanny pack hidden under my shirt. They can never take that. Oh, and there are con artists everywhere in China, but especially here. They see foreigners each and every day, so they know how to f*ck them over. Trust me. ALWAYS bargain and offer an unbelievably low price. Go up from there if necessary.