Taxi Fare etiquette, can someone tell me if I did something wrong?

Not well versed it taxi travel. Can probably count on two hands the number of times I have done so.
Last weekend took a short hop from B&B to Terminal; upon arrival the meter read $7.75. While removing the bags the driver said “That will be $10.00”. I assumed he was adding in his own tip which was fine since that was what I was going to hand him anyway. But he looked at me like I had just stiffed him.
So, just for my edification, was $2.25 an adequate tip on a $7.75 fare or is there an unwritten rule that there is a minimum fare and I should have tipped him a couple of extra bucks?