$20 tip? You guys are far more generous than I. When I negotiate a fare, that's the fare. I will usually buy the driver lunch because we always want to stop on the way, but that's about it. If it's someone I really like, I may add $5-10 but no more. Tipping on a negotiated fare just seems crazy to me. I do usually tip route taxi drivers though, since they're on a different sytem.