I don't really know, but I can tell you about my friends I guess. One took 21B freshman fall quarter, the other took 21C. First one is biomedical eng, second is chemical eng. Both of them are on track to graduate in 4 years with summer school.
Judging from that, you may have some trouble. I can't say though, but it might help if you look at your major reqs (UC Davis General Catalog | Programs and Courses
One plus about starting with precal is that you take 21A with people who also had to take precal. I think it's safe to say that che2a, mat21a, and mat17a are harder fall quarter because that's what the majority of freshmen are taking = a lot of premeds (mainly in the case of 17, but i don't know about 21) = more competition.
Oh, and I was just looking at these statistics today, but within the class of 2008 (people who entered in 04) the 4 year graduation rate was somewhere between 40-50%. 6 year was around 75%. Don't fret.