Car much slower with heavy fuel?

Is the setup so heavily dependent on tires and fuel load?
I ran tests to prepare the setup for Bahrain before messing with my race strategy.  My driver qualified 6th, but then was 6 seconds off the pace in the race!!! (I notice top speed was 15km/h lower than what it should've been). As soon as I pitted with hard tires again, and 61 L of fuel, the car gained 15km/h top speed. While doing test runs, the fuel strategy was set for 61L. I started the race with 74L (which I thought I would also qualify with).

What's up with that?  Can anyone explain to me what I did wrong, and what the proper setup procedure is?

thanks in advance! :)
-Dave.

You should think about possibly running more stints with less fuel as your overall time will improve. Tire longevity is less of an issue as well. I'm guessing you ran 61L because your tech director recommended it. Most Tech directors are completely wrong, the rest are so wrong they should be executed.

Fuel is crucial in this game. If you're going to run hards you never really want to run anything heavier than 50-55L.

I tried fueling only at my starting stint and not refueling throughout the race (What F1 nowadays does), I ended up 10-15 seconds off the pace... Never trying that again...

Yeah more fuel just doesn't sound right at 1st when you think about it.

True Tyson, it's a waste of money to have a top level Technical Director.