I've been doing a lot of Daily Race C today (The Vantage one-make race at the Roval), it seems like doing a fuel-only pitstop is the fastest way to race, about 4 seconds quicker than taking tyres too, it also seemed like I was the only one to notice so my DR has shot up (so now I'm playing against people I have 0% chance of beating).
Interesting... How long does not taking tyres save then?