I have observed that utilitarians will attempt to fudge the numbers to make the utility calculations come out the way they "should" inventing large amounts of anti-epistemology in the process

Welfare economics is the clearest example. It's the closest thing that exists to a rigorous formalization of utilitarianism. Yet economists of all ideological stripes have no problem at all coming up with welfare-economic arguments in favor of their positions, whatever they are -- and despite the contradictions, all these arguments are typically plausible-sounding enough to get published and win a group of adherents.

(Also, unsurprisingly, as much as economists bitterly disagree over these ideologically charged theories, they all just happen to imply that learned economists like them should be put in charge to manage things with their wisdom and expertise. Small wonder that Austrian economists, who are pretty much the only ones who call bullshit on all this, are so reviled by the mainstream.)