Test condition 1: No zombies, no wrecks, no mods, bring car to intersection of Rosewood fire/pd, quit and save, duplicate the save. Load back in, check gas, hold W and press nothing else until at Rosewood Spiffos, at which point press space, check gas.

Test condition 2: Load from duplicate save from test condition 1, but with Project Summer Car and Realistic Car Physics active. Check gas, drive same stretch of road.

Test 1, mid condition Dart: Vanilla used 0.231 L, PSC used 0.71 L for same distance. x3.07 increase in fuel usage

Test 2, very high condition Valuline: Vanilla used .19 L, PSC used .641 L. x3.37 increase in fuel usage

Test 3, low condition Taxi: Vanilla used 0.253 L, PSC used 0.996 L. x3.81 increase in fuel usage

So fuel usage is definitely substantially higher, within the x3 to x4 range it seems. I hope this helps!