The Riley turbo rocket uses twin (manual) turbos per engine to allow
near 100% power output at high altitudes. Considering these are 290hp
engines, there is no way 24gph total is going to cut it. Probably
more like 24gph per engine.

If you have a fuel totalizer in your Arrow, think about how much fuel
you burn on climbout, and then imagine operating the engine that way
at cruise.

I'd agree that 24gph is too low. The R/R 310 has TIO540s.
The norm-asp'd IO540's on our Aztec burn 34gph total at 75% power at 4000
ft.
6000 ft is about as high as we can maintain 75% power and we're burning 27
gph.
In fact, I just finished 6.3 hours between 6000 and 8000 includeing 2
takeoffs, climbs, descents, and landings and that is exactly what the fuel
burn was, 27 gph.
Above 6000 ft. our fuel burn goes down, but of course so does our power.
10200ft we burn about 21gph.

The turbos are going to push your power up beyond 75% continually. So I'd
guess that fuel burns around 45gph would be expected if you fly with the
throttles to the wall.

Jim