I recently blew a head gasket and had it replaced.?

Then it was running cold so I took it in and he said it was the coolant temp sending so I replaced that and now the gauge is where it's supposed to be. But I am getting around 24 mpg and before I was getting 35. The bad gas mileage has been bad since the mechanic fixed the gasket. But last time I filled up I got 34 mpg and the next time I filled up I got 25 then filled up again and got 33 then again and got 23. It's a 99 Honda civic ex with the d16y8 motor. Idk what else it could be please help me idk what to do it's sucking up my gas. Before the gasket blew I was getting 35 no matter what whether it was fwy or city. And I'm driving the same roads as before And my driving habits haven't changed at all. Thank you