I would guess 12-16 mpg would be closer to normal with a 235.
Are your brakes dragging?
Is your clutch slipping?
Carb or fuel line leaking?
Tires inflated properly?
The above items are the easy ones. A carb rebuild may be in order. It is possible to get poor economy with a bad carb, and still have the motor run smooth.
How are you measuring mileage? Are you using the dodmeter, or are you using a gps. It is possible that your speedometer/odometer is lying to you.
