Get a better idea of your MPG first so you have some hard date to go back to the dealer with. Fill up the tank, and calculate MPG. Take it back to the dealer if the MPG is clearly bad.
Here is the recall, it was supposed to IMPROVE performance, not retard it:
Recall ID # 08V429000 - FUEL SYSTEM, GASOLINE

ELIVERY:FUEL PUMP
Recall Date: AUG 25, 2008
Component: FUEL SYSTEM, GASOLINE

ELIVERY:FUEL PUMP
Model Affected: ELANTRA
Potential Units Affected: 65000 Recall Details
Recall Date:
AUG 25, 2008
Model Affected:
2008 HYUNDAI ELANTRA
Summary:
HYUNDAI IS RECALLING 65,000 MY 2008 ELANTRA VEHICLES WITH 2.0 LITER BETA ENGINES. THE FUEL PUMP MOTOR MAY NOT HAVE THE PROPER UNIFORM INTERNAL ELECTRICAL SURFACE CONTACT. THIS MAY CAUSE THE FUEL PUMP TO PRODUCE LESS PUMPING PRESSURE THAN REQUIRED TO SUPPLY THE FUEL INJECTION SYSTEM.
Consequence:
POOR ENGINE STARTING AND HESITATION MAY RESULT. OTHER ENGINE PERFORMANCE ISSUES MAY ALSO RESULT INCREASING THE RISK OF A CRASH.
Remedy:
DEALERS WILL REPLACE THE FUEL PUMP SUBASSEMBLY FREE OF CHARGE. THE RECALL BEGAN ON SEPTEMBER 15, 2008. OWNERS MAY CONTACT HYUNDAI AT 1-800-633-5151.
Potential Units Affected:
65000
Notes:
HYUNDAI MOTOR COMPANY