There are a number of things that affect ecconomy, one clue is whether the ecconomy dropped after having work done on the car.
Ignition timing - sometimes timing gets set at 10 degrees without the diagnostics link in place leaving thetiming retarded by 5-10 degrees. Unlikely to change on it's own unless the distributer is loose. Some ECU error codes also result in 'safe mode' retarding timing.
Fuel mixture - Poor ecconomy would usually mean too rich, unless it's so massively lean it's misfiring. Fuel filter would only make the mixture weaker so unlikely.
Rich mixture would usually be a failing lambda (O2) sensor, but could also be a faulty engine temperature sensor (ECU one is separate to the gauge one) or a leak in the exhaust system between engine and sensor.
If the new fuel pump is a high capacity aftermarket one, this will raise the fuel rail pressure slightly which will richen the mixture.
A few other possibilities:
Broken up cat or silencer baffles restricting the exhaust.
Vac pipe from manifold to fuel pressure regulator blocked / cracked / come off.
Vac pipe from manifold to turbo pressure senor (MAP sensor) blocked / cracked / come off.
If it's a 185 or 165 then splits / leaks in any of the intake pipework will cause mixture errors.
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