My '00 Mustang has around 55,000 miles on the odometer.

Lately he has been running kinda rough (not to mention 10 mpg city) so I took it to the local mechanic who said that Mustang needs replacement of spark plugs and cables at minimum.

Isn't 55,000 miles kinda early for tuneup? The book doesn't recommend it before 100,000+ miles.

 

The car IS ten years old. At the very least change your fuel filter, clean your throttle body, and maybe have the injectors cleaned. Maybe pull a couple of the plugs and investigate for any excessive carbon buildup on the electrodes.

 

I use the Gumout Regain when mine older rig gets kind of loaded up. Works within a quarter tank. Then I run it pretty hard for the rest of the tank. Might try it.

 

Don't go by the book. Fuel filter needs replaced every 24,000 miles and I would probably replace the plugs at every 24,000 and wires every 48,000. Oh....and do a radiator flush every 24,000 also

 

Seafoam the upper intake. The pcv system on the 3.8 pulls and condenses excess oil inside the upper intake manifold. Put one can in the tank and one can cleaning the upper intake.
Keep a clean fuel filter and air filter.
Plugs.