i had the same issue when i had my SPI. it was an 01...got it in 05'. it had the original fuel filter, air filter, spark plugs, wires, it was uuuuugly. was getting in the low 20's for MPg. Once i replaced all of those i was getting about 28-30 mpg. oh ya, if your egr hasnt gone out yet keep an eye on that. it's famous to break on the SPI.
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Mods - FS Werks Cool-Flo,CFM Crankshaft pulley/WP Kit, obx header/flex, FS Werks Stealth Flo Muffler with 2.75" straight pipe, & Tom's Tune SCT. FRPP kit & svt front sway coming soon!