Is there any fast cars l can get for 15k that gives about 20 or more mpg.
ex.srt4 neon 22mpg

Mitsubishi Eclipse or lancer. Or even a cobalt. Any of those cars with a few mods would blow that neon away in gas handling and power.

Here is a list in order of fuel economy.
Around 30 mpg:
1) Dodge Neon SRT-4 (MOPAR Stage 2 or 3 kit makes it really fast)
2) Chevy Camaro Z28 or SS ('93-'02)
3) Mitsubishi Eclipse GS-X
4) Toyota MR-2 Spyder
5) Mazda RX-8
These two (#4 & 5) r slow, but they achieve high mpg numbers & can be fixed up to be fast. The MR-2 Spyder especially, with a turbo or more powerful engine can be made to be fast, as it is quite light.

Around 20 mpg:
6) '93-'5 Mazda RX-7 (twin turbo)
7) Nissan 300ZX Twin Turbo
8) GMC Typhoon (fast truck/SUV)