Have two cars I work on regularly, one a 99 cadillac sls with a northstar 4.6 and the second a 97 GT with the modular 4.6. Both have about the same gear ratio and tire size. The caddy is much heavier than the GT. On the highway going about 75 mph the caddy gets 28 miles to the gallon, the mustang about 20. Both are running the same rpm (2000). OK I know the caddy has DOHC and the mustang only SOHC. My question is why is the engine efficiency so off on the mustang? Since we are tearing apart the engine on the mustang to rebuild it, I want to know if there is something I can do to improve the mileage? By the way the caddy is 275 HP so its not a matter of power. There has to be something everyone has done to improve things. Any comments?

 

The mustang can flow as much air as the caddy can. Port and polish the heads and just open up the air flow so it can breathe easier like CAI TB and Intake manifold etc would probably help..hell maybe even a tune...idk

 

I used to get well over 20 mpg at 2000 rpms do a full tune up.