makes sense... now my question is why am i getting 13mpg's on the highway...

my gryphon is set to 87 towing (canned... cant afford custom ones yet, plus im gonna do some more upgrades) and I previously had it on 91 perf, and both have shown about 13 mpg's... I measured the miles with the odometer and divided by gallons used when i filled a full tank each time, and the gryphon showed an average of 15ish. granted I havent had them on for very long, but the time they have been there they havent done anything for mileage

... any ideas? I did just put on a cat back gibson exhaust and that helps with weight and sound... but idk if it will do much for mileage...
any ideas? I am thinking of doing E-fans, and UDP's but those will wait for a bit, had to replace tires this week so im kinda strapped

thanks for the help.