I am running the 6spd with 4:10 and get about 17mpg with a mix of highway and city stock and that is driving realy easy . So you aren't to far from my fuel burn. You may want to keep a eye on the smarty to see how it turns out. It has the fuel saver program .

 

I would get injectors before the box. It sound like if you are getting that kind of fuel milage, your injectors might be going junk. But if you aren't conserned about your injectors, then I would get Bully Dog box or a Edge Box or Module. You will get better fuel milage when you do one of those things. Not much increase, but a little.

 

The van Aaken box would get you a better gain than either the Bullydog or the Edge box just because how the power comes on earlier.

 

I get around 14.5 around town in or out of throttle,on highway I get 12 unloaded doing 70-75 I have 4:10 auto and 35's,in stock trim I was getting 11.5, after the upgrades exhaust and comp box it went to 14.5 and I use howes additive but just bought a case of stanadyne to try and see if it helps