My input thus far testing the "velocity tuner:" (V6 Commander)
I think I drive 8 to 10 miles a day at a "constant" speed of over 40 mph without having to brake for lights or stop signs. Other than that, almost ALL of my driving would be city driving - and waiting a some rather long red lights.
The velocity tuner might work - I "was" getting 12 - 13 before. Now I'm getting more 14 - 15 readings from the on-board computer (people say that the mileage will get better over time, so that might be it as well). I also have installed a K&N High Flow Air Filter. I think they "help" with mileage, but they are not nearly as good as advertised. And, the directions on how to install the velocity tuner (came with the unit) were very hard to understand. I have tried a number of different places and I have turned it around a few different times to see where I get the best performance / gas mileage. I did notice (for real) that the engine seemed to have more power with the velocity tuner and the K&N filter installed. I noticed better response / power before the filter came in as well, so there might be something to the velocity tuner and power. And I like the K&N filter (lifetime filter). They cost around $50 on-line if you add in the cleaning / treatment kit.
Last week I did more open road driving and I did notice that the average mpg was over 15. However, I have never gotten better than 15.5 mpg reading from the onboard computer.
But keep this in mind: I bought my wife a Scion Xb - claims to get 30 city and 34 highway. She has almost 500 miles on it with an average mpg of just over 24 mpg - with her driving being a lot like mine (mostly city). Those window stickers are ALWAYS high. Seems to me that if you turn off your air conditioner, close your windows, slowly build your speed on a FLAT road to 55 mile per hour and stay at that speed for an hour, you "might" see 18 - 20 mpg with your commander (in my opinion).
But - I love my commander.
I drive the Xb every now and then, it is nice. Xb was cheaper to buy (about $5,000 less after all the incentives from Jeep and after all the add-ons from Scion). But I like the Commander a LOT better. Better ride, more comfortable.
I'll post more as time goes along. Friends of mine who paid $40,000+ for Chevy or Ford SUV's are getting about the same as me - even less mpg's. And they paid a LOT more for their cars out the door than I did.
All for now