It's pretty common for a tune to increase MPG because they usually advance the timing across the board.
More timing = more HP, which means you can use less fuel to keep the vehicle moving at part throttle.
The downside? It increases emissions, which is probably one of the main reasons the factory doesn't do it.
More timing = more HP, which means you can use less fuel to keep the vehicle moving at part throttle.
The downside? It increases emissions, which is probably one of the main reasons the factory doesn't do it.
Anecdotal observations of people who've had the tune for a "few days...seeing several more MPGs as well." Are absolutely unconvincing to me. In fact, they sound like shills. Especially since after being tuned I would expect you would be leaning on it more than usual- especially in the first few days.
The only way to increase mileage by this much (especially in mixed case driving) would be if you had been running bad low-octane gas such that the correction maps were dialing back timing significantly. Now, you put a tune on it, zero out the maps, add a good tank of higher octane fuel. Maybe. And only because the fuel mileage was being negatively impacted prior.