On my 06 Pony the shift points with the overdrive on are lower than if I have the overdrive off.
I thinks it's a nice option since driving around town I like to have it off and enjoy a little more acceleration from light to light. Even while driving at 55 between towns I think the car is more responsive since it does run at a higher RPM.
I'd like to find a way to switch the default setting from always on at restart to always off. Then I could chose when to turn it on, rather than always having to turn it off.
It lets the car rev higher before shifting gears when it is off. If your highway cruising, for example, it will let you travel at the same speed at a lower RPM when it is engaged.
i dont know if its true but i would think driving with it off you would get worse gas mileage in town because the engine stretches out the points where it would normally shift (to lower rpms in a higher gear) letting you hit a higher rpm range which i would take to mean burning more gas. i definitely notice a power difference driving with it off especially on those borderline points where it usually shifts - instead of punching it and waiting till the car shifts into a higher gear at lower rpms, you are already at higher rpms and just take off. i dont know if that was true or technical but thats what ive seemed to notice, correct me if im wrong