Just some guy
Join Date: Jun 2001
Location: SC foothills, USA
No flak from , I love having overdrive. But.
Technically you get slightly better mileage from a four speed just because in high gear it is "locked". A solid no-slip connection between engine and transmission. Where OEM's got the idea for a "lock up" in newer automatics. Old school automatics have converters that always slip a bit, thus worse gas mileage. The reality is that the MPG difference is very small in a car that's only driven lightly.
Overdrive is great, I am addicted to it. But it doesn't make actual financial sense. The gain in mileage is really only found at highway cruising speeds. THe math has been done, somewhere, but essentially you'd have to drive on the interstate regularly for years or see gas go up to ten bucks a gallon before you'd even break even on what you'd spend on swapping to an overdrive transmission.
Me, I'm not sensible like that. I just swap in overdrive transmissions for fun.