Pre-Planning the EcoBoost Swap: Transmission

After talking about how straightforward the engine choice was, the transmission decision was the product of far more research than I really want to admit. I started with my spreadsheet with gear ratios and wheel speed calculations that’s nearly a decade old–when the ST hadn’t yet hit dealer lots, and I was comparing it to gear ratios on my Mazda6 and Taurus SHO. At the time, I thought the ST could have used taller gearing in general, and even after driving one, I didn’t change my mind.

That spreadsheet considered 28 different transmissions, all capable of bolting up directly to the EcoBoost engine. It includes a few automatics, where I was getting a feel for how Ford geared the engine in the Escape, Explorer, and Taurus, as well as a number of foreign market variants mated to various tiny diesels abroad. The first two most obvious candidates were the ST’s MMT6, and the MTX75 used in the base Focus. Overall, I like the MTX75 better, but would have likely gotten terrible fuel economy and tired of the fast-revving engine with the gear ratios it shipped with. For reference, the ST’s MMT6 would put me at just under 3000 RPM in 6th at 80 mph, the base Focus’s 5-speed would have the engine at almost 3300 RPM. It’s not crazy high, less than 100 RPM higher than my Mazda6’s V6, but like that setup, a taller top gear would have netted better fuel economy without necessitating tons of downshifts in highway driving.

I’ll spare all of the details, but I ended up getting an MTX75, but not from a 2012+ Focus. Enter the MTX75 from the 2004-2005 2.3L PZEV Duratec Focus, with a taller 5th gear and final drive ratio. That same 80mph cruising speed will now be at under 2800 RPM–still quite a bit higher than in the Escape with the 6-speed auto, and I wouldn’t have to have it shipped in from somewhere in Europe. First through fourth gears have the same ratios as the base 2012+ Focus, so with the taller final drive max speed in gear goes up by 4, 7, 11,15, and 29 mph. All good so far. Relative to the MMT6, first gear is just barely taller, second picks up 5 mph (getting comfortably past 60 with one shift), and overall they’re spaced a lot more evenly. I also couldn’t decide whether Ford really thinks it’s OK to flat tow the MMT6 (i.e. behind a motorhome). Some do, the owner’s manual doesn’t distinguish between the two manuals, but the towing guide doesn’t list the ST as being flat towable. Kind of like where I was trying to figure out how big the ST’s fuel tank was as they were starting to hit dealer lots.

Another local salvage yard purchase, I got the PZEV MTX75 for $350.

Update: After running into the shift cables and exhaust trying to occupy the same space (with an Escape downpipe), I decided to take a step back and get an MMT6. Found one from an ’18 with low mileage for $500, plus $100 for ST axles and another $150 or so for freight. See *this post* for more on that.

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