Transmission Introduction -

I chose to go with a 5-speed transmission for my swap. Wade, the trail-blazer on the Miata-into-Roadster swap, used the 6-speed. His route has two major advantages. (1) The stock Roadster differential has the same gearing as the differential on the 6-speed Miata. (2) The 6-speed apparently has a much narrower tail section than the 5-speed. Point (2) is open to debate. The first-generation (NA) Miata 5-speeds may be narrow, too. It just so happens that folks who sold me my transmission shipped me a second-generation (NB) 5-speed. It bolts up more or less correctly, but it sure is bulky in the back. I had to cut a lot out to fit over the X-member. I also went with a different way of mounting the transmission to the frame. Wade drilled two holes in the X-member, with the body off, then sandwiched the X-member with bushings from a Volvo 122. I drilled two holes in the X-member, with the body on, then bolted a custom mount to them. The end result is a little cleaner looking and, if I've measured correctly, places the center of the Miata output shaft at the same height as the stock Roadster output shaft. Apparently Wade had to adjust his pinion angle to accommodate the greater output-shaft height that his bushings produced. The sole disadvantage to my approach is that the transmission is mounted too close to the X-member to fit a mechanical speedometer drive and cable. In fact, even the rather small electronic drive only fits with some shaving. If the transmission is mounted higher, like Wade did, you can use the mechanical drive if your Miata transmisison comes with one -- the Datsun Roadster speedo drive screws right in. If you have a later Miata transmission with an electronic sender, you can pull the mechanical drive out of an early RX-7 (along with the worm gear off the output shaft of the RX-7) and that will work, too. My transmisison sits so low that even the electronic drive barely fits. I may end up using that stock sender, slightly modified to fit, or an aftermarket inductive vehicle speed sensor, like the one Dakota Digital manufactures, which is widely available for about $50. For the time being, I've simply memorized the speeds for each gear at 3,500 RPM and let that and the surrounding traffic be my guide.