6th gear change 2005 MazdaSpeed?

How practical would it be for someone who knows transmissions to change 6th gear in my 2005 MX-5 to be more of a highway overdrive
gear, say 2500 rpm at 60mph rather than stock 3000 rpm. Seems such a waste of rpms to be cruising along at 3000 rpm when my 2001 Eldorado can do it at 1500 rpm and get 30mpg in the process. Thanks
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I don't think a taller sixth gear is available. You could swap the differential gearset for the 3.636:1 from an Australian 6-speed MX-5 (part #MA02) if you can find one.
But search the miata.net forum for definitive answers.
--
Lanny Chambers
St. Louis, MO
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It was possible to do this with the 5 speed. (Done that!) Never heard that parts were available for the 6 speed. In any event, the ring and pinion change is more effective. The speedometer calibration can be done by repositioning the needle without changing the transducer, or the transducer or transducer gear can be changed. 3.6 is a bit much, due to the low torque at low rpm of the Miata 1.8. 3.9 is a better match for "normal" use with an unmodified engine, and parts are easier to obtain. The Miata engine is, by design, a high revving engine. The usual American car engine develops quite a bit of torque at much lower RPM than the Miata 1.8, and generally does not get near the ~ 7000 RPM red line of the Miata engine.
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3.9 is what comes with a 6-speed, so he has one now. Note that he has a Mazdaspeed Miata; the OEM turbo will probably handle a 3.6 without a problem.
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Lanny Chambers
St. Louis, MO
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wrote:

The MSM has a 4.1 final drive- I guess to offset the larger tires. A switch to the much more common (than 3.6) 3.9 rear ratio will give about a 5% reduction in rpm vs speed. The RX-8 might use the same gearbox internals and have a different 6th gear ratio.
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Ah. I didn't know that. Thanks.

More likely, to make them seem actually quick, without dipping too deeply into the parts bin.

That's only a 150 rpm drop at 60. Doesn't seem worth doing.
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Lanny Chambers
St. Louis, MO
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wrote:

The MSM has a turbo, intercooler, 17" wheels, dunno about the brakes. But I'd say they dipped fairly deep into the parts bin.

Nope, it don't. Even the taller gear, the 3.63 only drops the rpm about 12%. The Miata engine doesn't have the power curve to drag the car along at 2500/60 mph.
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On Mon, 06 Jul 2009 18:20:36 -0400, Chas Hurst wrote:

The MSM does. I have one.
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What I can tell you is that my 99 (turbo,SBB,with 5th gear mod & 4.11 rear) can be annoying at times, due to the 5th gear change. Seems that around 45 mph, which is a common "speed limit" around here, 5th is a bit too high, and 4th a bit to low. Other than that, the gear change was useful, in that it helped reduce the noise at normal highway speeds, and may have helped slightly with fuel consumption during cruise. The tires, while larger than stock, are within 1% or so of the stock diameter.
One local club member has changed a stock NA to a 3.6 ratio, in order to save fuel. He was not concerned about other preformance losses. I haven't heard much about his satisfaction, other than the rear end is/was a bit noisier than it was before.
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Chuck. would love to hear from this club member about his satisfaction and about what rpm at what speed, he is seeing.
Chris 99BBB
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So would I. One of the problems is that the Miata speedometer and tachometer often are set from the factory to over read. In the process of doing a bunch of mods on my 99, we reset the speedometer and tachometer to be as acurate as we could. The speedometer now overreads by about 1 to 1.5 mi at 60 to 70 mph, and the tach about 150 rpm at readline. (My Garmin marine GPS and other instruments that can read the ECU digital output only read in one mph increments.) (I can change the rep rate on the marine unit, unlike the inexpensive automotive GPS units. The re "calibration" was done after the 5th gear change, and I did not (oops) record the values before we readjusted things. What I remember was that the speedometer was originally set by the factory right at the 5% high limit for 70 mph. I believe, but cannot swear to the tach before "calibration" was off in a similar manner at readline. (~350-375 rpm)
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