5-Speed vs 6-Speed NA/NB Miata Transmission

Posted by Zachary Visker on

Do I need a 6-speed? How much power can a 5-speed hold? I want a 6-speed because of the overdrive. 

One of the more common questions/concerns we have had during our used parts sales. Here we are going to lay out the details of a 6-speed NB transmission the pros/cons of the transmission and what to expect when installing one. 

Gear Ratios

The major difference between the 5 and 6 is an extra gear. Commonly assumed as the 6th being an overdrive over the 5, which is wrong. One of the more surprising facts about a 6 is the gear ratio of the 6th is actually higher then the one of the 5. Simply explaining that if you install 6spd into the same car with a 5spd your top gear will actually have the RPMs higher at a set speed compared to the 5th. Generally a 6-speed is complimented with the matching factory 3.9 diff to relate the gear ratios to a 5-speed/4.3/4.1 setup. 

Flyin Miata Gearing Calculator has an awesome representation to compare the transmission, differential and RPM/speed targets. 

Stronger Transmission

The strength of a transmission has many variables which include, torque curve of the engine, fluid condition, driver abuse, etc. Now assuming the variables are constant the reason many especially high horsepower Miata's upgrade to a 6-speed is they can hold the power better. While there isn't 100% factual proof that is true, in forum responses and personal experience I find the 6-speed seems to be more stout. Some examples of that may be that Mazda used a variation of the 6-speed in the Mazdaspeed which made more power then a natural aspirated Miata. 

Another indicator is the transmission casing notice how the 6-speed has more ribs in the casing compared to the 5-speed. 

As for power holding this is strictly an estimate. Which again varies based on how the car is driven and condition. But the 5-speed under moderate use can likely hold up to 200whp. the 6-speed is likely closer to 300whp.

6-Speed Above

5-Speed Below

The debate will continue on but for strength indirectly most agree the 6-speed seems to be able to hold more power then the 5-speed. 

Compatibility

90-05 Miatas are essentially Legos. This really applies to the drivetrains on the 90-05. The beauty of the 99-05 6-speed is it will fit any 90-05 engine. One small modification to the starter bellhousing on the 6-speed to fit onto a NA engine is needed. A 6-speed shifter is necessary. Otherwise a 6-speed will fit with any orientation of 1.6/1.8 engine, 1.6/1.8 clutch kit, open/LSD differential, etc. 

Availability

The 6-speed for the NA/NB only as available on 99-05 Miata's. 

NB1:

  • 1999 10 Year Anniversary
  • 2000 Special Edition

NB2:

  • 2001-2003 Special Editions
  • 2004-2005 Mazdaspeed
  • 2001-2005 Optional Feature

Pricing on the transmission vary with condition, mileage, version, etc. 

Do I need a 6-speed?

There are many reasons to consider a 6-speed. Starting with power holding. As stated above. Use your discretion and please use high quality GL-4 fluid. 

Another reason would be acceleration I would consider the 6-speed to be more of a close ratio 5-speed with an extra gear. Driving a stock NB 5-speed car vs 6-speed car. The latter seems to be slightly more peppy. That likely could be placebo but it defiantly feels that way. Putting in a 6-speed with a 4.3 diff would be quiet an experience. 

Overall I hope this educates everyone on the 90-05 6-Speed and helps the decision on if a 6-speed is necessary. 

 


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