Techno’s

 “Know your car” Series # 8

 

Mazda MX-5 Gear and Differentials Ratios

 and Tyre Sizes

 

Introduction

The engine is connected to the gearbox, the gearbox, via the tail shaft to the differential, and the differential via the axles to the wheels. Along this drive train there are gears which change the rotation speed of the next component.

 

The Australian delivered manual MX-5 has come in a variety of drive train combinations. There have been 14, 15 and 16-inch wheels, 4 differential ratios, 3 differential types (open, viscous and Torsen), 4 tyre profile heights and two different gearboxes. The combinations of these are shown below.

 

Gearboxes

There have been just 2 gearboxes, the 5 speed and the 6 speed. The 5 speed, introduced in the 1989 NA6A (1.6 litre Aug 1989 - Sept 1993), continued on to be the standard gearbox through the NA8A (1.8 litre Oct 1993 to Nov 1998) and NB8A (1.8 litre Dec 1998 to Oct 2000). The only exception being the NB8A anniversary models which had the 6-speed gearbox.

 

The 6-speed gearbox became standard in the NB8B (1.8 litre (Nov 2000 to present)).

 

Tyre Sizes

The NA6A and the NA8A all sported 14 inch wheels. The 1.6 had a 5.5-inch rim and 60-aspect profile tyres, the NA 1.8 litre came with 6-inch rims and 55 profile tyres.

 

The NB8A wore 15-inch diameter and 6 inch wide wheels with 50 profile tyres.

 

The NB8B introduced the 16-inch, 6.5-inch width wheels and 45 profile tyres.

 

Although the rim sizes increased from 14 to 15 to 16 inches, the profile of the tyre reduced from 60 to 45. Lowering of the tyre profile reduces the circumference of the tyre. The tyre profiles and sizes are detailed in "Know Your Car Series 5".

 

Differentials

The higher the ratio number the greater is the number of turns of the tail shaft to turn the wheels once. For example, a 4.3:1 ratio, as in the NA6A models, requires 4.3 turns of the tail shaft to turn the wheels once. The 1.8 litre cars in the NA and early NB models had a 4.1:1 ratio; the NB8B had a 3.636:1 ratio. Once again the NB8A anniversary models were different; having a limited slip differential with a 3.909:1 ratio. Limited Slip Differentials available in the 1.6 litre models were viscous LSD's. The viscous LSD contained a silicone fluid. When one wheel began to spin it caused the fluid on that side to spin and in doing so becomes thick and transfers the motion to the other side of the differential. The 1.8 litre cars came with Torsen type differentials (4.1:1 in NA8A models and 3.909:1 in the NB Anniversary 6 speed models). A Torsen LSD uses parallel helix gears to keep the torque balanced to each drive wheel.

 

The NB8B models have a 3.636:1 ratio open differential.

 

On the Road

The various combinations of gearbox and differential ratios when combined with tyre sizes require differing engine rotations to cause the car to travel a particular distance. Table 1 shows the engine (crank) rotations and the distances travelled for combinations of gears, differentials and tyre sizes.

 

 

Table 1 Crank Rotations, Tyre Sizes and Distance Travelled

Mazda MX-5 Australian Delivery

Gear, Tyre and Differential Data

Gear Ratios

NB8B

NB8A 10 AE

NB8A

NA8A

NA6A

1st

 3.760

 3.760

 3.136

 3.136

 3.136

2nd

 2.269

 2.269

 1.888

 1.888

 1.888

3rd

 1.646

 1.646

 1.330

 1.330

 1.330

4th

 1.257

 1.257

 1.000

 1.000

 1.000

5th

 1.000

 1.000

 0.814

 0.814

 0.814

6th

 0.843

 0.843

 

 

 

Diff Ratio

 3.636

 3.909

 4.100

 4.100

 4.300

 

 

 

 

 

 

Wheels

NB8B

NB8A 10 AE

NB8A

NA8A

NA6A

Tyre Width (mm)

205

195

195

195

185

Rim Inches

16

15

15

14

14

Tyre Profile

45

50

50

55

60

Tyre Circumference (M)

 1.85637

 1.80956

 1.80956

 1.79102

 1.81458

 

 

 

 

 

 

Wheel Rotations Per Kilometre

 

NB8B

NB8A 10 AE

NB8A

NA8A

NA6A

 

 538.687

 552.621

 552.621

 558.340

 551.091

 

 

 

 

 

 

Meters Travelled per Tail Shaft Rotation

 

NB8B

NB8A 10 AE

NB8A

NA8A

NA6A

 

 0.51055

 0.46292

 0.44136

 0.43683

 0.42200

 

 

 

 

 

 

Crank Rotations Per Kilometre Grey = 1:1 Gear

 

NB8B

NB8A 10 AE

NB8A

NA8A

NA6A

1st

 7,364.58

 8,122.34

 7,105.38

 7,178.92

 7,431.35

2nd

 4,444.21

 4,901.49

 4,277.73

 4,322.00

 4,473.97

3rd

 3,223.96

 3,555.68

 3,013.44

 3,044.63

 3,151.69

4th

 2,462.04

 2,715.37

 2,265.75

 2,289.20

 2,369.69

5th

 1,958.66

 2,160.20

 1,844.32

 1,863.41

 1,928.93

6th

 1,651.15

 1,821.05

 

 

 

 

The interesting feature in this table is the impact that the LSD and 6 speed transmission has on the Anniversary NB8A's performance when compared to the standard NB8A. Little wonder that the acceleration rate of the 10th Anniversary model is unmatched by it's 5 speed, non-LSD NB8A counterparts.

 


Graph 1 shows the distance travelled for one rotation of the crank in the different gears of the Australian delivered manual MX-5's.

 

 

 

The information contained above relates to Australian delivered manual transmission Mazda MX-5's and based on calculated tyre circumferences. Different brand tyres will alter the tyre circumferences by small margins.

 

Safe journey

 

 

 

 

Rob (Techno) Spargo

Mazda MX-5 Club of Victoria

November 2003