Differential Ratio selection
Page created by unknown Feb 26th 2005:
 Differential Ratio Selection:
Here is a quick guide to help you decide which differential ratio will best suit your needs based on an automatic, 3 or 4spd transmission
please be advised that this is a guide only and is to be taken and used at face value
2.60:1 - Super highway economy. Slug off the mark.
2.78:1 - excellent highway economy. sluggish off the mark
3.08:1 - Good economy. Average off the mark.
3.36:1 - Compromise between acceleration and economy.
3.55:1 - Good off the mark, average economy.
4.44:1 - Great off the mark. Flat out at 120 kp/h
 Automatic Gearbox Ratios:
On many occasions GMH fitted the next lowest diff to the automatic
versions of certain models with acceptable results.
i.e If the manual version came standard with a 3.55 diff,
the automatic version would have a 3.36 diff.
 Basic and often mis-understood concepts:
A tall diff is a smaller number eg: 2.60 ratio.
a short diff is a larger number eg: 3.89 ratio.
Tall diff = longer legs, higher top speed, less RPM at cruise, slower off the line.
Short diff = shorter legs, lower top speed, more RPM at cruise, quicker off the mark.
 Diff ratios:
Holdens with standard 14" rear wheels and tyres HK-HZ (say 195/75 SR 14)
will initially display the diff ratio on the tacho at exactly 100kph in top gear (1:1) in a Trimatic.
i.e a 3.55 will display 3550 rpm at exactly 100kph
Different diff ratios with 205/60/13 rear tyres:
2.60 = 41.8 Km/h/1000RPM or 2390RPM @ 100 Km/h
2.78 = 39.1 Km/h/1000RPM or 2560RPM @ 100 Km/h
3.08 = 35.3 Km/h/1000RPM or 2830RPM @ 100 Km/h
3.36 = 32.3 Km/h/1000RPM or 3100RPM @ 100 Km/h
3.55 = 30.6 Km/h/1000RPM or 3270RPM @ 100 Km/h
3.90 = 27.8 Km/h/1000RPM or 3600RPM @ 100 Km/h
 Speedo Driven Gears:
(The plastic gears driven by the gearbox and which the speedo cable connects to)
(Speedo Driving gears have similar colours but fewer teeth and are fitted to the gearbox tailshaft.)
For 13" wheels:
2.60 diff Pink 19 teeth
2.78 diff White 20 teeth
3.08 diff Dark Blue 22 teeth
3.36 diff Red 23 teeth
3.55 diff Yellow 24 teeth
3.89 diff Light blue 25 teeth
For 14" wheels:
2.78 diff Pink 19 teeth
3.08 diff White 20 teeth
3.36 diff Green 21 teeth
3.55 diff Red 23 teeth
3.89 diff Yellow 24 teeth
The above are all varied again by tyre and wheel sizes.
A 205/60/13 tyre with a 2.78 diff is about the same overall gearing as a 275/55/17 tyre with a 3.55 diff.
A 2.78 diff in a Torana with standard 13" wheels and tyres (say 185/75 SR 13)
will initially show 3080 rpm at exactly 100kph in top gear (1:1) Trimatic
(Note, the next lowest diff, 3.08).
A 3.08 diff in a Torana with standard 13" wheels and tyres (say 185/75 SR 13)
will initially show 3360 rpm at exactly 100kph in top gear (1:1) Trimatic
(Note, the next lowest diff, 3.36).
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