Fuel Economy and diff ratio

[Dr Terry]

I was thinking that ther might be some slip in the auto? Why would the diff on the auto be 2.78 and a manual in the same model be 3.36?

All torque converters 'slip' when they are working, this why the more modern units have lock-up type converters for improvements in economy on cruise etc.

Most older Holdens have a taller rear axle ratio for auto vs manual transmissions, because (as mentioned) it gives the clutch a chance to survive on take-off & also the converter gives a multiplier effect & launches much better than the manual.

Dr Terry