Fuel Economy and diff ratio

[Taily]

I think that tyre size wouldn't make any difference because you are comparing the engine revs to the final drive at the back of the gear box. It would work regardless of wheel or tyre size. Wouldn't work with a 5 speed ( over drive more tha 1 to1 final drive) wouldn't work if the cog on the end of the speedo cable was incorrect (speedo would be wrong to, I took the GPS to check that) and I think the final drive of a auto might be less than 1 to 1. Still think your a genius Beachy because I've been trying to work out my Diff ratio for a while. Was hopping that HK might give us a technical reason why this works.

Far as I know Frosty all late 60's thru 70's auto and 4 speed manual Holden stuff is 1:1 top gear (not sure about powerglide and early Hydramatrics though).

[Frosty]

Far as I know Frosty all late 60's thru 70's auto and 4 speed manual Holden stuff is 1:1 top gear (not sure about powerglide and early Hydramatrics though).

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?

[Taily]

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?

Fair points. The speedo drive is always on the output shaft of any box. Any slip would be in the converter, which would explain why you get a slight flash in rpm when you start to accelerate from an already steady speed in top gear. In a trimatic for example, the band is completely disengaged, the 2nd and 3rd clutch packs are buttoned down driving the output shaft at the same speed as the input shaft (otherwise there would be no engine braking on decell either). The diff ratio issue would be more to do with first gear start-off in a manual (think hill start and trying to reverse with too tall a diff ratio) - or she would shag the clutch quick smart.

[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