I think you have just proven the point that we can't all be expected to remember details over long periods of time.
Thats excellent work working out the speed and tyre diametres. Whilst the L34 used the 2.78 diff at Bathurst, Holden then felt with the A9X they should homologate the 2.60 diff as they knew they would be able to achieve greater HP and ulimately expect higher top speeds. What wasn't realised though was that the A9X also allowed a larger rear tyre to be fitted, which meant increased rolling diameter. With an increased rolling diameter, you would need to adjust the gearing to achieve the same result. Now if you look at the L34's racing at Bathurst, then look at the A9X's, particularly those from 1979 on the HDT cars you see a massive difference in rolling diameter. With those bigger tyres the cars were not able to achieve the speed up the hill or out of the corners with the 2.60 diff. BoB Morris once told me they tried the 2.60 diff in practice and realised the problem and changed back to the 3.08. HDT did the same thing and swapped the diff out of the promo car into Harveys car which ultimately had diff failure. One would assume the promo car had a spare race ready diff fitted as a time saving measure as they swapped the whole unit out. I was lead to believe it was an issue with the breather on the diff that lead to the failure.
You guys are unreal, tell me you never kept a secret (told a lie when put on the spot), done so much with your life you've forgotten trivial bits and fudged it or simply just didn't want someone to know the "truth" behind a story and given "your" account OR simply did not want to get dragged into SOMEONE ELSE'S SHIT FIGHT.
Never forget... never let the truth get in the way of a good story.
RIP PB.
Re tyre sizes as per Red's post above...
Not 1979, but from a road test of Garth Wigston's Roadways A9X hatch.............."front tyres were 23" rolling diameter, rears were Dunlop 418s , 12" wide, 24.2" rolling dia., ...the latest wheel arch flares will accommodate up to 25" if necessary....."
No date on the mag, but it has a full page ad for the new UC Hatchback with RTS on the back cover...so assuming its mid '78.