In 1968, LJK Setright published "The Grand Prix Car, volume 3", a companion to Pomeroy's earlier work. In it, he continued the analysis of the Pomeroy Index up to the 1966 season. Being Setright, the cars he chose were often somewhat eccentric, but I have added his conclusion to those of Pomeroy.
http://www.jumbani.demon.co.uk/Grand%20Pri...formance%20.htm
The fastest car listed is the 1966 Brabham with a Py Index of 197.5. This represent the car's relative lap speed over a variety of circuits. We can use it to evaluate the performance of the fastest car of 1967, the Lotus 49 by comparing their lap times on circuits where they both ran. There are six such circuits, the following shows the amount by which the Lotus was faster:
Spa + 6.6%
Zandvoort +3.7%
Nurburgring +3.3%
Monza +4.0%
Watkins Glen +4.3%
Mexico City +5.6%
I am inclined to regard the Spa figure as not representative, a combination of an extraordinary performance by the Lotus driver and a less than typical performance by the Brabham in 1966. If we take the mean of the other five circuits we conclude that the Lotus was 4.2% faster than the Brabham, giving it a Py index of 205.8. Reference to Pomeroy's an Setright's results show that this is an outstanding improvement for an era.
In the Grand Prix Car, Pomeroy put forward his celebrated theory, that if all else is equal, circuit performance will vary with the sixth root of engine power. He could give no reason why this should be so, merely observing that it was so, based on his records of almost 50 years' motor racing. If we assume that the Repco 620 gave 330 bhp and the 1967 DFV 400, the sixth root law suggests that the Lotus should be 3.3% faster than the Brabham. It seems probable that most of the remaining 0.9% improvement came from the tyres which were developing rapidly at that time.
In 1968, the Lotus was again the fastest car, and we can compare it with its 1967 stablemate on eight circuits:
Jarama -0.23%
Spa -19.2%
Zandvoort +0.9%
Nurburgring -21.0%
Monza +2.2%
Watkins Glen +2.0%
Mexico City +2.2%
Kyalami +2.9%
I have included Kyalami because in both 1968 and 1969 the cars which ran there were basically those which had finished the previous season. We should ignore the figures from Spa, where the 1968 Lotus never ran properly, the Nurburgring, where it rained throughout practice and Jarama where Team Lotus were understandably thoroughly disorganised following two driver fatalities.
The 1968 season was notable for the rapid development of wings and the Lotus had high, suspension mounted wings at the last four races listed. If we take the mean improvement of these, we conclude that the car was then 2.3% faster than in 1967. Apart from the high wings, the 1968 Lotus had modified suspension, greater drivability from the engine and improved tyres. We can assume that the 0.9% improvement at Zandvoort came from these things and the low wings and spoilers that were then used. This implies that the high wings were worth 1.4%. I would have expected it to be more.
A 2.3% improvement gives the high-winged Lotus a Py index of 210.5.
Incidentally, the fastest Lotuses at Monza, Watkins Glen, Mexico City and Kyalami in 68/69 were driven by four different drivers, which must prove something, but I'm not sure what.