The 2010 Tour de France winner's average was 39.585 kph.

Subtract from that what you feel is a reasonable estimate for the increase in speed brought about by 50 years of better nutrition. Subtract from that what you feel is a reasonable estimate for the increase in speed brought about by 50 years of better training techniques and physical conditioning. Subtract from that what you feel is a reasonable estimate for the increase in speed brought about by 50 years of improved French roads (fun fact: it was not until 1964 that the Tour was held entirely on pavement!) Subtract from that what you feel is a reasonable estimate for the increase in speed brought about by 50 years of improved team tactics, race radios and computers. Subtract from that what you feel is a reasonable estimate for the increase in speed brought about by 50 years of improved performance enhancing pharmaceuticals (hey, I'm just sayin'.)

Do you have a new, lower average winning speed in mind?

Now subtract the actual winner's average speed in the 1960 Tour de France from your number. BTW, that 1960 average winning speed was 37.210.

The difference left is your estimate of the difference that 50 years of improved bicycle technology has provided.