TR2/3/3A Vacuum advance questions

[6TTR3A]

My distributor model is 40698. My vacuum advance unit is 54412170 which has the specs as follows:
5 inch-pounds to activate the unit
11 inch-pounds that will affect the unit
7 degrees of advance which the unit provides.
However, Lucas says I should have model 54411480, which takes only 4 inch-pounds to activate the unit,
(the other numbers are the same for both models)
So my questions are: Does that one inch-pound really make that much of a difference?
and
The unit on my car is at least 31 years old. Does the diaphragm get stiff with old age (like I do!)
and in doing so change the numbers?
If I have to replace it I want to stay with the model I have because the vacuum lines are different
from mine on the "correct" one.
What do you experts say??
FRank

 

[CJD]

Vacuum advance is a mileage thing. The mechanical advance is for power. So, the worst you would see for an advance that comes in later is a very slight difference in mileage. Yes, the diaphragms do get old, stiff and often crack.

 

[Geo Hahn]

And I think that difference in mileage would mostly be when cruising at highway speeds with a light foot on the throttle. Under those conditions vacuum is strong and the engine can handle additional advance (which equates to better mileage).