Aged tyres = LRR ??

[D.O.G.]

G'day All.
I've had a jump in fuel economy in Zed over the last two tanks that has me wondering why.

My 3 tank average was 21.13mpg, then I put an old pair of tyres on the front of my car halfway through the next tank.
That tank returned 22.17mpg, a 4.9% increase and my best tank up to that point.
My latest tank (first full tank with these tyres) returned 23.18mpg, a further 4.6% increase, for a total of 9.7% from the old average.

These tyres still have lots of tread but the rubber is well cured (hard), could the rolling resistance have decreased as a result? Could it make that much difference?

Please understand, these tyres aren't unsafe. I can still push hard through a roundabout without under steering.

[CapriRacer]

I'm not a rubber chemist, but I would think that aging rubber would make the internal friction (hysteresis) higher and that is the largest contributor to rolling resistance. I would tend to think this would be a small change and certainly not as dramatic as you have posted. So I tend to think that the answer lies elsewhere.

One possibility is that given the wide range of values of rolling resistance within a given size, it is much more likely that the difference is caused by the inherent difference between the current set and the previous set.

[ConnClark]

Seeing that you stated that these tires have a lot of tread on them, I would say the increase in mileage is probably due to the belts not being worn out. When they are newish they are stiff and keep rolling resistance down. I noticed this when I had to throw on my spare recently. ( sadly it was 25 years old and shed the cap two weeks later )

[CapriRacer]

Opps, wrong thread

[D.O.G.]

Thanks for the replies.
Either (or both?) of you comments could be right.
Now I find myself looking forward to these tyres wearing out, so I can try a set of four "real" LRR tyres.

I filled up today, for a result of 23.59mpg. Another small increase, maybe as I get used to the better coasting?