Tires, Tire pressure and Gas Mileage

[Pinecone]

Have been seeing some interesting numbers on the Fiat. I run stock 16" wheels with the stock Pirelli P7 All Seasons, 195/45-16, in the winter. And the optional 17" rims with Pirelli P Zero Nero (Max Performance on Tire Rack), 205/40-17, tires for the summer.

I have been exclusively driving it, and with a pretty consistent mix of highway and city.

The All Season tires gave a combined gas mileage of around 35 - 36 MPG.

Going to the Summer tires dropped that to under 30 MPG. Then I checked the tire pressures and found them around 30 psi, versus my normal 40. After getting them back up, I am seeing around 32 MPG combined.

So around 10% difference between the summer and winter tires. And around 6.5% difference for tire pressure change of 10 psi.

I had not thought that the tire difference would be that big of a change in gas mileage.

Just thought I would share.

[Plaz]

I notice that as well... I average MPGs in the low-mid 20s with my 235/255 Pilot Alpins on 18" BBS REs, but in the high teens/maybe 20 with my 245/265 PSSs on 19" BMW Type 359s.

Combination of heavier wheels and greater rolling resistance from the bigger contact patches, I guess.

[bren]

Interesting experiment. I wouldn't have expected as much variation.

I look at the computer read-out every now and then out of curiosity, but I can't bring myself to care about gas mileage. Benefit of commuting <5000 miles year I guess.

[Pinecone]

I never paid that much attention, but I also never swapped tires for the seasons.

And the Fiat is giving me such good numbers, I like playing with what I can do.

BTW it gets double the mileage on the track versus the M3. 8.8 mpg for the Fiat, 4.4 for the M3. Not exactly apples to apples as the Fiat was on the Jefferson circuit at Summit Point and the M3 was at VIR.

[3LOU5]

The factor could be the outside operating temps as well, since we all know that cooler, denser air provides a more efficient combustion.

[Sharp11]

Originally Posted by 3LOU5

The factor could be the outside operating temps as well, since we all know that cooler, denser air provides a more efficient combustion.

I was going to point that out - he's comparing winter mpg to summer.

An interesting comparison would be to run a lighter, stock wheel during the summer months for a while, then switch back and measure the difference.

[JST]

Originally Posted by Pinecone

Have been seeing some interesting numbers on the Fiat. I run stock 16" wheels with the stock Pirelli P7 All Seasons, 195/45-16, in the winter. And the optional 17" rims with Pirelli P Zero Nero (Max Performance on Tire Rack), 205/40-17, tires for the summer.

I have been exclusively driving it, and with a pretty consistent mix of highway and city.

The All Season tires gave a combined gas mileage of around 35 - 36 MPG.

Going to the Summer tires dropped that to under 30 MPG. Then I checked the tire pressures and found them around 30 psi, versus my normal 40. After getting them back up, I am seeing around 32 MPG combined.

So around 10% difference between the summer and winter tires. And around 6.5% difference for tire pressure change of 10 psi.

I had not thought that the tire difference would be that big of a change in gas mileage.

Just thought I would share.

It's interesting that Pirelli makes both a P Zero Nero summer and all-season tire. The all-seasons that came stock on my Golf were also labeled as P Zero Neros.

They were reasonably good tires, though they ended up with noticeable dry rot after less than 4 years and about 25K miles. They still had decent tread depth at that point but were noisier than hell on the highway. Recently replaced them with Pilot Sport AS 3+; we'll see how those do.

[FC]

My fuel economy usually goes down (5-10%) in winter due to the fuel formula used in winter.

[kognito]

Originally Posted by 3LOU5

The factor could be the outside operating temps as well, since we all know that cooler, denser air provides a more efficient combustion.

Originally Posted by Sharp11

I was going to point that out - he's comparing winter mpg to summer.

An interesting comparison would be to run a lighter, stock wheel during the summer months for a while, then switch back and measure the difference.

Plus gas compounds also have different formulations for summer gas and winter gas

Edit, what FC said as we were both typing

[Pinecone]

Originally Posted by Sharp11

I was going to point that out - he's comparing winter mpg to summer.

An interesting comparison would be to run a lighter, stock wheel during the summer months for a while, then switch back and measure the difference.

Actually not, I was looking at the 3 or so tanks just prior to the tire swap to the 3 or so right after.