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Downsizing Wheels & Tires

6 messages,  Last post on Apr 13, 2010 at 3:52 AM

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What is this discussion about? Toyota Camry, Tires, Sedan

Messages Page 2 of 2

#3 of 6 Re: 2005 LE- Downsizing Wheels & Tires [capriracer] by matt_e_boy

Apr 09, 2010 (6:40 am)

Replying to: capriracer (Apr 07, 2010 3:32 am)
Thanks Capriracer,
       Though to explore the discussion I disagree with your load capacity belief. I used to sell tires in the early 90's and we watched the beginning of the 75 series aspect ratio phase out and in later years the 70 series and 14"tire. They were often replaced with 15"tires with lower aspect ratios as manufacturers would use this to increase handling as a selling point (for a while every car commercial I saw cars were drifting sideways!) Those cars' mileage was diminished however manufacturers still had to meet CAFE requirements so to compensate for their SUVs and performance cars and enhancements they were now building Prius' and other hybrid's to boost the overall gas mileage of their entire vehicle line. To the consumer this meant cars suddenly had wider, speed rated tires (like an H rating) that could take off ramps at 60mph because of the reinforced sidewalls but simultaneously phased out the average tire - suddenly average $60 tires were nonexistent or junk. The only decent ones I see nowadays are the foreign made ones that are in that price range. With the overall industry pushing performance everybody's tires are now wider and have more load capacity (especially with 44psi casings). But this also means lower mileage, increased hydroplaning (less pressure per square inch over the larger contact patch) and more expensive construction. Heck,I can remember the first dodge minivans had 195/75SR14s on their 4cyl model. So I do see the tradeoffs with tires but they are not with fuel mileage in mind on most cars. Don't get me wrong, the performance increase in the industry is actually nice as we have better performing vehicles overall and there are many more choices for the gearheads and tuners but that luxury has come at a price - both literally and figuratively.

#4 of 6 Re: 2005 LE- Downsizing Wheels & Tires [matt_e_boy] by capriracer

Apr 10, 2010 (6:27 am)

Replying to: matt_e_boy (Apr 09, 2010 6:40 am)
Matt,
 
Here is what I am basing my statement on:
 
http://www.energy.ca.gov/transportation/tire_efficiency/documents/2007-12-07_wor- - kshop/presentations/Lambilotte_Bruce_Task%204%20Rolling%20Resistance%20Testing.p- - df
 
This is a study performed by Smithers Scientific Services for the California Energy Commission to explore rolling resistance in tires and what effect certain laws might have on the general public.
 
On page 31 is a graph showing the Rolling Resistance Coefficient (RRC) for various sizes of tires but all the same make/model. I think it is easy to see that small tires (load carrying capacity wise) have better RRC.
 
They actually calculated a correlation value - and it was pretty poor at 50% - but the trend is there.
 
Conclusion: If you want to improve fuel economy by changing tire size: More load carrying capacity = Better

#5 of 6 Re: 2005 LE- Downsizing Wheels & Tires [capriracer] by matt_e_boy

Apr 12, 2010 (8:21 am)

Replying to: capriracer (Apr 10, 2010 6:27 am)
Ummm Capriracer, did ya see page 35?
Looks like a positive correlation between rolling resistance and load index to me.....
It's also interesting to note rolling resistance increases with UTQG ratings and tire weight.

#6 of 6 Re: 2005 LE- Downsizing Wheels & Tires [matt_e_boy] by capriracer

Apr 13, 2010 (3:52 am)

Replying to: matt_e_boy (Apr 12, 2010 8:21 am)
Matt,
 
Be very careful here. The way tires are tested for rolling resistance includes compensation for the load carrying capacity of the tire. What you get is rolling resistance force. To compare tires of a different load carrying capacities, you need to divide by the load to get rolling resistance coefficient (RRC). When you change tires on a given vehicle, the load on the tire doesn't change - so you have to use rolling resistance coefficient (RRC).

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