Quote:
Originally Posted by CapriRacer
Doug,
First, a 155/80R15 83S is 4 load indices lower in load carrying capacity. That means that all other things being equal, you will need to use 4 psi more than what is on the vehicle tire placard for comparison purposes.
According to Tire Guides, a book that summarizes vehicle tire placards, a 2009 Ford Focus with P195/60R15 87T's should have a tire placard that says to use 32 psi. Please check the vehicle placard to see if that is correct.
What this means is that even if you inflate the tires to the maximum, the tire has less load carrying capacity than the original tires. Not only is that going to hurt the fuel economy, but it's a safety issue, too. Less load carrying capacity increases the risk of a load related tire failure, which sometimes has tragic results.
PLUS, going down in speed rating (T to S) is also the wrong way with regard to safety.
While the amount of tread rubber is important, there are 2 other considerations for a tire's contribution to fuel economy: The rubber, and the amount of deflection.
As I pointed out above, the lower load carrying capacity means more deflection and worse fuel economy - all other things being equal.
The type of rubber is very important - and probably the most important thing. Tires designed for good fuel economy (like what came OE on your Focus), are going to use types of rubber compounds that give good fuel economy. That is generally at the expensive of traction (especially wet traction) and tread wear (or both!)
Going narrower also means a smaller footprint, and that has implications for traction.
So while going narrower may seem like a good idea, there's a lot of down sides to this. It has to be done carefully - which why you asked the question, isn't it!
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Interesting Capri. How is a load indices calculation performed? The focus is a fairly light car so I was surprised that this tire wouldn't have the load capacity. I'm also surprised that even with the tire filled to max it would give deformity degrading FE. 195 to 155 width is a substantial amount where deformation of that magnitude is surprising. BUT I really don't want to compromise safety, especially in a situation where I could harm someone else in what in the scheme of things ... is just a simple test.
As to the earlier comment on rims, yes, I would have to purchase narrower rims to go down to this width. I was looking at some cheap steel rims with some possible moon covers.
Doug