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Old 06-24-2009, 08:47 AM   #2 (permalink)
CapriRacer
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Adam,

First, in passenger car tires, tire size determines load carrying capacity. Peugeot's haven't been imported into the US for quite a while, so my books only have older entries for Peugeot's - and a 307 model isn't one of them.

Be aware that even within a given vehicle model, there may be different tire sizes used - and sometimes this is a reflection of the different vehicle weights.

So the first step is to find your tire placard that lists the original tire size and the proper pressure for that size. It's a law in the US to have this placard, but my understanding is that tire placards are pretty universal worldwide.

So using a 185/65R15 is actually going down in load carrying capacity and that increases the risk of a load related tire failure, which sometimes has tragic results. I don't recommend you continue this practice.

Second, a tire's rolling resistance is only slightly connected to tire size. It is more connected to the amount of deflection (load vs inflation pressure), the amount of material being deflected (basically the amount of tread rubber), and the material properties of the material being deflected (mostly the hysteresis of the tread rubber)

Within a given tire line, rollling resistance coefficient doesn't vary much between tire sizes, assuming the load on the tire is the same. You should take this to mean that there isn't any real advantage to going down in tire size. In fact tires with larger load carrying capacity tend to have better rolling resistance coefficients - and while width is a factor, it seems to be small compared to overall load carrying capacity. There is a school of thought that says that for good fuel economy, you want to get the tire with the largest load carrying capacity you can.

But differences between tire lines can be HUGE!! But there is a price to be paid.

The classic technology triangle is Rolling Resistance / Treadwear / Traction (especially wet traction)

BTW, most car folks use the term "hub" to mean the part that the wheel attaches to, and not the "rim" or "wheel" which is what attaches to the "hub" and is the thing the tire is mounted on. Tire specs routinely refer to the dimension in question as "Rim Width".

Don't pay much attention to the max pressure listed on the sidewall of the tire (at least for Standard Load passenger car tires). There isn't a universal rule about how this is derived. I talk about it here:

Barry's Tire Tech

One of the problems you are going to face is availablility of information about the specific tires you can find in your locale.

First, OE tires (tires delivered to vehicle assembly plants) tend to have better RR than tires designed strictly for the replacement market. OE tires ARE available in the replacement market, but tend not to be identified as such. Further, since a given vehicle will have a limited range of tire sizes that are applied - and the tire specs will be different between vehicle models - even tires within a line of OE tires will vary all over the place for RR.

Even further, different parts of the world may have the same tire name, but the tire itself will be geared towards a different performance goal. Put another way, you can't be sure that a "Goodstone FireEagle GT" is EXACTLY the same tire in the US as it is in, say, Australia.

Not to mention that certain tire lines won't be available on a worldwide basis. There are country specific regulations that might prevent this.

OK, that addresses the basics of your posting - and I'm sure others will be able to point out tires that might suit your purposes.
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