tires and gas mileage
 

I just replaced the worn out Dunlop SP 5000s that were on my '98 when I bought it with a set of stock Yoko Avid V4S's. I'm real happy with them except that I'm noticing a substantial (10% or so) drop in gas mileage based on the 1 full tank I've gone through so far. Could be a sour tank of gas or a coincidence, but I'm wondering if the new tires are to blame. I know there are some tires, like the Michelin MXV4, that are supposed to have reduced rolling resistance, which helps mileage, but I was not aware that the Dunlop was such a tire. It might be my imagination, but it feels like the car doesn't gain speed the way it used to when coasting down big hills. Anyone else noticed such a big change in mileage after replacing tires?

Even though these tires are softer (i.e. somewhat more grip in cold conditions and snow) than your ordinary "summer season tire", there shouldn't be that much of a difference. Are you sure they are exactly the same dimension?
E.g. here in Sweden using dedicated winter tires does make a noticable difference to gas milage, but even with the coarsest type of winter tyre (with small hard metal studs/spikes in them) there shouldn't be more than 10% difference. (Just for the record, these swedish winter tyres are classed for no more than say 110mph as opposed to the >=V class tyres which are classed for >150mph. Kind of academical, as it takes an idiot to drive a 2 tonne car with winter tires at e.g. 130mph or more. Having said that, these idiots can be seen almost every day. :) )

//Björn

The tires are the same size (225/60-16), and I would have thought they are of comparable softness (the Dunlops are listed as "ultra-high performance" whereas the Yokos are simply "high performance", suggesting less softness). I just refilled the gas tank and am hoping there will be some improvement on this tank.

To elaborate a bit: I take it CA means California, so the normal tires sold there are probably quite different than the ones sold in northen Europe. (Yes, I know that parts of California gets snow in the winter.) Anyhow, from what I've found out about the Yokos they are "year around" tires. I.e. they can somewhat cope in snowy conditions too. My guess is that tires made for "normal" californian conditions don't take snow into the calculation.
What I mean with the term "soft" is that the tyre is relatively soft in cold conditions. This usually means that the tyre is too soft for a warm summer day, so that it becomes somewhat sloppy compared to a "summer" high perf. tyre. So, there are soft performance tires and winter time tires and the two are soft for very different reasons. A winter tyre is somewhat like a rain race tyre, i.e. more or less useless on dry tracks.
Please note that I'm writing this from a Swedish perspective. Around here the whole country is currently covered in snow and anyone who drives around without approved winter tires can pick up a ticket (some $250 or so).

//Björn

Stupid thought (perhaps not), but are inflations EXACTLY the same?

Some tires will drag the car down a bit more, but not more than a single mpg or so given the exact same size.

I was very surprised with the outstanding fuel economy afforded to me by Goodyear Viva Authority FuelMax tires available at Wal-Mart, in the '95 Lexus. Better fuel economy at stock pressures than hardened Cooper Performance GT-H tires at 50psi in my '96 LS4.

I seem to be doing better on my second tank; maybe the tires need to break in a bit or wear down rough edges or something. Tire Rack does say they need to break in some for max. performance; maybe the same is true for gas mileage. The inflation pressure should theoretically be the same, although during most of the last tank, we were having a cold snap, at least by Northern Calif. standards (30s [F] during the day, which may be a heat wave by Swedish standards, but is probably cool enough to make a difference). I should probably recheck the pressure. I don't think it has anything to do with being California-spec tires, as they are classed as "high-perf. all-season," not "summer-only," and I did not buy them locally; I bought them from Tire Rack, based in Indiana. The old Dunlops are classed as "ultra-high performance," which to me suggests they should be softer/stickier than all-seasons, althought the fact that they were nearly bald may offset that.

There is another thread on winter tires, which is good reading.
Anyhow, what happens with the soft "summer-only" tires in the winter is that they get very hard and thus have very poor grip. It's obviously hard to make tires behave like mulitgrade oil, i.e. soft when cold and harder when warm.

//Björn