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Discussion Starter #1
So I have ALL the pertinent info, I will lay out the entire story, check it out if you are interested.

I gradually got the old (2000 Zetec) Focus up to decent gas mileage, important because I was driving 6-8K miles a month. Stock car, crate motor now with 27K miles, on full synthetic. KN filter, SVT airbox.

I drive to St Louis from Louisville a few times a week, and every trip my MPG was amazing consistent. I got 32.3x on a little less than one full tank. Seriously, I got 32.3 something on 7 straight trips, exact within a tenth of a MPG. I drove exactly the same each time, cruise control at 8 over speed limit, same time, same traffic, etc.

OLD tires were well worn General Altimax HP's, stock size, bought from Tirerack. In the end, the car was severely out of alignment, pulling VERY hard to the left. Wet traction bad, so I bought new tires in an emergency (during a rain storm) from a tire store while out of town. Loved the HP's, but shopped price and what they had.

NEW tires are Kuhmo Solus, again stock size. Also got the car aligned, now it floats along nicely.

But my new MPG for several tanks doing the exact same thing is 27.5-28.5 ?!? I guess tires with less grip can be more fuel efficient, that seems possible. But I figured the bad alignment alone was scrubbing MPG off, and expected to stay close to old MPG. On this trip, it amounts to about an extra 60 miles driven per tank. Not to mention I just completed my driving season (watching kid play college ball) and just drove 8,500 miles in last 5 weeks.

Any thoughts on why the huge drop? Do you think it will improve as the tires wear? Any other thoughts welcome as well.
 

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Shiny Metal Titanium
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The new tires may weigh more adding to rotating mass of the wheels
They may also be grippier than the old ones causing more friction and loss of MPG
 

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Floods suck.
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I was thinking this could be something to do with it as well as what everyone said above-(I kow you are from Louisville). My gas mileage is slowly starting to drop, so I was thinking maybe winter gas is starting to arrive. I thought it was a bit early, but who knows.
 

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i've always heard air temp outside can effect gas mileage in small amounts. i was getting about 28-29 mpg with my SVTF with bad tires and a slightly agressive driving style. i just put new continentals on. i havent checked my mpg's yet but this makes me curious.
 

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Got Bewst?
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I had the same thing happen to my 2000 Pontiac grand prix (my previous car) I would get about 25ish to the gallon and after I got new tires it dropped to almost 20 mpg. This was not completely due to just tires but also winter gas, so the exact mpg drop is unknown, but the tires were almost wore through about 8 months later and I was back up to 25... That couldn't have been the rubber either. The tires were cheapest off-brand discount tires you could find in stock size, so my guess is rolling resistance of actually having a sizeable amount of tread on the ground.
 

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Shiny Metal Titanium
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Plus as the tire wears, weight is shedded
weigh it when it's brand new and weigh it after it's well worn. Then multiply by 4 to get the approximate unsprung weight lost
 

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Tire pressures?
 

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Tires with little tread get better mpgs....less tread squirm. You have to tailor the pressure to the tire...Kumhos in my experience (limited) require more PSI than some other tires. Run mine at 40 or so.
 

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Discussion Starter #10
Wow, a lot of responses and advice, some I considered and some I did not. In order.

I did get a print out of the alignment, if I can find it. I waited on alignment while trying to squeeze life out of old tires, and it pulled so bad before that if I let off the wheels it would lurch into the guard rail, and now it drives true. Seat of the pants tells me my old alignment had to be worse than new one, but I could be wrong, and cannot guarantee it is perfect. Goodyear did it.

I didn't think of comparing weights, but wouldn't think they would be off by that much, or that is would cause that much dif. It would be more of a problem if I changed wheel sizes and wider tires, etc, but this is all OEM stuff, should be similar. I do think the newer tire is grippier and part of the problem.

I didn't think about winter gas. For those who don't know, Louisville does use reformulated gas for emissions control, and there is a summer blend and winter blend. Although, it is my understanding winter gas is better. However, this change started two days after change from one long trip to the next. And it also includes a fill up in St Louis and a measurement back home. They have much cheaper gas, which I assume is not reformulated, but mileage wise it was all the same.

I'll look into tire pressures. They installed them at factory specs. Oddly enough, I played with pressures on my old tires, including running them high, to get better mpg. On the old tires, it had no effect, it was very consistent. I figured higher (and harder) would raise mpg.

Other than that, it may just be a normal occurrence I haven't dealt with before. If any Tirerack folks have an opinion, it is greatly appreciated. This wasn't the tires I was planning to buy from you guys, and I was very close to buying, but I just got in a bind on one trip.

Thanks for all the feedback. I guess I can track my progress in the next year or so, and see if wear does affect it this much. Can't wait to get the summer wheels back on, car looks much better that way! [grinking]
 
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