New tires and a mileage penalty... ???

[boogetyboogety]

Finally had to get two new front tires (sidewall damage, not wear-related, I could have gone another 10-15K miles) at ~39,500 miles, and my gas mileage immediately dropped 10-12 mpg over the last four weeks/500 miles. They are the original Bridgestone Potenzas, with my normal 50 pounds pressure. The original back tires are still holding up the rear, I never rotate tires on a front-driver, and they have another 25-30K miles left of life.

I have long suspected that the mileage drop may have to do with the tires having a break-in period (see http://www.insightcentral.net/forum/vie ... sc&start=0 ) but have never been able to prove it. Unfortunately, since the new tires coincided with the advent of cooler weather for us, I can't in all good scientific conscience determine whether the new tires have something to do with my mileage drop. But in years past, cold weather has not dropped my mileage so drastically.

Have any of you noticed a mileage drop with new, original-issue tires? And if so, did it eventually (I hope) go back to normal? How many miles elapsed before your usual mileage "came back?"

[eastender43]

IMHO there are several factors at work here:

Cold weather,especially with WIND,will really cut mileage...I am looking at 47 with my MT insight...

I also just got new tires,,,,had been averaging 59

New tires are significantly larger in rolling diameter,so the computer thinks you are traveling more miles....that is,a worn tire will rotate more times per mile then a new tire,so you get somewhat better displayed mileage...New tires also have softer rubber than tires aged 6 years,but it is the tread,useful for traction but detrimental to rolling resistance,which is the biggest influence on rolling resistance.

..It is widely accepted that slick tires have less rolling resistance than tires with a tread design...This is well documented with bicycle tires...Worn out tires are slicks..Believe me,mine had 95K on them...

And additionally,new tires have additional resistance because the new outer surface is really quite rough when viewed under magnification..After 5000 miles it improves(Assuming the tire is driven on a smooth surface)

BUT....optimal mileage will always be obtained with skinny slick( or worn out) tires,at max pressure,with perfectly neutral alignment(NO toe in) in warm weather,with no wind(or a tail wind)...

I would not be alarmed by your drop in mileage...it is mostly weather related.But the tires themselves will continue to roll imperceptibly better as they age,and by the time they are worn in,and summer is here,your mileage will return to within 5% of where it was...JMHO,east

[trunkout]

now that I think about it my mileage has gotten better with tire age...
hrmmm.....

[bfivelover]

I got new tires about a month and a half ago. My mileage dropped 5 to 7 MPG for about three weeks, then suddenly went back to normal. Pretty much same weather conditions. Now it's much colder and I'm still getting nearly the same mileage. I drive about 700 miles a week, so it was about 2000 miles before the mileage went back to normal.

[trunkout]

Originally Posted by bfivelover

so it was about 2000 miles before the mileage went back to normal.

maybe it takes that long for the squirrels to grow bigger muscles......

[met-head]

Just read this post and had to put in my 2 cents worth. I'm in the tire business, and I am getting tired of Prius owners complaining that when they get new tires, their mpg drops by 2 or 3. The is caused by two factors. First, as already mentioned, the diameter of the new tires is about 1/2" taller than worn tires, which effectively changes the final drive ratio by roughly 2%, which the speedometer does not take into account. You actually go further and faster than you think and put a slightly heavier load on the engine. The second factor is additional friction due to tread squirm. The deeper the tread, the more squirm. That is one of the reasons manufacturers are hesitant to start introducing low rolling resistant tires to the market, because the trade off, will be longevity. Would you be willing to gain a 5% increase in fuel mileage on a tire that lasts only half as long? From what Michelin tells me, a tire works best when it is about half worn out. Obviously the current trend to lower, wider tires on bigger rims is totally contrary to efficiency. More rolling resistance, more frontal area, more unsprung weight, it's like adding 4 more flywheels. Even the new Prius's have gone to 16 in rims. If you don't believe me, jump on a mountain bike and take a ride aound the block (26X1.5" tires at 50 psi). Then jump on a road bike and do the same thing (700X23 tires at 120 psi). See what I mean...

[red1dr]

If the greater circumference of a new Prius tire has a 2% effect on the mpg, figuring typical 50mpg for a Prius gives a 1mpg drop with new tires. If the owners are complaining about a 2-3mpg drop, that leaves 1-2mpg to come from somewhere else, squirm being a likely candidate.

Yes skinny tires would give better FE but unfortunately wide tires look sexy, which sells cars. Ideally manufacturers would go to narrow high pressure tires but this would also probably mean recalibration and possibly even redesign of suspension to absorb road shock that is now handled by tires. Higher pressures might improve the longevity of LRR tires??

[met-head]

Actually manufacturers are working very hard to recalibrate suspensions so that 35-40 series tires can still give a supple ride, (quite a feat). Tire manufacturers love the current trend. A ten year old base Camry used a 195/70R14 witha speed rating of T; you can buy a new 80,000 mile Michelin for about $79. A new base Camry takes a 215/60R16 with a V speed rating. A 50,000 mile Michelin in that size will cost you about $170 each. I can buy 4 (60,000 mile) tires for my '72 Corvette for less than it costs for 1 ( 25,00 mile) tire for a new Corvette . It's all about the Benjamins.

[boogetyboogety]

Thought I'd chime in... I just went back on my records to see what happened with my tires after driving a while... and within 2500 miles, my mileage went back to previous levels... and increased by an average of 2 mpg! My LMPG frustratingly remains at 55.6 @ 55,000 miles, though, after dropping from a high of 56.2 after a fifteen-mile, triple-digit dash home during a medical emergency about two years ago...

I agree with met-head on most things, although I had a hard time wrapping my head around a tire being a larger diameter when new, it's something I never even considered. Doh! But tires do change over their useful lifetime, and tread squirm surely has the same effect on mileage as driving on a pebbly road vs. a nice level tarmac, or driving through rain. Slight but measurable drops in mileage...

[rusty41251]

I really did it. I bought Goodyear Integritys. Same as used in the Prius but in size 175-65-14. Thats ok because the diameter is only slightly more than than the 165 RE-92s. At only $47 (they were on sale) a tire why not? Well I used to have to indicate 61mph for a GPS 60 and now indicate 60 for the same, so no real change right? It is quieter and smoother. Will it do better in the snow? I hope so. Even with cables the RE-92s were junk. Butttttttttttttttt it squirms badly in rain grooves and the milage dropped badly. It runs now only in the 50s and low 60s where I always could count on 70s and 80s with the RE-92s during their last half of tread. So, will the milage pick back up. I hope so. Now I also put in new brake pads. Can anyone think of anything I could have done to screw it up? The car doesn't seem to have more drag coasting but I drain the battery much more and hills require an extra down shift. All this could still be to to the larger diameter or extra width but gee I took pains to keep them close. Compare the specs at Tire rack or online tires. Is it that sensitive?
Rusty