New tires/ worse miileage!

[Gardenarian]

Hi -
I have a 2004 Scion XB and was consistently getting 33-34 mpg. It used to be that the only tires you could get for this model were Bridgestone, which wore out really fast (30k miles). Well, now Scion has said that it is okay to go a little bit larger in tire size (I think it's 10mm larger) so I bought a set of Michelins with an 80k warrantee. But since I've had the new tires on, I'm averaging only 29-31mpg.
Anyone have a clue why this might be?

It could be coincidence and something else is causing my decling mileage, but I can't imagine what. I had the oil changed when I had the tires put on. Aside from needing a new battery 6 months ago, I've had no problems with this car.
Ideas??

[jp1]

I'm not any sort of engineer or tire guy, but if I had to guess I'd say it is the tires for 2 reasons. Well, one reason with 2 factors. First, depending on how big your old tires were, perhaps 200mm, this would be a 5% bigger tire, hence more rolling resistance on the ground. Second, if the bridgestone tires are higher performance tires, which seems likely if they're only good for 30k, then they probably have a smaller flat spot on the ground compared to the new ones (all tires have a small spot on the ground where they have depressed down and created a flat spot. Think of how much of the tire is flat on the ground when the air pressure gets low for an exaggerated example. Even with full air pressure there is some flattening happening on the ground, just not as much). Increased flat spot on the ground from being a less rigid tire would also equal more rolling resistance.

Personally, if I were you I'd call NPR's cartalk and ask them. They handle quirky questions like this all the time.

[Float On]

Yes, it's your tires. I was told that if I put a little smaller, lighter tire on my Durango that I'd get better gas mileage. I'm not sure that going from 12 mpg to 13 mpg was really worth losing about 2" of height on my truck or not. (Soooo can't wait until I can get something with better mileage). I can see where adding larger/heavier tires would decrease your mileage by that much.

[CathyA]

It definitely could be the tires. I went to one that had more traction for winter, and my mileage dropped a couple miles/gal.
I also think that as a car ages, its mileage goes down. I have a 2001 Honda Odyssey van and for the first half of its life, I was getting 24-26 mpg.
Now I get 19/20.....which is depressing.
And I take it in for its servicing when its due......but I think its just getting old.

[early morning]

Ok, maybe this is stupid, but wouldn't a larger tire size mess up the odometer and speedometer readings? I always thought they were calibrated to tire size. Might that incorrect reading be a bit of the problem with larger tires? The bigger tire would cover a bit more road in one revolution, so at the end of 100 miles or a tank of gas or whatever, you would have gone further than the odometer says.

[1tolivesimply]

Early Morning is correct.

[Bronxboy]

Ok, maybe this is stupid, but wouldn't a larger tire size mess up the odometer and speedometer readings? I always thought they were calibrated to tire size. Might that incorrect reading be a bit of the problem with larger tires? The bigger tire would cover a bit more road in one revolution, so at the end of 100 miles or a tank of gas or whatever, you would have gone further than the odometer says.

Correct. It has both good and bad effects; taller gearing from the larger tire is nice on the highway.

Downside is that odometer and speedometer will read slightly low (or less high) with the new tires. Odometers/speedometers are usually set up to read high as it slightly reduces warranty costs and blame for speeding tickets. My current Ford's odometer is at least 3 mph high at 70 mph.

Your tires are 10 mm wider, 185/60R15 to 195/60R15. Per http://www.1010tires.com/TireSizeCalculator.asp, your diameter difference is 5%. This would account for about 1.5 mpg in indicated difference. Colder weather, thicker oil (10W-30 vs 5W-20 or 5W-30), or increased rolling resistance of the larger tire (as jp1 notes) could account for the rest.

[Gardenarian]

Wow! Thanks for all your feedback. I'll have to study up on this a bit - pretty technical!

[jp1]

Yes, it's your tires. I was told that if I put a little smaller, lighter tire on my Durango that I'd get better gas mileage. I'm not sure that going from 12 mpg to 13 mpg was really worth losing about 2" of height on my truck or not. (Soooo can't wait until I can get something with better mileage). I can see where adding larger/heavier tires would decrease your mileage by that much.

Taking early morning's comment below into account I wonder if you're actually getting an extra 1 mpg? Perhaps you're actually not driving full miles anymore but are instead calculating your mileage based on a shorter distance.

[jp1]

Downside is that odometer and speedometer will read slightly low (or less high) with the new tires. Odometers/speedometers are usually set up to read high as it slightly reduces warranty costs and blame for speeding tickets. My current Ford's odometer is at least 3 mph high at 70 mph.

I'm surprised your speedometer is that far off. Since we use a GPS whenever we drive, and we don't own a car so we use it in an assortment of zipcars and rental cars, I've had the chance to check the accuracy of speedometers in a variety of cars. Every car we've driven has been pretty much 100% accurate.