GenVibe - Community for Pontiac Vibe Enthusiasts

It's been kind of warm since switching to my 195 snows

Running about 7.9L/100km (29.7mpg) vs 8.5 (27.6mpg) with my 235 summers.

I'm surprised.

Could be you are actually going less distance. If wheel diameter and aspect ratio are the same, the tire diameter of the 195 is probably about 1.5" less than the 235.
http://www.csgnetwork.com/speedocalibcalc.html

Wider tires often give less fuel economy. There's a reason the Honda Insight uses 175s. Also: diameter difference.

Chiadog wrote:Could be you are actually going less distance. If wheel diameter and aspect ratio are the same, the tire diameter of the 195 is probably about 1.5" less than the 235.
http://www.csgnetwork.com/speedocalibcalc.html

Ugh, wrong

kumquat wrote:Wider tires often give less fuel economy. There's a reason the Honda Insight uses 175s. Also: diameter difference.

I figured it'd be better, but its better than I'd thought

Sample size is way too small, and too many other variables at play, to determine the cause of the observed effect.

thebarber wrote:

Chiadog wrote:Could be you are actually going less distance. If wheel diameter and aspect ratio are the same, the tire diameter of the 195 is probably about 1.5" less than the 235.
http://www.csgnetwork.com/speedocalibcalc.html

Ugh, wrong

He's 100% right, but if you're using the same aspect ratio on 235s and 195s, then your diameter is way off and your MPGs would be severely inflated.

Still, chances are that the diameter of your summer and winter tires are different by at least a couple of percent, which could account for most or all of the variation, especially once you take into account other factors not controlled for (temperature, length of warm-up time, small variations in traffic and trips, type of driving, use of AC/defroster, rolling resistance differences, etc).

kumquat wrote:

thebarber wrote:

Chiadog wrote:Could be you are actually going less distance. If wheel diameter and aspect ratio are the same, the tire diameter of the 195 is probably about 1.5" less than the 235.
http://www.csgnetwork.com/speedocalibcalc.html

Ugh, wrong

He's 100% right, but if you're using the same aspect ratio on 235s and 195s, then your diameter is way off and your MPGs would be severely inflated.

Still, chances are that the diameter of your summer and winter tires are different by at least a couple of percent, which could account for most or all of the variation, especially once you take into account other factors not controlled for (temperature, length of warm-up time, small variations in traffic and trips, type of driving, use of AC/defroster, rolling resistance differences, etc).

1.3% difference in speed variance does not equate to 8% better FE

195-65-15 to 235-45-17

FYI, got same 8.5L/100km on 225-45-17

If you think anyone would run the same sidewall profile on a 195 to a 235, give your fucking head a shake

I commute 110km each way to work, I set the cruise at 125kph/77mph..I thought the first tank was a fluke, but now have gone thru a couple tanks in similar weather conditions.... Skinnier tires equals 2mpg or 0.6L/100km...even on softer winter rubber

thebarber wrote:

kumquat wrote:

thebarber wrote: Ugh, wrong

He's 100% right, but if you're using the same aspect ratio on 235s and 195s, then your diameter is way off and your MPGs would be severely inflated.

Still, chances are that the diameter of your summer and winter tires are different by at least a couple of percent, which could account for most or all of the variation, especially once you take into account other factors not controlled for (temperature, length of warm-up time, small variations in traffic and trips, type of driving, use of AC/defroster, rolling resistance differences, etc).

1.3% difference in speed variance does not equate to 8% better FE

195-65-15 to 235-45-17

FYI, got same 8.5L/100km on 225-45-17

If you think anyone would run the same sidewall profile on a 195 to a 235, give your fucking head a shake

Well 1.3% difference in diameter equals a 1.3% difference in circumference and a 1.3% difference in measured speed/distance traveled and therefore a 1.3% difference in fuel economy.

That's just a calculated diameter, anyway. The effective diameter of a tire can only be calculated within some tolerance. Not all 195/65/15s are going to be the same effective diameter. I have found a difference in indicated speed of up to 5% with different models of the same "size" tire.

And, believe me, I have learned never to underestimate the stupidity of people, particularly when it comes to doing non-standard things to their vehicles!

Filled up last night to the tune of 8.0L/100km and its been near freezing in the mornings the last couple days (aka how long it takes me to go thru a tank)

1.3% means it a potential of 0.1L/100km less use....which I guess isnt insignificant, but takes my 8.5 to 8.4....we're still talking 0.5L/100km difference in similar conditions for narrower tires

Fwiw, I'm already sick of the 195's....so narrow