The mpg difference 15s vs. 17s

Well, if you're sure you don't drive more aggressively with the 17s - I know I would - that eliminates one possible explanation for the difference in mileage.

What's the weight difference between the two sets of wheels and tires? Rotating mass is more critical than static weight, so any differences here would have a greater effect.

What's the rpm (rotations per mile) difference between the 15s and 17s? I would imagine the 17s are lower profile, but if the outside diameter is significantly different from the 15 inch wheel and tire combination, you may be noticing a measurement difference, not a real difference in gas consumption. I'm sure you've thought of this, but you didn't mention that part, so I thought I'd ask.

btw, have you ever measured a mileage difference due to using a high powered stereo system?

 

Years ago on a non hybrid vehicle i switched from 15's to 17's and had a similar reduction in mpg. A slightly different note, on my other non hybrid vehicle with LRR tires i have improved mpg using techniques learned from driving my Prius.

 

Yes F8L it is great with any vehicle and it took a Prius ,to teach an old dog like me, a few new tricks.

 

One trip of data is not enough, my febrile friend.

Three or four days a week I drive ~ 25 miles one way to work at speeds mostly 50 - 60 mph, and a decline of I think 300 feet. Since temps have cooled down and AC is off, I average about 68 - 70 mpg, but twice in the past month have hit just over 80 mpg.

 

Interesting info F8L. I wonder exactly what part of the bigger tires it is that causes the MPG loss. Is it the lower profile? Is it the fact that they're wider? Or is it merely that the construction is not optimized for LRR?

From what I've read there doesn't seem to much in the way of noticeable MPG difference between the same model LRR tires in 185/65/15 and 195/60/15. So I wonder what it is that makes 17" tires so noticeably different.

 

from what i've seen.. they make a huge difference. i say huge because the cap with 17's on a genII tends to be in the high 40's.. (for me).. while with 15's i used to be able to get into the 50's...

the one oddity i've seen: when i was towing long distance... basically up to semi valley and back to culver city (roughly)... i was getting 48mpg... (steal 5x8 trailer loaded 4 ft high with pipe insulation.)

as a reference back to Hyo, when i was running my sub turned up all the way and blasting the stereo (till eventually the cone ripped on my sub)... i was getting a good 5mpg less then i'm getting now. but there could be a lot of things involved in that (i drive a little less aggressive at times now)

going back a few years... aggressive driving with 15's never made me less than 42mpg... now my low is 38mpg... i could blame that on needing a throttle body cleaning and such... or the point that i'm always making shorter commutes than i used to make... shrug... idk

 

LOL, yeah, I missed the repeat testing part. Apologies for doubting your scientific approach

I hope Qbee ventures an opinion, since the physics of weight changes in tyres frankly eludes me. I know bicyclists moan and groan over extra grams in their tyres. Certainly the tyre width plays a part in extra friction -- the overall tyre friction between two tyres that only differ by widths should be proportional to the ratio of the widths. Tyre friction is going to be most apparent right where you are testing: not fast enough for air friction to account for the lion's share of friction, and not in the city where brake use dominates mpg.

I know you can read the tyre spec format (I keep forgetting), so what is the ratio between your 17 and 15 inch tyre widths ?

 

OK, if I read correctly the first number is the cross-sectional width, so 215 mm for the 17 inch tyres and 185 mm for the 15 inchers.

215/185 = 16% !!
According to Wayne Brown, half of the friction at 60 mph is not air, and we know that the drivetrain uses about 3% of the energy to move the wheels at that speed, so tyres account for 40 - 45% of energy losses. 40 - 45% of 16% = 6.4 - 7.2%.

So just the width difference will drop 64 mpg to 59 mpg.

 

So, part of it is a measurement difference. It's reasonable to 'knock off a few mpg', but this isn't scientifically accurate. Is there a better way to do the testing? A scangauge may not work, because it still uses wheel input. Could a gps be used, along with precise measuring of the gas consumed?

I've teased F8L before about having a ten horsepower stereo. I'm curious to know if there's a real difference. Part of it may well be the fact that driving aggression seems to match the music. The same seems to be true of caffeine levels, too.

Each? Meaning 20 lbs heavier for the set of four?

I've noticed a gliding difference just with tire pressures. With this car and the driving techniques typically used, the gliding is bound to make a big difference - it's not just accelerating the higher rotating weight up to speed.

Definitely. When you're supplying all the hp (human power), the differences are very noticeable. My mountain bike has big knobbies on it that sound like 4x4 truck tires on pavement. Off-road, it's obvious they're designed for traction in dirt, and that's where they come into their own, but on-road, they're painfully slow. On the road bike, I have to balance lightness with durability. Having a super light tire blow or puncture doesn't save any time or effort, and becomes a significant risk in traffic or on high speed downhills. Wet traction and cornering profiles are also important considerations, in addition to weight.

 

someone did a test on here that calculated hp used to sustain at a certain speed.. for the 15's it was somewhere around 12hp... but for the 17's it required almost 20hp to maintain the same speed.

sage... you're right. the first number is the width of the tire in mm... people don't realize how much of a difference that much more tire can make.

 

Here I think is a bit more accurate math analysis of increased friction from a wider tyre, again using Wayne Browns overall energy consumption and aero contribution results, and knowing that a 215 mm tyre has 16% more ground contact than a 185 tyre just from the width (ignoring increased ground contact from weight):

At 60 mph, 215 wh/mile using 15 inch tyres,
air accounts for 102 wh and non-air is 112 wh.
4 wh used by the drivetrain, so 108 wh by the 185 mm tyres,
and 108*1.16 = 125 wh by the 215 mm tyres.

Adding everything back up, (102+4+125)/(102+4+108) = 1.079439252

8% hit in fuel economy, so a 64 mpg on thinner tyres becomes 59 mpg on the wider version.

 

I wonder if this testing was at constant low speeds where tyre friction becomes a larger fraction of total friction losses

Addendum -- Nah, that does not make sense. The limit should be 16% more fuel/distance from width considerations only.

 

F8L, perhaps you can measure longitudinal ground contact of your two sets of tyres ? Part of the answer here is additional work involved in tyre deformation as it rotates.

One other thought: what does LRR mean, anyway ? If it is a measure of compound, then tread differences are not taken into account. I would not be surprised if 17 inch tyres are designed to have better grip than smaller tyres since they appeal to the 'performance' crowd.