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range impact of wheels/tires

Posted: Mon Jul 25, 2011 12:16 pm
by defiancecp
So I've seen some discussion where most people are theorizing that the range/efficiency should be minimally impacted by tire rolling resistance - I just wanted to provide a quick counter-example.

I've recently changed the wheels/tires on my leaf from stock to a set of advanti denaro b2 black 18x8 wheels and kumho ecsta spt 215/40/r18 tires.

The wheels are 19.4 lbs (compared to stock at 21lbs per the measurements taken in this forum), and the tires are 23 lbs (compared to stock @ 19lbs per the ecopia web site)

I have two routes I can take to work - a hilly 4-lane 40mph road with lots of stop lights or a relatively flat interstate. Before, I would get about 4.3 mpk pretty much every time (as indicated on the dash) on the hills and either 4.1-4.2 on the interstate. Driving exactly the same, with the tires changed, both numbers dropped quite noticeably. In no way does it threaten to be a range limitation issue, but I was still disappointed with how much of an impact it was.


So that's 2.5 lbs each over stock, but of course the existing weight is moved more to the outside of the tire. Prevailing theory would be that the weight would be the strongest contributing factor to any change in range.


But here's the thing that leads me to believe otherwise - The hills dropped from 4.3 to 4.0 or 3.9 - so a little under 10% loss of range.
The interstate dropped from 4.1-4.2 to 3.7, a more significant loss in range.

To me, it seems like loss of range due to rolling weight would not increase at a higher speed (and might even be more noticeable on the hills) - a increase in efficiency losses at higher speed to me seems much more consistent with resistance-based losses. Or am I thinking about that wrong?


Anyway, from a purely aesthetic perspective, the wheels look amazing on this car, and the tires ... well, don't. I stuck close to the factory overall tire diameter to avoid speedometer reprogramming, but now that I'm looking at old pictures and the current setup, I can see that the outside tire diameter is really just not a good fit for these wheel wells; it needs another inch? or maybe two? Having a larger diameter wheel (and therefore a smaller sidewall) actually highlighted this.

On top of that, 215 width is simply not a good match for these wheels; there's a visible 'retraction' of the tire as it comes off the rim, which makes the tire look 'too small' to an even greater extent.

So, I'll drive these 'til they wear out (paid for now :) ), but next go round I'm going to just deal with a speedometer change and get a larger diameter, get LRR tires, and get them a tiny bit wider so they don't 'suck in' from the rim lip.

In fact, I just noticed ecopia 422 is available in a 225/50R18 - Just enough additional width to fill out the rim, and 1" larger diameter should make it look nicer in the wheel well without adding too much.

Anyway, I know a lot of people are more concerned about maximizing efficiency than I am, so I thought this would hopefully help contribute to that knowlege base.



Actually, one thing I need to confirm - I haven't confirmed the tire shop got the pressures right. If they underinflated I'd never be able to se it at this tire size, so I need to make sure I'm not skewing the results with low pressure.

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Posted: Mon Jul 25, 2011 12:33 pm
by rawhog
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Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 12:39 pm
by TomT
Assuming that the difference numbers are correct - it is hard to draw conclusions without a long term study as even small non-obvious daily differences can affect the numbers - I would say that the difference is caused purely by the different tires increased aerodynamic drag and rolling resistance. The difference in weight is all but statistically insignificant!

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 12:47 pm
by Skywagon
mogur wrote:Assuming that the difference numbers are correct - it is hard to draw conclusions without a long term study as even small non-obvious daily differences can affect the numbers - I would say that the difference is caused purely by the different tires and their rolling resistance. The difference in weight is all but statistically insignificant!
I would not agree on this one as statistically I am seeing a 6-8% shift in range data over the first 3000 miles with the stock wheels and now 4000 miles with the lighter wheels I am running (I am counting the 2500 miles without running the A/C for comparison). I will have more data once it has cooled off this fall. Inertial mass is significant.

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 1:10 pm
by DaveEV
defiancecp wrote:So that's 2.5 lbs each over stock, but of course the existing weight is moved more to the outside of the tire. Prevailing theory would be that the weight would be the strongest contributing factor to any change in range.
In the city weight makes a difference - but on the highway it's mostly wind resistance. And bigger wheels typically have HUGE gaps between the spokes leading to a LOT more wind resistance. 10% seems about right recalling some tests I've seen.

BTW - you have to remember that 2.5 lbs of rotating mass is also considerable - and you have to consider where that mass is located now - with larger diameter wheels/tires, that mass is located farther away from the center of the wheel - and the effort to accelerate that mass goes up by the radius squared - meaning that if your 12.5% increase in wheel diameter would increase the inertia by 26%! Then consider that you added mass considerably to the tire - the farthest point away from the wheel and it's no wonder energy consumption has gone up.

Not to mention the non-LRR tires you've put on - those alone could account for 2-4% increase in energy consumption.
defiancecp wrote:Anyway, from a purely aesthetic perspective, the wheels look amazing on this car, and the tires ... well, don't.
Pics always help. :)
defiancecp wrote:Actually, one thing I need to confirm - I haven't confirmed the tire shop got the pressures right. If they underinflated I'd never be able to se it at this tire size, so I need to make sure I'm not skewing the results with low pressure.
Definitely invest in a tire pressure gauge so you can check them yourself. I'd suggest 40 psi front and 38 psi rear to start.

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 3:17 pm
by defiancecp
Sure, pics:
Image
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Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 3:30 pm
by DaveEV
Yeah - see those huge open spaces between the spokes of your new wheels? Those are perfect for sucking up and spitting out large volumes of air. Not good for aerodynamics.

8" wide wheels are definitely a bit wide for 215 width tires - 7" wide wheels would have been better (hmm - looks like only 7.5" are commonly available in 18" wheels). Narrow wheels are also slightly more aerodynamic. :)

Guys over on Priuschat have reported an increase in fuel consumption of 10-20% when going to large wheels - going from high 40 mpg before on stock 15" wheels/tires to low 40 mpg on 17-18" wheels/tires is fairly typical.

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 4:50 pm
by DeaneG
drees wrote:..Guys over on Priuschat have reported an increase of 10-20% when going to large wheels - going from high 40 mpg before on stock 15" wheels/tires to low 40 mpg on 17-18" wheels/tires is fairly typical.
You mean decrease?

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 5:10 pm
by DaveEV
DeaneG wrote:You mean decrease?
Meant increase in fuel consumption, fixed.

Re: range impact of wheels/tires

Posted: Mon Jul 25, 2011 5:33 pm
by evnow
Skywagon wrote:I would not agree on this one as statistically I am seeing a 6-8% shift in range data over the first 3000 miles with the stock wheels and now 4000 miles with the lighter wheels I am running (I am counting the 2500 miles without running the A/C for comparison). I will have more data once it has cooled off this fall. Inertial mass is significant.
Your m/kwh is statistically significant - but 2lb change is not. Just load a 2 lb weight to the car and check it out. It has to be the rolling resistance or the aero change as was pointed out above.