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21" Wheels Less Efficient?

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mknox

mknox

Well-Known Member
Aug 7, 2012
10,104
1,901
Toronto, ON
#1
I've read in several threads that the 21" Performance wheels suffer a bit of an efficiency penalty compared to the 19's. I'm curious as to why this would be. I always thought the whole point of the larger wheel/smaller tire combo was to reduce unsprung weight for improved performance. In other words, I thought the 21" wheel/tire combo was lighter than the 19" wheel/tire combo. Why would less weight translate to poorer efficiency?
 
#2
I think most of the difference is actually tire choice. The 21" come with sport tires, which grip more, but also drag more than standard all season tires that come on the 19".

I would expect if you put the same model tire on both the 21" and 19" rims you would get effectively no difference in electricity economy
 
#3
Well, if efficiency is calculated with all factors involved (including the price of the tires), then the 19 is easier on the pocketbook.

Yokohama - ADVAN sport - 245/35R-21XL - $451 each (so without any fees, $1804 for 4)
Yokohama - ADVAN S.4. - 245/45R-19 - $189 each (so without any fees, $756 for 4)

- which doesn't reflect the life of the tire, the driving style of the owner of said tire, and does not include any sensors, disposal fee, etc.

So if you are looking for just rolling efficiency, don't pay attention to me. If you are talking about efficient use of your pocketbook/wallet ....

... well ....
 
#4
sublimaze1 said:
So if you are looking for just rolling efficiency, don't pay attention to me. If you are talking about efficient use of your pocketbook/wallet ....... well ....
Click to expand...

I was actually thinking about the rolling efficiency...had been reading talk of slightly reduced range etc. with the 21's.

I guess as ElSupreme says, it's likely more to do with the tire than the tire/wheel combo size.
 
#5
mknox said:
...reduce unsprung weight for improved performance. In other words, I thought the 21" wheel/tire combo was lighter than the 19" wheel/tire combo. Why would less weight translate to poorer efficiency?
Click to expand...

It's the opposite. With all things being equal, a 19" wheels weighs less and hence has less unsprung weight than a larger wheel. Think about it this way...there is more "metal" on the larger wheel making it heavier.

Spoke pattern and hub thickness also effects weight, but if they are similar in "design" the smaller wheel is lighter.
 
#6
7racer said:
It's the opposite. With all things being equal, a 19" wheels weighs less and hence has less unsprung weight than a larger wheel. Think about it this way...there is more "metal" on the larger wheel making it heavier.
Click to expand...

Okay... I had always assumed the little bit more aluminum weighed less than the corresponding little bit more rubber on the 21's vs. the 19's (since both have the same overall diameter), but never actually researched it.
 
#7
Last edited:
#8
ChadS said:
I just weighed a 21" silver wheel with the standard Continental 245/35R21 Extreme Contact DW tire on it...61 pounds. Oof. Continental says the tire is 26 pounds, so that means the wheel must be 35 pounds.

Anybody have a 19" wheel/tire sitting around they can weigh? Let us know which tire it is, thanks. I've got the Pirelli 245/45R19 Sottozero Serie II winter tires on my 19's; Pirelli says the tires are 28 pounds, but I don't know the wheel weight.
Click to expand...

It's not just the weight of the wheel, it's how close that weight is to the centre of rotation. The closer the weight is to the centre of rotation, the less force it takes to spin the wheel. So a 15 kg 19" wheel might be just as efficient as a 13 kg 21" wheel--it all depends.

But if you're really concerned, Tirerack has some sub-9 kg wheels for the Model S on sale for $4000 (no tires).
 
#10
If you want max. range then use LRR tyres.
If you want max.acceleration the use low weight wheels & tyres. (actually low moment of inertia)

In Europe we have a new system for classifying tyres for rolling resistance. The labels are A (low rr) to G (high rr).
I made some calculations to show the effect of different tyres / labels on range for Tesla model S.
Best 19" has C label, best 21" has E-label.
Label A and B do not exist for 19" and 21" yet. But I think it is a matter of time.
Sorry for km's ( 90 km/h = 55 mph, 100 km/h = 62 mph, 120 km/h = 75 mph, 1 km = 0,62 miles ).

Does anyone know the exact rrc for the 19" and 21" tyres that Tesla uses ? I assumed C and E label, but could very well be E and F label.
The values in the table below are valid only on flat roads, 70F and no wind.

View attachment 19073
 
#12
From other cars I know this here and I have personal experience:

Tires have a less density as rims.
That means the 21' + tires are heavier than 19' + tires, because you have more tieres material and less rims material on the axle.

But the important point is that with the 21' you have a higher mass on a increased radius around the axle. To get this mass in rotation you need more energy because it is more mass outside from the axle.


I hope you understand what I wanted to tell, if you want I can sketch it.
 
#13
And, the Goodyear All Seasons will be 3% (or so) more efficient than the Continental High Performance due to tread design and rubber compound. Combining the increased rotational inertia of the 21" combination with the rolling resistance improvement for the All Season tires can easily reach 5%. In fact, I wonder if this is the major contributor of the unexpected range delta between the 60 kW and 85 kW batteries. We know that the 85 kW was tested with the 21" and I'll bet the 60 kW was EPA tested with the 19".
 
#15
tdiggity said:
This thread shows that the 19's are more efficient than the 21s
Click to expand...

I'm no statistician, but I bet there's a positive correlation between choosing 21" wheels and being a lead foot. I'm not sure we should blame the tires for that without more evidence.
correlation.png
 
#18
on my car I got a better efficiency since I put the 21"... but I think it's due to the increase of outside temperature (switching from winter tire to summer tire / switching from 20's F to 40's F.
The handling (especially on high speed curve), is much much better with the 21"
 
#19
I wish we had rrc codes required for tires sold in the us. I am using Bridgestone Pole Potenza RS970As all season 19" tires as my winter tires here in the Philly area. They get great reviews and I love them after driving them now for a few months. However, Tesla says they tested them and rejected them due to their poor rolling resistance performance.
Does anyone know of a European code table that could be used to see how these tires rank compared to the Tesla "approved" 19" and 21" tires?

Sent from my iPad using Tapatalk HD
 
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