A 10% reduction in rolling resistance = ? MPG increase.

Rolling resistance's total effect on fuel economy is highly dependent on vehicle speed - at lower speeds it makes up a larger percentage of total friction than at higher speeds when aerodynamic drag makes up the majority.

Tesla has a nice chart which shows how much power is required to move the Roadster at different speeds, and you can see that rolling resistance very roughly accounts for about 1/3rd (33%) the total resistance below 45 mph and between 15-20% total resistance at freeway speeds.

So I'd have to guestimate that at low speeds a 10% reduction in rolling resistance would improve efficiency by about 3% and at high speeds would improve efficiency by about 2%.

The real question is - how do you plan on reducing rolling resistance by 10%?

 

Just to muddy the waters....Rolling resistance also includes brake drag, wheel bearings, universal joints and part of transmission losses. All together, those are probably significantly less than tire rolling losses.

 

Those are typically considered "drivetrain" losses, not rolling resistance.

If you look at the Tesla blog post I linked to above, you can see that drivetrain losses are accounted for separately from other friction sources.

 

thats a good chart
it indicates precisely what we should expect. Since the question relates to MPG, which is a function of power required, we need to consider that aero drag is a cube root law, that is it takes around 8x the power to achieve twice the speed.

So the original question needs to be modified to include at what speed. For clearly RR will have a greater impact at low speeds than at high speeds where ultimately aero drag becomes a far larger component.

Looking back at the chart, aero drag is virtually nil at 5mph, the crossover comes at around 42mph, and is around double the RR at 60mph.

And a word to the wise, averaging your speed wont give you the a correct feel for the numbers. This is because of that cube root factor that again will have a greater influence at higher speeds

 

it is a typical scenario though to batch a tyre RR to mechanical losses, and I take it that is how he considered it. But you are also right to pull it back to the question which it seems is based on tyre RR

 

While I realize that at higher speeds, say 60mph, that RR plays less of a role than does aero drag. RR still does have a large effect so it should not be ignored. I've noticed that on freeway overpasses I can enter a glide on the downhill side MUCH easier with my LRR 15" tires than with my LRR 17" tires. I also realize there is aero drag losses associate with my wider 17" tires but my point is simply this, ensure you have reduced RR as much as possible because it is one of the easiest fixes you can do. Aero mods are more difficult and expensive to achieve and they often make the car ugly unless you're really into boat tails.....

 

yes, Im a little surprised to see these quantities on the scale of that chart, given I would imagine Tesla looked into RR pretty closely, but now I see they specified 215/35 ZR 18 front and 255/30 ZR 19 rear.
2009 BRABUS Tesla Roadster Price, Specs & More | RSportsCars.com

other models are toting 16" and 17" designs

also very true, we are indeed fortunate that on the Prius the aero design results are quite good. I havent layed any money down on rims yet, I just installed Clazzio's yesterday and thats been keeping me busy.

 

Tesla had to balance the RR and traction values of a wheel and tire choice. With so much torque down low they would fry our 195/65/15s! Since the car is also designed to be sporty they had to go with a short sidewall tire so an 18 is a reasonable choice.

I'm jealous of the Clazzio seat covers. Those things are NICE!

 

yes their car is mighty quick, I wonder about the consequences of multiple rim diameters should you need a spare tyre though Would be nice to have real quantities to attach to low RR figures advertised wouldnt it.

OT: Have a few good tips on Clazzio installs I will get to in the near future. Katskins are nice equipment too, its just that when imports +transport exceed A$1,000 here we have to pay tax and duty, so it was Clazzio for me. Its nice how much they tidy up the inside of the car and make it look new again

 

Basically true but a little confusing. Aero drag power is a cube function of velocity, but actual aero drag and also work to overcome aero drag over a given distance is a sqaure function.

It takes 8 times as much power to over come aero drag to go twice as fast, but since twice the distance is covered in the same time, it only takes 4 times the work to cover the same distance at twice the speed. All else being equal (it isn't), that would be 4 times the fuel.

 

you edited out
'Since the question relates to MPG, which is a function of power required, we need to consider that aero drag is a cube root law, that is it takes around 8x the power to achieve twice the speed.'

 

I edited that out to shorten the quote because it didn't help clarify anything.

Aero drag vs velocity isn't a cube function, it's a square function.

Aero power vs velocity is a cube function, not aero drag vs velocity. I assumed you knew the difference, my bad.

 

Those are not the stock wheels/tires, those are aftermarket.

Stock Tesla Roadster wheels/tires are 175/55-16 front / 225/45-17 rear.

As F8L mentioned, larger/wider wheels also have a significant aerodynamic effect in addition to just an increase in rolling resistance. Not only is the frontal area larger the wheels also have much larger gaps between the tires/spokes which increases turbulence and drag as well.

Tesla is supposed to have an aero wheel option for the Model S which will increase range by about 6-7% - very curious to see what these look like compared to the standard wheel.

 

For a nice explanation of the relevant math see:
Ch A Page 254: Sustainable Energy - without the hot air | David MacKay

 

I can see I wasnt plain enough

'Since the question relates to MPG, which is a function of power required, we need to consider that aero drag is a cube root law'

there we are

 

yes that will be interesting
thanks for pointing out the wheel size changes