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Old 08-08-2019, 12:22 PM   #11 (permalink)
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The BMW is also rear drive so it is less likely to tear up tires accelerating compared to the Spark EV. Apparently just about any speed under 40 can produce tire spin on the Spark. The BMW overall looks more like the set out similar to Tesla to do what's right to make the best EV even if it's outside the norm so I wouldn't discount the weirdness as being wrong. Tesla doesn't do it because they market their acceleration and handling performance as much as their economy.

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Old 08-08-2019, 02:57 PM   #12 (permalink)
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Originally Posted by oldtamiyaphile View Post
I find it telling that (almost) no no other MPG car has gone that route. All the current day EV's have pretty wide tyres, so I wouldn't read too much into the i3's tyre spec.
VW's XL1 has, though.

Many EVs go "wide" because of the excess power and torque they "need" to lay down ...

New Leaf, like over 200HP ... silly
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Old 08-08-2019, 03:08 PM   #13 (permalink)
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Originally Posted by rob.e View Post
What confuses me is that the BMW tyre (bridgestone ecopia EP500) gets a fuel economy rating of only "B" (eu tyre labelling) which i was surprised about given how skinny it is..

Tyres i have on my honda are way wider (225/40 18) and are rated better for fuel economy - they are "A"; pirelli p7 blue.

How is the pirelli better than the skinny super-eco bmw bridgestone? Or am i missing something?
The EU rating is not absolute , but relative

"Same" tyre, but with an XL load rating will often return a better FE rating

For sensibly small tyre sizes, like 175/65/14 it's hard to find an A rated tyre - there's just one ... even though the tyres are modern.

205/55/16 ? A's aplenty ...
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Old 08-08-2019, 03:16 PM   #14 (permalink)
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How is the pirelli better than the skinny super-eco bmw bridgestone? Or am i missing something?
Ultimately it's not RR that counts, it's RR force you need to overcome
So factor in the weight
C on a lighter vehicle will cause less drag (force) than A on a heavier one


Then add extra aero drag from wider wheels
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Old 08-09-2019, 03:56 AM   #15 (permalink)
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The EU rating is not absolute , but relative
Relative to what? I don't understand that statement.
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Old 08-09-2019, 04:00 AM   #16 (permalink)
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Did some research. It's absolute, not relative

https://eur-lex.europa.eu/LexUriServ...46:0058:en:PDF

So, to answer my own question, yes my 225/40 tyre with A rating has lower cooefficient of friction than a skinny bmw bridgestone that "only" has a B rating.

BMW i expect though did the maths and decided that best option for them was compromise of narrow for aero, but grippy compound for safety/putting the electric torque down even if it meant marginally higher rolling resistance.
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Old 08-09-2019, 08:51 AM   #17 (permalink)
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Quote:
Originally Posted by euromodder View Post
The EU rating is not absolute , but relative …..
No, I don't think so. The grade is determined by the RRC (Rolling Resistance Coefficient), which is determined by a test.

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Originally Posted by euromodder View Post
….."Same" tyre, but with an XL load rating will often return a better FE rating ……..
That only means that the test gets a different result if the tire is different - one of the peculiarities of trying to do RR ratings. Size and Load Range are examples of "different" tires. The EU didn't try to address this in the regulation.

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…… For sensibly small tyre sizes, like 175/65/14 it's hard to find an A rated tyre - there's just one ... even though the tyres are modern.

205/55/16 ? A's aplenty ...
Yup, larger tires get better RRC - and that is reflected in the ratings.
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Old 08-09-2019, 02:58 PM   #18 (permalink)
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I think the idea is similar to comparing crash "stars" on different classes of cars. Crash a 5 star compact into a 3 star pickup and the difference in weight is going to make a loser out of the 5 star. So a big tire might have a better RE score but a smaller tire give you better feul economy because there is more to it than RR.
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Old 08-10-2019, 04:47 AM   #19 (permalink)
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Originally Posted by rob.e View Post
Relative to what? I don't understand that statement.
Load index
So it's become popular to increase LI to achieve better RR class
Your car of course has a given (max) weight.

Upside is the higher LI means less deformation, usually allows higher PSI
If your suspension can cope with it without bouncing about.

Measurements are done by manufacturers
No control tests, no penalties.


Wet braking is related to a 2􏰗􏰞􏰣􏰠􏰧􏰗􏰞􏰣􏰠􏰗􏰗􏰞􏰣􏰠􏰧25/60/16 reference tyre
Making it hard for smaller tyres to get the best results

Test conditions can vary wildly in temp and road condition.
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Old 08-10-2019, 05:02 AM   #20 (permalink)
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So a big tire might have a better RE score but a smaller tire give you better feul economy because there is more to it than RR.
Exactly

My C rated winter tyres return the same (even slightly better) FE than the 10mm wider summer B rated ones on larger & heavier rims.

The winter tyres' smaller size, lower weight is more than compensating for their worse RR and less favourable driving conditions.


Go for (or stick with) light (wheels) & narrow if your goal is FE.

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