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Old 03-15-2018, 04:15 PM   #1 (permalink)
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Tire width impact on fuel economy / MPG

Has anyone quantified tire width impact on FE?
Currently looking at tires and rims. Would like to go about 7% larger circumference both to get my speedometer more accurate and to effectively get taller gearing in my 98 Odyssey. It's a 2.3L 4 cylinder v-tec and it revs a bit under 3000 rpm at 100km/hr.
I could go with more rubber (taller tires) or more rim. More rim helps with rotating mass and unsprung weight, but the skinniest tires I can find are 215's for a 18" rim, whereas if I went with 16" rims I could get 205's but take the hit on weight (and looks).
Thoughts?

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Old 03-15-2018, 04:26 PM   #2 (permalink)
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Quote:
Originally Posted by westygo View Post
Has anyone quantified tire width impact on FE?
Currently looking at tires and rims. Would like to go about 7% larger circumference both to get my speedometer more accurate and to effectively get taller gearing in my 98 Odyssey. It's a 2.3L 4 cylinder v-tec and it revs a bit under 3000 rpm at 100km/hr.
I could go with more rubber (taller tires) or more rim. More rim helps with rotating mass and unsprung weight, but the skinniest tires I can find are 215's for a 18" rim, whereas if I went with 16" rims I could get 205's but take the hit on weight (and looks).
Thoughts?
Are you trying to go heavier or lighter?

My personal experience has been with swapping from 17" wheels that were fairly aerodynamic and weighed 48 pounds with tire down to 16" wheels that are not aerodynamic and weighed 36 pounds with tire. Overall, the smaller wheels are more pleasant to drive with, the car accelerates easier and faster, and braking is noticeably better(much appreciated!) too! When I went to 16" wheels I also went with dedicated LRR tires, the Bridgestone Ecopia EP422+. Highway fuel economy has stayed the same, and lower speed coasting has improved slightly. I went from 215/50/17 tires down to 205/60/16 tires.

I also found 11 pound 15" rims a while ago that would fit my car, so when I need new tires(assuming I keep the car that long), i'll be going down to a 205/65/15. My current 16" rims weigh 16 pounds each, and the 17"ers were 23.5 pounds.

There are those on this forum that will say a wider tire will give you less rolling resistance, which I find hard to believe, and haven't seen any information on why that is.
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Old 03-15-2018, 06:45 PM   #3 (permalink)
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Quote:
CD --- tire & rim size (for a 1991 BMW 318i)

0.293 --- 155 R 15; 5 1/2 Jx15 St. with wheel covers
0.294 --- 165 R 15; 61 2 Jx15 St. with wheel covers
0.297 --- 175/70 R 15; 6 Jx15 St. with wheel covers
0.305 --- 185/65 R 15; 61 Jx15 St. with wheel covers
0.311 --- 205/60 R 15; 61 2 Jx15 St. with wheel covers
0.314 --- 205/60 R 15; 7 Jx15 LM
0.319 --- 225/55 R 15; 7 Jx15 LM
From thread: tire width vs. drag (Cd)

The problem is you're changing multiple variables when you change wheels/tire size. Is the replacement tire going to have better or worse rolling resistance? How will you know?

It's a bit of a crapshoot.
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Old 03-15-2018, 08:12 PM   #4 (permalink)
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Quote:
There are those on this forum that will say a wider tire will give you less rolling resistance, which I find hard to believe, and haven't seen any information on why that is.
Name them so we can point and laugh. While we explain that Cd is not rolling resistance.

To your point though, the modern trend is tall and narrow. See Neil Blanchard's thread, it's the most comprehensive. Bridgestone Announces Large Diameter Narrow Tires
Quote:
All else being equal, a large(r) diameter narrower tire will have lower rolling resistance and lower aero drag than a small(er) diameter wider tire.

One reason is that a larger diameter tire will 'bridge' across unevenness of the road better. Another is that since a tire is essentially 'wedging' the weight of the car uphill constantly - a larger diameter tire has a mechanical advantage, because the 'wedge' it provides is a lower angle.
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Forum member Vekke is running them. I would be too, if I could afford the wheels. Prolly 1952 MG-TD wires with knock offs, right?

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Old 03-15-2018, 09:16 PM   #5 (permalink)
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It's well established that wider= lower rolling res.

Quote:
One reason is that a larger diameter tire will 'bridge' across unevenness of the road better.
Same can be said for wider. You can look at the Cummins data for rough vs smooth roads. A narrow, high pressure tyre is a bit like bringing a rough road with you everywhere you go.

If we want to explore rolling res you can look at bike racing. On an indoor velodrome (wood boards) you'll see them running 19mm tyres at ~150psi (220 is max side wall!).

Road racers used to run 23mm at ~120psi, but since new data has come in, most pros now run 25mm at ~90psi. 28mm at 70psi offers lower rolling resistance still, but 25mm is seen as the best compromise between res and aero drag.

Thing is we all 'knew' that higher pressures were 'faster'. That's how it 'feels' and 100% of riders 'know' that. Now dyno and wind tunnel testing have shown those seat of the pants instincts to be mistaken.

In short, if you have particularly rough roads you may have better results in a more compliant, wider tyre running at a sensible pressure, if you drive on polished concrete go narrow and pump it up till they bulge!

Now having said all that, if you look at Metro's chart, I think you'd be silly to fit wider tyres chasing RR if they throw your Cd out. Those Cd gains are massive -especially as they also reduce CdA.
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Old 03-15-2018, 10:56 PM   #6 (permalink)
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What you say may be true. (155-)175/19 are OEM on the i3 electric. Maybe they sacrifice ride for range. The multivariant nature of the answer Metro pointed to includes rim width (affects sidewall stiffness) and ride height (affects CdA).

My own experience was running 145/15s (down from 165/15 OEM) on the front of numerous Types I and III. The best tuning was on the 1964 Notchback with 5 1/2" rims and a $200 (four-)wheel alignment. It never failed to go where I pointed it.

On the Squareback, not so much. I found out that downhill in the Coast Range I would run out of tires before running out of brakes.

Now the Superbeetle has OEM 165/15 on the back and 165-50/15 on the front. The low sidewall makes the suspension setting legal.

Never heard what overall diameter westygo requires.
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Old 03-16-2018, 01:02 AM   #7 (permalink)
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I hadn't considered that so many variables came into play. Definitely hard to isolate only the aero penalty, then cross reference with rolling resistance...
I am looking for a tire diameter of 27.2" ideally. Plus a point or 2 would be OK.
My tire guy tells me that 16" and 18" tires are more common than 17", so I was mainly looking at those sizes.
That said, I don't think it's that big of a deal. 215-60r17 gives me the exact diameter to make my speedometer correct. 205-70r16 is close, but the cornering is already pretty squishy. Those 70 profile sidewalls would add to that.
Not planning on lowering the car, but maybe if it ever needs struts.
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Old 03-16-2018, 01:06 AM   #8 (permalink)
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The BMW electric with the large diameter and narrow tires looks like it is a go cart with bicycle tires, I guess that is just getting used to what efficiency looks like. I've heard of disappointing range in those cars, seems BMW didn't get a few things right on that one.
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Old 03-16-2018, 01:14 AM   #9 (permalink)
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Shall we go further down the rabbit hole?

Vehicle weight per wheel for the tire and PCI, the bolt circle, and offset for the wheel. Rim width affects turn-in, a handling issue. I went with 165-50/15 over 165-45/15 for payload weight.

Wikipedia lists the i3 weights
Quote:
Curb weight
60 Ah: 1,195 kg (2,635 lb)
60 Ah: 1,315 kg (2,899 lb) (with range extender)
edit: I was looking at the Wikipedia article. About the range issue:
Quote:
In May 2016, BMW announced that the 2017 model year (MY) BMW i3 will have a 33 kWh battery, up from 22 kWh in the previous model, allowing to increased range. The battery pack capacity was increased by more than 50% without changes in exterior dimensions.... The improved battery has an upgraded electronics package that has new software mapping for the battery cooling system and the electric motor. The 94 Ah battery pack fits both the all-electric i3 and the i3 with the range extender.

The Range Extender (REx) variant will also feature the same higher capacity battery as the all-electric model, with a corresponding all-electric range increase. The 2017 REx model will offer additional range in the American market thanks to a fuel tank that is 25% larger than the previous model with capacity for 2.4 US gal (9.1 l; 2.0 imp gal). This is actually the same tank the i3 has always been manufactured with and used outside the U.S., but BMW had locked out the tanks last half-gallon of capacity in the American market to meet California's ZEV requirements for vehicles with range extender, as the car had more gasoline-powered range than all-electric range, which would affect its status as a zero-emissions vehicle (ZEV) in California.
Heh.
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Old 03-16-2018, 06:27 AM   #10 (permalink)
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Quote:
Originally Posted by oldtamiyaphile View Post
Thing is we all 'knew' that higher pressures were 'faster'. That's how it 'feels' and 100% of riders 'know' that. Now dyno and wind tunnel testing have shown those seat of the pants instincts to be mistaken.

In short, if you have particularly rough roads you may have better results in a more compliant, wider tyre running at a sensible pressure, if you drive on polished concrete go narrow and pump it up till they bulge!
My understanding of this is that "faster" on bicycle tires isn't strictly rolling resistance. Having very hard tires increases rider fatigue, even if the bike rolls easier, so you find people can't put as much power to the road for as long. It's a human limitation which motors do not share.

Correct me if I'm wrong, but a smaller contact patch will result in less rolling resistance, and there are several ways to achieve this. Adding pressure reduces sidewall flex, and sidewall flex eats less forward motion. However, sidewall flex is also what gives a suspension effect, because it also eats vertical motion. The reason we use hollow tires filled with air is not for speed, but for comfort, and to prevent irregularities in the road from tearing our vehicles to bits.

All else being equal, it's better for rolling resistance if the contact patch is longer and less wide, so if you're shooting for lowest rolling resistance possible (often at the expense of ride quality) you'll want very narrow tires with a relatively large diameter - approaching the shape of a bicycle tire.


Last edited by Ecky; 03-16-2018 at 06:32 AM..
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