Tire size and MPG

[Piwoslaw]

I'm posting here since this is the freshest of the many threads with similar title.
This morning I found this local article, Opona wymyślana na nowo. The Google translation appears to have a problem with formatting (the ads on the right cover part of the text), so here's what it's about:
Michelin sponsored a tire conference in Rio and among the new trends is a very small diameter tire - 175/70R10. Using a 10 inch wheel shaved 40kg (88lbs), which should help fuel economy. Here's the translation:

The first one is a very small, 10-inch tire and wheel (175/70 R10), which provides the same performance (traction, braking ability) as a 14-inch tire (175/65 R14). Thanks to miniaturization, the total vehicle weight was reduced by almost 40 kg, which translates into reduced fuel consumption and reduce carbon dioxide emissions.

And that's it. No word about lower rpms, gearing, etc. They didn't mention that the savings from switching to ATV tires would only be in stop and go traffic.

[mcrews]

pete said: "[B] I figure the added wear on tranny and axles is easily offset by the reduction in rpms/engine life. [B]

The real concern is in the small stuff. bushings and brakes.
But with the majority of your driving being highway you probably don't need to worry

the tires weight about 3.5 lbs more and the 15 rims about 4lbs.
So you at 7lb x 4 = 28lb of moving extra weight.
the old set up was about 125lb moving and the new is around 148lbs.

[pgmomni]

Originally Posted by mcrews

pete said: "[B] I figure the added wear on tranny and axles is easily offset by the reduction in rpms/engine life. [B]

The real concern is in the small stuff. bushings and brakes.
But with the majority of your driving being highway you probably don't need to worry

the tires weight about 3.5 lbs more and the 15 rims about 4lbs.
So you at 7lb x 4 = 28lb of moving extra weight.
the old set up was about 125lb moving and the new is around 148lbs.

I am aware of the added unsprung weight and the increased inertia to get things rolling.
There was a very detailed thread on Toyota Nation regarding the effect of larger diameter tires/heavy wheel-tires.

What a day, drove down to NYC, worked from 11:30 to 8:30 and drove back home to Maine. The summer repaving has started and there is road work all over the place. Lots of traffic jams. Took 5 hrs to get down and 5 hrs to get back.
Scangauge says 35.9 MPG. Will fill up tomorrow as
today my new Full Moon Racing Hupcaps came from Hupcap King!

Pete

[Ecobalt]

Originally Posted by pgmomni

... They are 6.3% oversized. Stock was 195/70-14, now using 205/70-15.

Lowered my RPM's almost 200 rpm at 75 mph.
Wheel/tire setup is slightly heavier but it dont feel it with the 5 speed manual tranny. ...

Gained a whole inch of ground clearance on the Camry. I can drive right over most normal sized curbs now if i need too!
...
Mounted them tires myself with a crowbar and Stanley Wonder Bar.
Need to get someone to edit the video clips doing it.

Pete

You could put "stiffer springs" on the car to keep the wheels from striking the top of the inner fender. The author of the Canyon Hiking Guide to the Colorado Plateau did just that to his VW Rabbit. He wanted to prove that you don't need a 4WD vehicle to travel unpaved, rutted roads. There are tons of "lowering springs" available out there. I have not seen stiffer springs. You might buy air-assist or some sort of coil-over shock absorber.

I wish I could see that video of you changing tires with a tire iron.

[WA1DH]

Thanks for all the input. Looks like I sparked quite the discussion..

I wound up with P185/70R14 tires. I got a good deal on 2 different pairs of like-new tires (>90% tread remaining) in that size. I've only put maybe 30-40 miles on them, but if my Scangauge is accurate (I adjusted the offset for the new tire size), it uses more gas to accelerate, but less gas at cruise. I'm guessing this is due to higher gearing with the taller tires. Not very scientific just watching the Scangauge right now, but I'll post back with some real numbers after a week of commuting with them (~600 mi).

[mpgx2]

Tyre width has an enormous impact on fuel economy.

Narrower tyres get you less rolling resistance and are
more economical. Wider tyres get you more grip but
it comes at a fuel consumption cost.

If you want more cornering traction, get a wider tyre.

If you want more economy get a narrower tyre.

Also, some brands are better than others. Some
european brands like continental and michelin
are worth the extra money.

Final consideration is how do you drive? Are you
a ding dong who can't control a car? go for the
wide tyres. If you drive a bit slower and are not
into powerslides (narrow tyres are better for that
too actually) then one size thinner than what
the manufacturer suggests is a good idea.

I'm always amazed that people put wider tyres
on cars than is needed..

[CapriRacer]

Originally Posted by mpgx2

Tyre width has an enormous impact on fuel economy.

Narrower tyres get you less rolling resistance and are
more economical. Wider tyres get you more grip but
it comes at a fuel consumption cost.

If you want more cornering traction, get a wider tyre.

If you want more economy get a narrower tyre.

Also, some brands are better than others. Some
european brands like continental and michelin
are worth the extra money.

Final consideration is how do you drive? Are you
a ding dong who can't control a car? go for the
wide tyres. If you drive a bit slower and are not
into powerslides (narrow tyres are better for that
too actually) then one size thinner than what
the manufacturer suggests is a good idea.

I'm always amazed that people put wider tyres
on cars than is needed..

First, tire brands is NOT an indicator of RR. Every manufacturer produces a wide range of tires - and some are good for RR - some are not.

There is a technology triangle: Rolling Resistance, Traction (especially wet traction), and Treadwear. These are all traded off to get the final result - and it's always a compromise.

Second, while it seems intuitive that tire width would be important for RR, load carrying capacity seems to be more important. Going larger in load carrying capacity is directionally the right way, while just going narrower doesn't seem to pay much benefits, and especially if you are also going down in load carrying capacity, you're going the wrong direction for RR.

[cfg83]

CapriRacer -

Ok, this is cool. I have been noting load carrying capacity but I have not been associating any significance in terms of RR.

CarloSW2

[Piwoslaw]

It appears that narrower tires always have lower load capacity, or at least in the cases I've seen. I'd already be 1-2 sizes narrower, but I need the max load capacity of the car once every few months.
Also, a smaller tire may also have lower max sidewall pressure (eg 45 vs. 51 psi).

[gone-ot]

...often, similar diameter "truck" tires with equal or greater load capacities have narrower widths due to their typical "dually" mountings.

...might be worth looking into what they offer!?!