Why does running one size bigger tires increase fuel economy?

[GasSavers_worthywads]

Originally Posted by Compaq888

http://www.miata.net/garage/tirecalc.html

I came back to this website and I figured the less revs per mile you have the less the tire spins. So my old 195/65/15 tire was better fuel economy if I were to pump it up.

So...
195/65/15 Revs/mi:807
205/60/15 Revs/mi:817 -Current tire
205/65/15 Revs/mi:791 -Clear winner

So would using 205/65/15 give me more of an advantage since it spins less per mile?

Yes the 205/65/15s will be lower revs than the 205/60/15s.

A better option could be 195/70/15 @ 783 Revs/mi.

By moving wider from 195 to 205 you're putting a wider patch on the road and increasing rolling resistence.

My wife's Honda Element manualhas 215/70/16s @ 724 rev/mi and I'm investigating 215/75/16 @ 703 rev/mi. It's geared to be at 4000rpm @ 80mph, I could bring that down to 3884 @ 80. I'd rather not increase width though.

[krousdb]

Originally Posted by MetroMPG

it would be cool to know the relative MPG for the other gears at that speed.

Yes. But you would have to find a long flat road with a low speed limit. Nothing like that around here. ;-(

[diamondlarry]

I went from 175/70/14 up to 185/70/14 when I replaced my tires last year. I found that I needed to add 5% to my mileage to be accurate. That is why you see some really wierd numbers in my gaslog. My Saturn has enough torque that I can still cruise at as low as 25 in 5th. To do this, the road has to be flat and I can't accelerate much at all.

[MetroMPG]

By moving wider from 195 to 205 you're putting a wider patch on the road and increasing rolling resistence.

...and also increasing both frontal area (aero drag), and wet weather resistance by *small* amounts.

then again, the wider tire may let you corner more comfortably faster, so you save energy there

Yes. But you would have to find a long flat road with a low speed limit. Nothing like that around here. ;-(

plus you would have to drive inefficiently, briefly... ON PURPOSE! AGGGH!

i *almost* went out and did the run this afternoon, but it was around -13C and the block heater wasn't plugged in, so i changed my mind. unless someone else beats me to it, i'll do it next time i go on an errand run. it's a quick 'n' easy test, and will be interesting.

[MetroMPG]

just had another thought... will a car with an auto tranny see an equivalent benefit from going to taller tires as you would see in a car with a manual transmission (=direct engine-to-wheels connection)?

or will the auto torque converter just slip more to keep the engine rpm up where it normally is?

[Compaq888]

Yes the wider tires do help me corner better, what I do on the freeway is when a turn is at 35mph onto a different freeway i cancel my cruise control and let it coast 60mph in that turn and they hook up great. So do you think the 205/65/15 would be the best choice?? Since it will give me a wider patch and less revs per mile.

[Compaq888]

Originally Posted by MetroMPG

just had another thought... will a car with an auto tranny see an equivalent benefit from going to taller tires as you would see in a car with a manual transmission (=direct engine-to-wheels connection)?

or will the auto torque converter just slip more to keep the engine rpm up where it normally is?

My rpm stays the same at any speed, but the mpg just go up. Doesn't matter what tire I'm using.

[MetroMPG]

Originally Posted by Compaq888

My rpm stays the same at any speed, but the mpg just go up. Doesn't matter what tire I'm using.

remember though, the mileage benefit from a taller tire comes from reduced engine/transmission rpm. so if your rpm doesn't drop with a larger diameter tire...

[GasSavers_worthywads]

Did a little research on tire sizes for the Element.

Stock size Goodyear Wrangler HP 215/70/16 27.8Diameter, 750rev/m.

No such thing as a 215/75/16?

Replacement Goodyear Fortera SilentArmor 225/75/16 29.3D 714rev/m.

So this could drop my rpm from 4000rpm@80mph to 3808@80, 4.8% reduction.

But it will also increase tire width 4.7% and raise vehicle height by .75 inch.

My guess is it's a wash, anyone else want to speculate?

[SVOboy]

So this could drop my rpm from 4000rpm@80mph to 3808@80, 4.8% reduction.

But it will also increase tire width 4.7% and raise vehicle height by .75 inch.

My guess is it's a wash, anyone else want to speculate?

Prolly a wash I'd say, or an expensive experiment it'd be hard to ever know the results of. What's the bolt pattern on the element?