Why does running one size bigger tires increase fuel economy?
 

I remember this from way back in the day, but I still dunno why this would help? Can anyone tell me? I know larry is running this so mehbe he'll chime in.

i asked a similar question a
 

i asked a similar question a while back, relating to swapping to a "taller" transmission. the concept is the same.

https://www.gassavers.org/forum_topic/transmission_swapping_for_mpg.html

if you put a taller tire on the car, it has the same effect: lower engine RPM for a given road speed = reduced losses to internal friction (engine, transaxle).

you just have to make sure you have enough torque to compensate for the taller tranny/tire. (for most cars this won't be an issue; for my car it is an issue)

when changing tire dia., you would also have to correct your speedo/odometer, or compensate manually when doing fuel consumption calculations.

i tried to come up with a way of thinking about this that makes sense intuitively, and this is what i got: are there times you wish you had an additional (higher) gear (e.g. 6th)? if yes, then you could probably benefit from a taller tire/tranny.

Hmm. Interesting, still a
 

Hmm. Interesting, still a little puzzling to me though, but I think I got it.

you got me thinking about
 

you got me thinking about "losses due to internal friction", and wondering about just how large or small they are.

a quick, easy and illuminating data-gathering experiment for someone with an instant FE readout would be to record MPG at a constant speed (say 40 mph), on a consistent grade, in gears 2, 3, 4, & 5.

would this not reveal the magnitude of losses from internal friction at various rpm for that car's engine & transmission?

and then, wouldn't you be able to plot a curve and estimate changes in FE from further reducing rpm with taller tires/tranny?

any takers?

if not, i'll try to remember to do it next time i saddle up the blackfly (in a day or 3 probably).

What'd I do to make you
 

What'd I do to make you think that? :p

Dan did his first (with corrections), I think, commute on the superMID m-1 and told me via pm that he hit 56 mpg, which is nice. Also, he said that times when he wanted to be in forth it was so much better to be in 5th, and that despite a bit a lugging he was getting 70-80 mpg in fifth from 35-40 mph.

Re: What'd I do to make you
 

i believe that. i've seen similar figures in my car in high gear/low speed situations. (though i haven't yet specifically looked for my "sweet spot" for the absolute best mpg/steady speed combination).

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

So if my stock tire size is
 

So if my stock tire size is 195/65/15 and now I'm using 205/60/15, how big can I go to get a mpg boost??? My rim is 6 inches wide.

https://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?

Re:
 

Yes, but your odometer wont show that you have travelled farther. You would have to add 2% to your odometer reading when you calculate MPG. That doesn't mean your MPG will increase by 2% however. I wouldnt go to the larger tire size unless you need new tires anyway. If that is the case, you would be better off with 195/70/15 or 195/75/15 if you can find them.

Re: What'd I do to make you
 

I did show 56 MPG but it wasn't my commute. It was a trip to the in laws which is even more FE friendly than my commute. What was really sweet was my trip from the gas station to home yesterday. 67 MPG with the ICE on only 52% of the 3 mile distance. It was downhill so not really fair. Sweeeeet!

Re:
 

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.

Re: What'd I do to make you
 

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

I went from 175/70/14 up to
 

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.

Quote:By moving wider from
 

...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 ;)

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.

just had another thought...
 

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?

Yes the wider tires do help
 

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.

Re: just had another thought...
 

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

Re: just had another thought...
 

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...

Did a little research on
 

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?

Quote:So this could drop my
 

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?

Re: Quote:So this could drop my
 

Is 5x114.3 the bolt pattern?

I'm at 20,000 miles and will be needing new within the next 10,000 it looks. I won't be deciding until the oem Wranglers are toast, but they are wearing fast.

Yep, search from some
 

Yep, search from some lightweight rims for that stuff, :p, that's the same bolt pattern as the ITR so you can get some nice stuff, ;)

Re: Yep, search from some
 

ITR?

Is there a ROI on lightweight rims?

ITR = Integra Type R ROI = ?
 

ITR = Integra Type R
ROI = ?

Re: ITR = Integra Type R
 

ROI = Return on Investment.

lighweight rims will help as it reduces rotational mass, but the ROI is unknown. It's probably one of the "hard core" modifications for extreme enthusiasts.

Re: just had another thought...
 

I didn't really pay attention to the rpm with my old tires. All I know is my OD always starts at 37mph.

Re: just had another thought...
 

Waiiiit... I thought you just said :

It seems that you either paid attention to your RPM or you didn't. Both of your statements can't be correct.

Re: just had another thought...
 

I explained to you in the IM the difference between them.
I'll explain for the rest here too.
My OD went on at the same speed with both tires, but the 3 other gears are way different.

Next taller tire is actually different. I went to the track a couple of times with my 195/65/15 tires and I got consistent 2.4xx 60'. With my kumho's that are 205/60/15 my 60' actually got worse because they are shorter. My 60' was 2.5xx-2.6xx with the 205/60/15. The kumho's are wider so I should have more traction and some of you are going to say that the other tires were of better compound and hooked up better and that is where you're wrong. My other tires would barely hook up in the rain for regular street driving and I almost wrecked 3 times in one month because of them. I even bought the kumho's because I eventually did wreck because of them. Now the kumho's have great traction, I can make turns at higher speeds and they have great wet traction. I actually have to go WOT just to spin my tires in wet traction.

So the conclusion is having a bigger tire makes a difference at the track and it should improve your fuel economy because it spins less per mile than the smaller tire.

Re: ITR = Integra Type R
 

I don't understand the issue with rotational mass and FE. Sure, you invest more energy to accelerate, but you get it back while coasting. ;-)

Re: ITR = Integra Type R
 

I think the FE gain with lightweight rims is all in city driving. It requires less energy (ie, fuel) to spin the tires. For stop and go traffic, that would be a major bonus.

I'm also sure that it takes a slightly less amount of energy to maintain a cruising speed with lightweight rims.

Quote:I don't understand the
 

When I studied physics last year and we were doing intertia one of the first things was about a car that had a 2000 pound mass and 1000 pound flywheel. If you invested in getting it up to speed it would take four km to wind down or something. However, you gotta know that eventually you will lose that energy to braking, much like having a heavier car. If you invest in moving 300 extra pounds you gotta stop it sometime and that investment will be wasted.

If I hung 10 pounds on my rims and it didn't affect drag the coasting would probably last longer, but seeing as how energy is not converted perfectly to get the wheels moving the savings wouldn't be there in my mind.

Re: Quote:I don't understand the
 

Brakes? You use brakes?

I agree that if you use your brakes a lot, lighter wheels would make a difference. But I use my brakes so little that I doubt the difference for me would be measureable.

It's too bad Matt didnt hove those HX wheels on ebay about a month ago. I would have had those light weight wheels and lighter weight 14" tires. Oh well. I can't complain with what I have.

Get some damn vx rims, mine
 

Get some damn vx rims, mine will be delivered tomorrow. :)

Re: Get some damn vx rims, mine
 

If I decide to go all out then maybe VX rims. But that also means stripping the interior, filling the tires with helium and switching to aluminum wheel weights. ;-)

By the way, did you finish the rear springs? Post some pics.

I would not call 100 for
 

I would not call 100 for sexy *** rims all out and helium is just being lazy about checking/filling.

I finished the ****, had to jack the car back up a bit. Will post some pictures tonight and write the DIY tomorrow night for lack of time tonight.

Need to align it myself, which should be fun! DIY on that too.

Also, I found some coilovers for you if you want them.

Re: I would not call 100 for
 

Look forward to the DIY's. Thanks for offering the coilovers, but I won a set on ebay and they will be here on Wednesday.

Haha, damn, was gonna get
 

Haha, damn, was gonna get this set for 20 and they sell for 150 shipped on ebay, :p

LOL you get everything for
 

LOL you get everything for low price. Too bad you can't get altima parts that cheap. I've been looking for G20 rims that weigh 13lb stock for a while now.

note, see new related
 

note, see new related "experiment" thread:

rpm/gear choice VS MPG at constant speed

damn all this tire talk, all
 

damn all this tire talk, all i know what to do is pump them near the 50 PSI limit. By the way do alloy wheels cost more to replace than standard steel wheels?