driving on 4 narrow spare tires (or similar) to reduce rolling resistance

[mans]

Hi guys,
while driving on the street I see the car in front of me with a doughnut on his rear wheel.
I can't help but notice how narrow the doughnut spare tire is, it looks like 1/2 the width of the normal tire.
At the minimized ground contact + the higher pressure of doughnut tires I thought to myself there should be way less rolling resistance if you were driving on 4 of these... tho you'd need to find ones that have the same outside diameter of your full size tires so you don't decrease your engine rev's.

my car is 2500 lbs, and i have a 4cyl. my tires are 195/R15.

I did some research and found I can purchase tires that are only 125 mm wide instead of my 195's (these 125's are made for some types of very old vw's and similar old small cars) and I can get the proper wheels that would mount these tires to my car's bolt pattern too.

my regular tires are 195's. -thats 195 mm wide.

the new ones would be 125's. that's 125 mm wide that's almost 35% narrower then the stock tires

(as to safety - the load capacity of the 125's are adequate and I wouldn't care about a possibly bumpier ride that the 125's might bring since they require higher pressure)

making this switch would cost me around $425.

worth it?

anyone know what kind of mpg gain the decreased rolling resistance would equal?

right now I have no idea if it's 1% or its 20% or so I'm having trouble deciding if I should go for it.

can anyone shed any light on this?


(I do know that 10 speed bike tires are really low rolling resistance, so the smaller your footprint is the better your mpg's should be but I have trouble calculating what the gain would be for this car tire project, I welcome all feedback)

[drmiller100]

there are actually 3 numbers on tires.

the 195/85/15 for instance. The middle number is the ratio of height to width.

if you want a narrow tire, you want the middle number to be high. 85 is about as narrow as it gets.

[mans]

the size of our tire footprint is almost all decided by the first number.

185/70/R15 tire will be 185 mm wide.
185/85/r15 tire will also be 185 mm wide.
the '85 will be a little taller but the footprint will be the same size.

185's are the narrowest you will find in a tire store today.

I have managed to find 125's from a supplier that sells to people who are restoring vintage vehicles and many really old ones had narrow tires that you don't see today.
the thickness of the tread is 2.75" wide instead of almost double that on my 195/70/R15's.

[BHarvey]

http://ecomodder.com/forum/showthrea...eet-19126.html

[mcrews]

there are several posts on the 'spare tire' issue.
Read them.
while you are not using the 'spare tire' -thank God- there are lessons to be observed from running a toooo narrow tire.
ANY decrease in overall diameter will result in lower mpg.
Will the new size have a smaller od?
THe lose of handling will be huge. especiaaly at higher speed.

[Ryland]

The 1,900 pound weight of a VW beetle is much lighter then your current car, so just like how I'm to heavy for 10 speed bike tires to work well you might find that 125mm wide tires are to narrow for the weight of your car.

You don't say what kind of car you have or even your full tire size so it's hard to say if there is a better match, you should be able to go narrower and not see a drop in mileage but 70mm narrower seems a bit harsh, your best bet would be finding a low rolling resistance (LRR) tire and lighter wheels that have a smooth face, something like Saturn SL1 alloy wheels.

[mans]

my current tire is a 195/60/R165

the car weighs between 2,400-2500 lbs.

ecomodder doesn't let me post links yet I don't have enough posts.

To see these very interesting and highly unusual tires please see:
"cokertire.com", and
click on "catalog"
then click on "tire size"
and go to R15(you must go to R15 it's not available in other sizes)
then click on 125/R15 or on 135/R15 (the 135 has the higher load rating)


one is a 125mm wide, the other is a 135mm with a substantially better load rating.
it was a giant pain to find wheels that would fit these tires and fit my bolt pattern, but I found them.

these tires are so narrow they looked like the dream mpg tire,
however, the info I see from metro mpg's tests show these might not work and it just baffles me. I really just can't understand how the doughnuts@70 psi didn't just rule the contest they're so skinny.

there is no center number for this tire,apparently they didn't have those back then. my guess is its like a 125/150/R15

[euromodder]

Originally Posted by mans

worth it?

MetroMPG has tested 4 narrow spare tyres, and it didn't work out well.
Check out the link BHarvey posted.


The narrow size will help, but it'll only be worth it if you can get narrow tyres that also have lower rolling resistance than the tyres you're running now.
Pattern and compound make more of a difference than size.

If these narrow tyres were made for vintage cars, chances are LRR wasn't on the list of design requirements.

[mcrews]

your asked to tell us what car you have........and you dont......odd.

You ignore the risk to poor handling and braking.......oddd.

When asked the od .....you ignore that?

Kinda have your own agenda.........why are you asking for help again?

[Ken Fry]

"Narrowness", in today's tires has little to do with rolling resistance. There are 165 85 15 tires (old VW tire) with nearly twice the rolling resistance of the 185 65 15 Prius tire.

Rolling resistance varies from about .006 to .014, with sticky, sporty tires being at the high end.

The temp spare donuts are often bias ply rather than radial, and the compound is not designed for low rr -- the spare is already dangerous enough without compromising traction in favor of low rr. So even at sky high pressures the temp spare does not work well as a low rr tire. (And in every other respect besides weight, they are awful.)

There is a fair amount of science involved in designing low rr tires. The tread compound is the most obvious difference, and one that leads many people to be not all that thrilled with low rr tire handling characteristics.

There is no substitute for finding the coefficient of rolling resistance for tires. Guessing does not produce reliable answers.