[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)