Quote:
Originally Posted by achang1
i want to say getting thinner tires got me an extra 1-2 mpg, more on the 1 side than 2.
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Ah I see what you are saying. You are talking about their horizontal surface. How does that affect the tire's lifetime?
If you drop from 215 75r15 to 215 60r13 you see much bigger gains that if you shrink the tire width. The reason for that is if you shrink total wheel-tire diameter (dropping tire profile 15 mm and wheel diamter 2 inches(at least looking at my tires r stands for diameter. . .?)) you drop a substantial amount of weight on the tire total and none of the tire is where all of your tire used to be.
On a Grand Cherokee 13s probably are not possible, but 60's instead of 75s definitely are.
Just for a feel in the change my 18 inch wheels weighed over 50 lbs each and my 13s are easily under 30 lbs. The tires themselves are about the same weight because the 18s were 40 mm and the 13s are 60 mm. The 13 is slightly lighter because its a 185 instead of 225.
Ok then we aren't really arguing about the real physics here. Unsprung weight is probably a poor term for talking about the tires because if you had something else just hanging on the car unsprung it would not heavily influence the car's mpg. adding a 120 lbs of weight at the bottom of the springs would not drop my mpg back to 25, maybe 34 ish. Adding tires that had an additional total 30lbs each would radically reduce your mpg just because of the uniform force needed to maintain uniform acceleration(25 MPG).
But also even if you can't reduce the weight of your tire but you can reduce wheel-tire diameter(on your next tire swap) you will see gains on MPG because its reducing the force needed to spin your tire(also reducing your ground clearance and making your underbelly drag slightly less).