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other factors... (archive)

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Posted by YLO///M3 on September 14, 2000 at 21:25:51:

In Reply to: Interesting math for 17 -> 18 in wheel conversion posted by Jonny Estoril on September 14, 2000 at 21:04:19:

The 18 inch wheel has a greater rotational moment of inertia and that means it takes more torque to accelerate it. I think this might consume the extra gas. Also, the larger tire has greater drag (a lot of drag is actually associated with tires because the spin so fast). For the same wheel type an 18 inch wheel tire combo will be heavier also increasing the rotational moment of inertia and consuming more gas.

All in all I do not think a bigger wheel is more efficient although very small wheels are bad as well because the bearing friction becomes a bigger part of the equation as the diameter decreases.

Also interesting - look at Indy cars. They have like 13 inch wheels with very wide tires. I believe the reason for this is to decrease rotational mass, unspung weight, to help with handling acceleration and fuel consumption. Every little bit helps in long races.

If you are not an engineer than you will probably prefer not to read all this.

Ok well I may be upgrading to a set of 18" wheels in the future. Being that my car is in Florida and I go to school in Boston. That means i'd have to drive the car 1500 miles to get it up. I figured maybe I would theoretically get better mileage with the 18" wheels on such a long trip so I decided to do the math.

Here are the facts:
1 mile is 5,280 feet which converts to 63,360 inches. The distance to Boston is 95,040,000 inches. The diameter of an 18" wheel is 25.401 inches while the Diameter of a 17" wheel is 24.716 inches. (Both measurements were calculated WITH tires) To get the circumference, you must multiply the diameter by Pi. Cirumference for 17" and 18" wheels are 77.65 & 79.80 in respectively.

An 17" wheel will rotate 815.97 times per mile, so 1,223,955 times over 1500 miles.
An 18" wheel will rotate 793.98 times per mile. so 1,190,970 times over 1500 miles.
The 17" wheel rotates 32,985 times MORE than the 18". So, when added in if they rotated the same amount, the 18" wheel went 41.54 miles more.

I figured if I get 25mpg with the 17"s over the entire trip, it would be a reasonable estimate. Which means I would use approximately 60 gallons of gas.
Subtracting the 41.54 miles from the mileage, I would attain 25.7mpg if I held the same speed. If that was the case, I would save 1.63 gallons. Not much of a savings in fuel but I think the distance difference was more interesting.

Of course, this was all done on paper and didnt count in such factors as exrta drag, changed ratios, etc. If anyone can correct me on this, feel free to. I am open to feedback as this was just an experiment =)

Jonathan V.
99 Estoril M3
Euro clears, Triflo DTM "loud"

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