A friend of mine told me recently that she knows someone that put bigger wheels on his car to lessen the number of miles that would be put on his odometer. His new wheels, she reasoned, use less revolutions than his old wheels to take the car the same distance.
I guess the question boils down to this: how does an odometer figure how many miles a car has travelled? If there’s a formula that X revolutions = Y miles, then maybe the bigger wheels idea has merit. But it also seems that the car and/or odometer would compensate for the size of the wheel and/or axle, and the milage would be the same no matter what the size of the wheel.
Also, I know that if the bigger wheel theorem holds true it’s probably a very insignificant amount (like saving him 1 mile in 10,000 or something like that), but I’m just wondering if the principle is true.
I believe the take-off for the speedo and other instrumentation is usually on the gearbox and simply counts revolutions of the main drive - the instrumentation has no way to ‘know’ that larger wheels are fitted and that compensation is necessary.
Suppose the standard tyre is 24 inches in diameter, the circumference will be 24*pi inches (75.39 inches)
Fitting a tyre that is an inch larger in diameter adds pi inches to the circumference, so the it is now 78.54 inches.
Which means (in this example) that for every revolution of the wheel, you travel a little over 4% further, a little over 4% faster.
Ignoring deformation of the rubber, increased friction on the gearing, torque blah blah…
As an average speed, relative to the ground, no, but relative to the axle, yes, the point closer to the centre HAS to move more slowly because it has less distance to travel in the same time as a point nearer the rim.
I remember reading one of Cecil’s columns (http://www.straightdope.com/classics/a1_258b.html) regarding how we got the standard speeds of records (33 1/3, 45, etc). I believe the answer to your question is no.
Its rotational speed will be lower, as it has less far to travel to make a revolution. Think of being in the middle of a child’s roundabout, vs. being at the edge of it. At the middle, to make a full revolution, your nose will rotate, say, one metre in five seconds. At the edge, your nose will have to rotate maybe 20 metres in the same amount of time.
It depends on how much bigger the wheels/tires than the standard ones for that vehicle.
As Mangetout said, adding only 1 inch to the diameter adds roughly 4% to the circumference on a 25" tire. Adding 2.5" would add 10%.
This 10% will translate directly to the speedometer, hence the speedometer would read 54 mph while actually traveling 60, and the odometer in that for every 60 miles traveled, the odometer would only count 54.
After 10,000 miles then odometer would read 9,000.
Interesting fact: With respect to the ground the center of the wheel is travelling forward as the same speed as the car.
The bottom of the wheel is not moving at all (obviously, unless it is skidding) and
The top of the wheel is traveling forward at twice the speed of the car.
A chunk of tire which would detach from the tire at the point of contact with the road ould remain stationary. A chunk of tire which would detach at the highest point would shoot foward at twice the speed of the car.
There are handy Java-based calculators on the web to figure out how much a change in tire size affects the speedometer. Plenty of non-Java ones too, that’s just the first one I found; Google “speedometer error calculator” for more.
As others wrote, the principal is correct. This assumes that when you say wheel you mean wheel + tire, because what matters is the outside diameter. If you switch from a 15" wheel to a 20" wheel, but at the same time switch from a tire that has a 5" high sidewall to one with a 2.5" sidewall, you haven’t changed the outside diameter (which in both cases would be 25").
However, even if you use the original wheels and tires, your odometer is nowhere near 99.99% accurate (which is equivalent to 1 mile in 10,000).
Tires start off with anywhere from 10/32 to 13/32 tread depth, and are supposed to be replaced when they get to 2/32. That’s 8/32 to 11/32 difference in radius, which is 16/32 to 22/32 difference in diameter. Given that typical car tires are about 25 inches in diameter, that’s a difference of 2% to 2.75%, giving a
potential difference of 275 miles over 10000 miles just from tire wear.
OK. So it matters. When I get the wild hair to replace my 12" tires with 14" tires (I have a small car) what are the odds that anyone will suggest I change my speedometer gear (or whatever it’s caled)? How about when I replace the 13" street tires on my pickup with giant oversized wheels & lift kit? Is changing the gear ever done?
You can change your rim size from 12" to 14" inch, but that doesn’t mean the tire height will change, you can just use a low pro tire on the 14" rim, if you wanted, and nothing really changes.
Now if your going to make a drastic change like replacing your 13" tires with some monster truck tires and a lift kit, it would be wise to change out your gears so your engine does’t work so hard to go the same distance as before.
You can’t go up all that much in size - you are limited by the wheel wells. When you up the wheel size, typically you’ll also go for a lower profile tire (i.e. shorter sidewall), so the difference won’t be much (5%?). Going the jacked-up truck route, you can drastically change your tire size, but unless you know what you are doing the handling will be so messed up that you’ll never have to worry about going too fast.
But yes, speedo gears are changed. Not too often, mind you, but it can be and is done.