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In Reply to: Re: Not affected by tire size? R u sure? posted by Steve on September 11, 2000 at 04:53:42:
A 245/40 series tire gives you 10 feet of travel time while spinning the axle 1 time in your vehicle. Let us say that spinning the axle 10 times makes the computer think that
the car has traveled 100 feet. Now lets say you change that tire to a 275/40 series tire, thus increasing your sidewall height and the diameter/circumference of the tire. So now 1 rotation
of your 275/40 tire gives you 11 feet of travel. And this turns your axle 1 time. Why? Because your axle is fixed to rotate at the same rate that the tire is. So your vehicle
still thinks that 10 times of rotation actually gives you 100 feet, when you really traveled 110 feet.
To prove my point in the real world, taxi cabs often can be found with almost no tire pressure. Why? Because this will increase the amount of rotations that it takes the
wheel to cover a given distance, due to a smaller tire, thus increasing your fare.
Hi Michael,
I'm not sure if it is possible for any "distance travelled" sensor to still be accurate if you change your tire size. It has to make assumptions based on what the car does - such as number of wheel rotations. Right?
Unless of course it uses some form of radar or gps or other motion sensor that doesn't depend on wheel rotations - but I doubt it...
Well sure, but in this case we know that our car's odometers are not linked to gps systems or anything. So I beg to differ with Mike's comments (Mike's my buddy and we give each other a hard time) that tire size is irrelevant. It IS relevant because tire rotation and distance covered is directly related. When you change tire size, nothing is recalibrated, yet the circumference changes. What you're doing is very very slightly making things inaccurate. But I'm sure it's all within operating ranges. :)
Steve
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