Odometer readings (New vs. Old tires)

[Brian D.]

"Thanks", MetroMPG for the writeup on Odometer readings & Google Earth's uses. Reading it reminded me of something I haven't thought of for a while. I'm sure this is like splitting hairs, but where else could I take this question--where I wouldn't get laughed at? Here goes:

Has anyone ever wondered, or better yet FIGURED what the odometer difference would be between a given set/size of NEW tires versus balding tires? Since the circumference of a new tire is certainly larger than a bald tire, there is obviously a difference to be calculated; I just wonder what it is. Once again, calling all geeks on this one. I'd love to know the scale of difference, but I've not done the experiment myself (yet).

[lindermant]

I'll take a stab at it... using info from this link and back-tracking:
http://www.miata.net/garage/tirecalc.html

assuming a stock tire size of 185/60-14:

diameter = 22.7 in
circumf = 71.4 in
revs/mi = 887.0

lets say you have completely bald tires and assume the stock tread depth was 20/32 in (0.625 in); we'll subtrack 1.25 in from the stock diameter to get 21.45 in.

bald tire size 185/60-14:

diameter = ~21.45 in
circumf = ~67.35 in
revs/mi = ~940.7

I'm probably dead @$$ wrong, but hopefully I'm at least leaning in the right direction...

[krousdb]

Most tires have 9/32 to 11/32 of tread depth where the last 2/32nds are considered illegal. So a tire at the legal limit would be about 1/2" less diameter. So for the example given previously you would have 909 revs/mile. That would lead to a 2.4% odometer over-registration. That is assuming that your odometer was dead on to begin with.

[lindermant]

ayep - kbd is right... a quick look at tirerack showed a tread depth of 10/32 vice the 20/32 I mentioned earlier.... close but no cigar for me

[Brian D.]

Good enough though. Thank you, both. Works for me!