# Attn Bored car nerds and math wonks **URL:** \ **Category:** Factual Questions\ **Created:** [August 13, 2015, 10:47pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782 "2015-08-13T22:47:17Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [August 13, 2015, 10:47pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/1 "2015-08-13T22:47:17Z")
Long story short, I had to replace the front tires on my car with slightly smaller tires. How much does this throw off my speedo and odo meters? Details: useful and otherwise 2001 civic DX manual 144k miles proper tire size 185/70 14" new tire size 185/60 14" I believe I’m correct in thinking the meters are calibrated and measured off the drive wheels, yes? This is a beater car we sort of inherited, so when I was “subtly persuaded” to get new tires, I didn’t want to spend the big bucks on actually brand new tires and instead just got a cheap set of used, and the garage I go to only had the smaller size. note: “subtly persuaded” here means “complete tread delamination at 70 mph on the 101 freeway skidding across 3 lanes of traffic” Also note, I _could_ probably calculate the answer on my own, but I am giving my fellow dopers a chance to figure this out because they love to solve problems. Also, I am lazy. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 13, 2015, 10:54pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/2 "2015-08-13T22:54:39Z")
Easiest way is to use a comparison calculator like [this](http://tire-size-conversion.com/tire-size-calculator/)one ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [August 13, 2015, 11:09pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/3 "2015-08-13T23:09:22Z")
> [@bob\_2](#): > > Easiest way is to use a comparison calculator like [this](http://tire-size-conversion.com/tire-size-calculator/)one right on, thanks! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [August 13, 2015, 11:17pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/4 "2015-08-13T23:17:19Z")
The calculation is really easy, but I don’t understand the notation you’re using for the tire sizes. The odometer and speedometer will be off by the same proportion as the tires. So if you take the ratio of tire sizes, that’s also the ratio of what your odometer will read to what it should. ---
**Author:** ![snfaulkner](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/snfaulkner/32/433_2.png) [@snfaulkner](https://boards.straightdope.com/u/snfaulkner)\ **Post date:** [August 13, 2015, 11:28pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/5 "2015-08-13T23:28:16Z")
> [@Chronos](#): > > The calculation is really easy, but I don’t understand the notation you’re using for the tire sizes. The odometer and speedometer will be off by the same proportion as the tires. So if you take the ratio of tire sizes, that’s also the ratio of what your odometer will read to what it should. I believe, tho not completely sure about, that 185 is the tread width in mm? and the 70 or 60 is the sidewall height as a percentage of the treadwidth, and the 14" is the wheel radius. I’m not completely sure that is all correct because it bugs me a lot that there is both mm and inches in the same notation. Any one know for sure? ---
**Author:** ![zombywoof](https://avatars.discourse-cdn.com/v4/letter/z/9e8a1a/32.png) [@zombywoof](https://boards.straightdope.com/u/zombywoof)\ **Post date:** [August 13, 2015, 11:38pm UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/6 "2015-08-13T23:38:32Z")
> [@snfaulkner](#): > > 185 is the tread width in mm? and the 70 or 60 is the sidewall height as a percentage of the treadwidth, and the 14" is the wheel radius. I’m not completely sure that is all correct because it bugs me a lot that there is both mm and inches in the same notation. Any one know for sure? The width is from sidewall to sidewall (which is not necessarily the width of the tread), and 14" is the wheel diameter, but otherwise you’ve basically got the right idea - see [here](http://www.discounttire.com/dtcs/infoSidewall.do). ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [August 14, 2015, 1:44am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/7 "2015-08-14T01:44:10Z")
> [@Chronos](#): > > The calculation is really easy, but I don’t understand the notation you’re using for the tire sizes. The odometer and speedometer will be off by the same proportion as the tires. So if you take the ratio of tire sizes, that’s also the ratio of what your odometer will read to what it should. [Here you go.](http://www.hankooktire.com/global/tires-services/tire-guide/sizes-specs/passenger-cars-tires.html) The way he has it is standard notation in the US (as I’ve always seen it) for tires. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [August 14, 2015, 3:10am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/8 "2015-08-14T03:10:10Z")
> [@zombywoof](#): > > The width is from sidewall to sidewall (which is not necessarily the width of the tread), and 14" is the wheel diameter, but otherwise you’ve basically got the right idea - see [here](http://www.discounttire.com/dtcs/infoSidewall.do). Nope. The 14" is the diameter of the wheel. In other words the hole in the center of the tire is 14" in diameter. To further complicate matters tires are like shoes. Two tires from different makers listed as the same size can be physically different is sizes. Just like I buy one brand of shoes I wear a 7.5 in other brands I wear a 10. So without a measuring tape you won’t get a rigorous answer. ---
**Author:** ![zombywoof](https://avatars.discourse-cdn.com/v4/letter/z/9e8a1a/32.png) [@zombywoof](https://boards.straightdope.com/u/zombywoof)\ **Post date:** [August 14, 2015, 3:16am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/9 "2015-08-14T03:16:09Z")
> [@Rick](#): > > Nope. > The 14" is the diameter of the wheel. In other words the hole in the center of the tire is 14" in diameter. No shit - that’s why I said “14” is the wheel diameter", as opposed to snfaulkner’s “14” is the wheel radius". ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [August 14, 2015, 4:03am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/10 "2015-08-14T04:03:29Z")
> [@zombywoof](#): > > No shit - that’s why I said “14” is the wheel diameter", as opposed to snfaulkner’s “14” is the wheel radius". My bad I read it wrong. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [August 14, 2015, 8:05am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/11 "2015-08-14T08:05:22Z")
> [@snfaulkner](#): > > note: “subtly persuaded” here means “complete tread delamination at 70 mph on the 101 freeway skidding across 3 lanes of traffic” That sounds like a confession to a crime… ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [August 14, 2015, 8:18am UTC](https://boards.straightdope.com/t/attn-bored-car-nerds-and-math-wonks/727782/12 "2015-08-14T08:18:44Z")
The 14" is the \*\*rim \*\*diameter. Confusion is caused by describing it as the \*\*wheel \*\*which to most people would include the tyre. tire size 185/70 14" = 24.2" wheel diameter = 833.9 revolutions per mile tire size 185/60 14" = 22.7" wheel diameter = 887.3 revolutions per mile 60 mph on the larger tyre would read 64 mph on the smaller one.