# How fussy is tire size? **URL:** \ **Category:** Factual Questions\ **Created:** [January 15, 2004, 6:50pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622 "2004-01-15T18:50:00Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![Jann](https://avatars.discourse-cdn.com/v4/letter/j/839c29/32.png) [@Jann](https://boards.straightdope.com/u/Jann)\ **Post date:** [January 15, 2004, 6:50pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/1 "2004-01-15T18:50:00Z")
I’ve got an old car with tires marked P205/70R15 M+S and someone offered to swap with a new set of P205/65R15 M+S (they are sending that car to the junkyard because of engine problems) That’s as close as I’m going to get to the same right? So it should be fine? The owner’s manual doesn’t specify size but refers to a “decal on the right door post”, which is long gone. ---
**Author:** ![Phlosphr](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@Phlosphr](https://boards.straightdope.com/u/Phlosphr)\ **Post date:** [January 15, 2004, 6:58pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/2 "2004-01-15T18:58:56Z")
You should be fine **Jann**. 205, 70’s to 205,65’s is a jump down in Aspect ratio… Your wheel size it the same so you’ll be OK…The way tire numbers work is like this: P205/70R15 M+S p205 is tire width 70 is aspect ratio 15 is wheel size… ---
**Author:** ![collinsc](https://avatars.discourse-cdn.com/v4/letter/c/f6c823/32.png) [@collinsc](https://boards.straightdope.com/u/collinsc)\ **Post date:** [January 15, 2004, 7:12pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/3 "2004-01-15T19:12:29Z")
Take a look at [this calculator](http://pw1.netcom.com/~sgalaba/tiresize.htm) to see if the differences are enough to mean anything to you. Your car will sit about a third of an inch lower, and your speedo will be off by more than it already is. Other than that though, not much difference. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [January 15, 2004, 7:23pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/4 "2004-01-15T19:23:32Z")
On my old Honda Civic, I had to replace a busted wheel, so I got one at a local junkyard off of a Saturn. Same lugnut rim specs, same wheel diameter. Only the tread was half an inch less than the other tires…but that didnt affect its performance. ---
**Author:** ![the\_first\_supraliminal](https://avatars.discourse-cdn.com/v4/letter/t/f475e1/32.png) [@the\_first\_supraliminal](https://boards.straightdope.com/u/the_first_supraliminal)\ **Post date:** [January 15, 2004, 7:34pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/5 "2004-01-15T19:34:46Z")
Based on **collinsc** ’s chart, if you went 50 mph your speedometer would read 51. But it probably reads that now because your worn tires are 1/3" shorter due to loss of tread top and bottom. Besides being nearly invisible, the reading is in the right direction not to give you tickets. As for the car being 1/3" lower, the same applies to worn tires. ---
**Author:** ![venterap](https://avatars.discourse-cdn.com/v4/letter/v/a9a28c/32.png) [@venterap](https://boards.straightdope.com/u/venterap)\ **Post date:** [January 15, 2004, 8:07pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/6 "2004-01-15T20:07:06Z")
> [@Phlosphr](#): > > You should be fine **Jann**. 205, 70’s to 205,65’s is a jump down in Aspect ratio… Your wheel size it the same so you’ll be OK…The way tire numbers work is like this: > > P205/70R15 M+S > > p205 is tire width > 70 is aspect ratio > 15 is wheel size… Adding to this, 205 is the rim width in mm measured at the tyre bead seating, 15 is the rim diameter in inches, again measured at the bead seating, the 70 is the aspect ratio of rim width to tyre sidewall height, i.e the tyre sidewall in this case is 70% of 205mm, or 143.5mm Using this you can calculate the influence that a different size tyre will have on your car. Let’s take your current tyre first: Overall tyre diameter = rim diameter + 2xsidewall height = 15x25.4 + 2x143.5 (there is 25.4 mm to an inch) = 381 + 287 = 668 mm This gives us a circumference = pi x diameter = 3.14159 x 668 = 2099 mm i.e. every revolution of your tyre will give you a (_theoretical_) travelled distance of 2099 mm Strictly speaking we don’t need to calculate the circumference, as the diameters alone will give us the correct answer, but it’s easier to visualise. Besides, it’s more fun… Now for the new tyre: Tyre sidewall height = 65% of 205 = 133.3 mm Overall tyre diameter = 647.5 mm Circumference = 2034 mm Comparing these two figures gives us a % difference of 100 x (2099 - 2034) / 2099 = 3% This means your odometer and speedometer is going to overread by 3% compared to your old tyre, which is negligible in most cases. I want to add a bit of (_unasked for_) advice here. Your tyre’s “M+S” markings are for a “Mud and Sand” tyre, so I’m guessing the car is a 4x4. Is this right. With 4x4’s you must make sure that you have the same **size, model and brand tyre on every wheel** , including the sparewheel if possible. Having different sized tyres or tyres of different manufacturers or tread patterns can do your differentials a lot of (expensive) damage in the long run. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [January 15, 2004, 11:09pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/7 "2004-01-15T23:09:59Z")
Indeed. That’s why the space saver donut spare tire cautions to drive no more than 50 miles…not because the tire is substandard, but because the wheel is smaller and will cause the limited slip differential to wear out. ---
**Author:** ![Anachronism](https://avatars.discourse-cdn.com/v4/letter/a/e56c9b/32.png) [@Anachronism](https://boards.straightdope.com/u/Anachronism)\ **Post date:** [January 16, 2004, 5:27am UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/8 "2004-01-16T05:27:28Z")
> [@venterap](#): > > I want to add a bit of (_unasked for_) advice here. Your tyre’s “M+S” markings are for a “Mud and Sand” tyre, so I’m guessing the car is a 4x4. Is this right. With 4x4’s you must make sure that you have the same **size, model and brand tyre on every wheel** , including the sparewheel if possible. Having different sized tyres or tyres of different manufacturers or tread patterns can do your differentials a lot of (expensive) damage in the long run. In the States M+S stands for Mud and **Snow** and this marking is on all ‘all season’ car tires. The majority of tires sold in the US are marked M+S. The only tires **not** marked M+S are summer only tires, in most cases these are high performance tires. ---
**Author:** ![venterap](https://avatars.discourse-cdn.com/v4/letter/v/a9a28c/32.png) [@venterap](https://boards.straightdope.com/u/venterap)\ **Post date:** [January 16, 2004, 8:34pm UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/9 "2004-01-16T20:34:28Z")
> [@Anachronism](#): > > In the States M+S stands for Mud and **Snow** and this marking is on all ‘all season’ car tires. The majority of tires sold in the US are marked M+S. Ah, so sorry. My bad. In South Africa we rarely get snow, but we have a LOT of sand, so… Same advice apply, though. ---
**Author:** ![Art\_Barber](https://avatars.discourse-cdn.com/v4/letter/a/edb3f5/32.png) [@Art\_Barber](https://boards.straightdope.com/u/Art_Barber)\ **Post date:** [January 18, 2004, 12:42am UTC](https://boards.straightdope.com/t/how-fussy-is-tire-size/224622/10 "2004-01-18T00:42:48Z")
Good rule o’ thumb: If you increase/decrease the aspect ratio of the tires by 10, you decrease/increase the section width by 20, giving you a wider tire that is not taller. For example, 175/70R14 can be changed with 195/60R14. Keep in mind, BTW, that tire sizes are not strictly exact, brand to brand: since tire specs are based on measurements made INSIDE of the tire, they are subject to external differences because of tire thickness. Of course, you must make sure your wheels can handle the extra width, and that the additional width does not cause interference with the body and suspension during turns. Art