Come On Baby Size My Tire

December 2, 2007 4:52 PM

Tire size substitutions?

Even after reading the excellent advice in percussive paul's question, I have questions about the allowed sizes of my tires. My current tires are Michelin MXV4s, and their size is 205/60 R 15 91 H (M+S). I had a flat last weekend, and figure that it's high time I replaced all four tires (not just one) given that I am still driving on the tires I had replaced in 1998. I walk to work and only drive on weekends on highways or logging roads.

I am looking at the Costco site, which irritatingly only allows me to select tires according to my car's year and model. Costco is offering two tire sizes for my car's make/model/year, neither of which is the size of my current four tires.

My car's user manual stipulates either 185/65R 15 88 or the size that I have now, which is 205/60R 15 91 H

Costco's choices list either 185/65R 15 or 205/65R 15. I feel that the second size is wrong for my car, although the folks at Costco told me (on the phone) that 'this is the specification, I see it on the computer, therefore it is correct'. Ugh.

My question to the AskMe tire experts: should I only consider the 185/65 tires from Costco or can the 205/65's also be safely used on my car? I should add that both tire size choices include the Michelin model that I currently have (just not the identical size).

What would I be trading by going to a 185 tire instead of a 205? And could I use a 205/65 instead of a 205/60, eg would the /65 aspect ratio 'fit' on a 205 tire size, on my car?

I found this cool tire size applet and I see that different tire sizes will impact the accuracy of my speedometer - but my specific question remains, can I buy the 205/65 even if it is not in spec (does it matter) or should I order the 185/65s instead.

I like Costco's prices - certainly *much* lower than going to my car dealership and getting replacement tires there, and also less expensive than other tire stores in Vancouver BC.

Please explain tire sizes to this tire newbie. Thank you.

posted by seawallrunner to travel & transportation (12 comments total)

Here is an article explaining tire sizes from Tire Rack; they also have a "find tires for your car" function that you can use to compare results with Costco's.
posted by Forktine at 5:02 PM on December 2, 2007

Seconding that you look at Tire Rack's "find tires for my car" function. But your wheels may not be able to go from 185 to 205. You need those specs, too.
posted by Cool Papa Bell at 5:10 PM on December 2, 2007

FWIW, Costco's tire people are universally spectacularly unhelpful. I find it bizarre considering everyone else at Costco is generally so useful.

When I went to buy tires there, I asked what they had that would fit my car and he told me about the 3 models and the prices. When I asked what the difference between the 3 tires were, he looked at me like I was a moron and said 'That one is $130 per tire, this one is $120 per tire and that one is $90 per tire.' Uh, thanks.
posted by jacquilynne at 5:52 PM on December 2, 2007

I had no problem using 195/65s on my stock rims designed for 185/70.

I went with that particular size because it kept my speedometer reasonably accurate, while still letting me get the particular tires I wanted (BFGoodrich didn't make a Traction T/A of any sort in 185/70 at the time)

There are several sites out there that will show you graphically what the difference is. Whether a particular tire can be mounted on a narrower than spec rim depends on the sidewall's flexibility. BFG's literature said that I ought not do that, but I asked a reputable tire shop and they thought it would be fine, which it turned out to be.

Basically, the best thing to do is look at the tire manufacturer's literature, since they know how their tire is made.
posted by wierdo at 5:53 PM on December 2, 2007

Oh, I didn't answer the whole question..a wider tire will (generally) result in slightly lower gas mileage and more 'tracking,' where the tire will tend to follow any grooves in the road.

In your case, a 205/65 will have a radically different profile than a 185/65, and be both wider and have a taller sidewall, which will lead to more sidewall flex, which may or may not matter depending on how you drive.
posted by wierdo at 5:56 PM on December 2, 2007

thanks guys for the responses so far. weirdo, what is the main difference in the profile between a 205/60 and a 205/65?

and would my wheel well be able to accommodate either? Or do I really only have a choice between the 185/65 and the 205/60?
posted by seawallrunner at 6:10 PM on December 2, 2007

The first number is the width of the tire in millimeters, and the second represents the height of the tire from the edge of the wheel to the outside edge of the tire, as a percentage of the width.

That means 205/60's are 205 millimeters wide, and .6(205) or 123mm tall* whereas the 205/65's are 133mm tall. The total increase in diameter of the tire is 20 mm.

The suspension of your car is designed around a specific diameter of tire. I personally wouldn't put 205/65's on a car made for 205/60's.

The 185/65's will be 120 mm tall, which is a probably close enough height-wise, but they are so much narrower, I don't think they're going to be right either. Is it possible that they are for a lower/cheaper version of your car that came with narrower wheels?

Do you have a Discount Tire near you? They honor The Tire Rack's prices. They'll also order tires if they don't have them in stock.

*(there's a term for this measurement, but I can't remember it or find it; forgive me)
posted by found dog one eye at 6:53 PM on December 2, 2007

section height, duh
posted by found dog one eye at 6:57 PM on December 2, 2007

I can't give info related to tire sizes, but given your proximity to the US border, and the low value of US dollar, you may find what you need at a lower price in the US.
posted by ShooBoo at 7:05 PM on December 2, 2007

*(there's a term for this measurement, but I can't remember it or find it; forgive me)

It is also called 'Aspect ratio'. The width of the tyre is more critical for the grip you will get (as long as it is not significantly different to the width of the wheel on which it is mounted). The aspect ratio really only affects the rolling radius (and hence circumference) of the tyre which (within reason) only affects the accuracy of your speedometer - which has legal implications.

In summary: No, the extra 20mm will not make your car dangerous at all. It will give better road performance (through extra rubber) than the 185 option, but Yes, it will make your speedometer wrong.

I'd try and find the right size tyre from somewhere else, personally. If I couldn't, I'd fit the 205/65 and just accept that my speedometer would read slightly under.
posted by Brockles at 8:56 PM on December 2, 2007

Here is an excellent tire size calculator. Plug-in your OEM stock wheel/tire size on one side, then play with possible replacement wheel/tire size combos on the other side. The graphic tells you just how far off the OEM diameter you are.
posted by Thorzdad at 4:47 AM on December 3, 2007

Thank you all for your responses. I appreciate every comment and every response.

Here's what I ended up doing: from the Costco drop down menu (that I need to use, to select my tires) I selected another model for my car brand, one whose tire selections match my tire size. I selected the tires that I wanted, ordered four of them on the Costco online site, and also used a $100 off coupon also available on their site, for Michelin tires. By signing up for a Costco card (which I did not yet have) I am also putting the purchase on Aeroplan points.

Previously I had also sent an email to their tire department, asking about the sizing of the tires. I had also indicated that my owner's manual states that the size I currently have is the correct one. I will only have to bring in that manual to the warehouse once my tires arrive.

So there you have it - I was able to learn a little more about tires, I got a good deal, and now I'll be able to purchase pillow-sized bags of marshmallows for the rest of the year :)

Thank you all again for your insights, wisdom and good advice.
posted by seawallrunner at 3:35 PM on December 3, 2007