Tire Nomenclature

Factual Questions
1

What do all the numbers and letter mean regarding the size or a tire.

Particularly, what are the differences between a 275 40R17 and a 235 55R16. How much bigger/wider is one from the other. The second is a smaller tire, but I’m thinking the “55” has something to do with height? Because there seems to be more space between the rim and the treat on that tire, than the other. But how much difference? Are these numbers in centimeters? Inches? I know the last number is rim size. Anyone have the straight dope on this?

2

Tire sizes and classifications

3

(From http://www.tyres-online.co.uk/techinfo/sidewall.asp )

So the first tyre has a profile that is 40% of the diameter (0.4 x 17) = 6.8"
The second tyre has a profile that is 55% of the diameter (0.55 x 16) = 8.8"

4

Awesome. Thanks. I couldn’t find anything that wasn’t trying to seel tires. And none of those sites had adequate information.

5

Sorry, I screwed up there - the aspect ratio is related to the width of the tyre, not the diameter of the rim.

So, 275 40R17 has a profile (section height) of 0.4 x 275 = 110mm (4.33")

and a 235 55R16 has a profile of 0.55 x 235 = 129.25mm (5.09")

I thought those tyres seemed a little chunky!

6

So if a car used to have the 23555R16, his speedomoter is set for a tire diameter of 26.18". But if that car switched to the other tires, with a diameter of 25.6", how far off would the speedometer be? Or is it negligible?

7

And which would you rather have on your car. I’m pretty sure of the advantages of wider tires. What are the disadvantages? What are the advantages/disadvantages of a tire with a bigger profile?

8

If you have a wider tire, it could possibly rub against your fender. We actually had a Navigator in our shop recently that had 24" wheels that rubbed through a wiring harness and shorted out his engine computer. Ford actually had a service message about this. Also, these low profile tires(anything with an aspect ratio of 45 or lower) will make your ride quality suffer, and if you have to drive in snow, will make you skid around like a bumper car.

9

You need to compare the circumferences:

Original tyres: C = π x (26.18/2)[sup]2[/sup] = 538.3 inches

New tyres: C = π x (25.6/2)[sup]2[/sup] = 514.7 inches

So the speedo will over-read by (538.3 - 514.7) / 538.3 = 4.38%

(it will over-read because the new tyres are smaller and will thus be spinning faster)

10

Not always. My winter tires are 215/55/R16s (similar profile to my summer tires which are 215/50/17).

The only real disadvantage to wide tires is gas mileage (reduced due to extra friction) and slightly reduced ability to counter hydro-planing and on-snow traction due to surface area. You do have an increased ice/wet/dry traction though which is why I stuck with a wide snow tire.

Also…price.

A slight increase in tire width should not cause rubbing. You should of course check out online what the max width of tire you can use on your vehicle. Most +1 tire increases are safe (generally an extra 10 mm wider). My next tire will be a 225/45/17 (profile need to be reduced to maintain overall tire diameter).

If you aren’t auto-x’ing or tracking your car, you might not want to take this route. A high-performance tire can be pricey and wear quicker than a “normal” tire.

11

I don’t doubt your math, but is this an accurate assumption? IS the speedometer calibrated for a certain size tire? Or do they send all the cars of that model with a generic average calibration?
For instance, my Mustang. It comes from the factory with several different tire options. Are the speedometers on each Mustang set according to which tires it shipped with? Or is it just averaged out?

So now, when it says I’m going 90, I’m really going about 86?

mike1dog, you make it seem like the new wider tires suck big time. What are the benefits of a lower profile? Why would someone make tires in that manner if it’s just going to make the ride quality suck? Please tell me it improves performance or something.

12

Less sidewall flex allows the tire to maintain shape when turning.

13

As far as getting the new tires, it’s too late for that. I’ve already got them, they’re on the car and the work fine. The tires and wheels are both Ford factory, so I knew they would fit on my Mustang. I think they’re supposed to come on the Cobra or Saleen or something. But definitely not a little v6 like mine.

I hit a curb recently that destroyed two of my rims and tires. I’ve been driving with two doughnuts on the front all week. WHAT FUN!! They’re each rated for 45mph, and I figure since I have TWO of them, that I can do 90! Right? Well… I hit 75 anyway.
But I cam across a pretty good deal on a set of 4 pretty new (as new as the ones I lost) tires, all on the same kind of alloy rims I had before. The only difference is these are 17 inch rims and not 16 inch. I was pretty happy with the deal, and with the tires.
Now I have two almost new tires on 16inch ford alloys, and nothing to do with them. eBay maybe?

How is the wider tire better for ice/wet/dry traction but not better if you’re hydro-paning. Better on wet surface unless you’re already sliding? This seems contradictive to me. Please explain.
Also, isn’t wider things better on snow? Why does more surface area have worse on-snow traction?

14

So, then, as far as “quality of the ride”. Is this talking about comfort? The wider profile tires add performance but take away from comfort? Is that an accurate statement?
If so, I can deal with that.

15

The simplest way to check speedo accuracy is on an interstate or other highway with miles indicated on delineators beside the berm. Set cruise on 60 and note the second hand on your watch when passing a milepost. In theory, if your speedo is dead-on, you’ll be one mile further down the road one minute later. If it is off by a significant amount, you may be able to correct it by changing the speedometer cable drive gear in the transmission, which is how manufacturers compensate for different rear axle ratios.

16

Generally (operative word), wider tires result in stiffer sidewalls, since a greater portion of the tire is tread, and a smaller percentage is sidewall. Stiffer sidewalls on the ‘low profile’ tires (50, 45 and 40 series and so on) are more responsive when cornering and make handling more predictable. Generally, a tire with a ‘relatively’ short sidewill will transmit more bumpage to the passengers.

Just as soft shocks cushion the road, a tire set up that uses a taller tire can help smooth the ride.

The whole thing comes down to the balance desired. Early 85 Vettes road like pogo sticks, because the engineers wantes pure numbers to advertise: numbers in the slamon, the total g’s pulled in cornering and so on. Early vettes had very very low profile tires/tyres for their day, and later models had ‘softer’ suspensions to compensate. Obviously, ‘softer’ purely relative, since in a vette it’d still be quite firm.

Again, generally, you can upgrade tires without affecting very much --even the speedo, and this might help you understand how diff tires go on the same production vehicle.

Say you have tires like this: 205/70 - 15’’. You have a modest tire on a 15" rim. Not very low profile by any means, since 70 shows that a large % if the tire is sidewall. You could go to the tire dealer, and buy up a set of 215/65 - 15" tires. What you would wind up with is a tire that is almost as tall as stock (helps keep ride height consistent and susp travel consistent…helps keep speedo pretty dead-on)…yet you would have a wider tire (but VERY likely one that isn’t too wide for the car wheel openings/clearances. VERY likely it won’t be.

You have a stiffer tire now…one that is just about as tall as the original, has a greater % of rubber as tread and a slightly stiffer sidewall. …all on the same rims.

You can even swap out rims like this, too, and not affect things. You increase to a 16" rim, but the height of the tire doesn’t change, but more ‘meat’ is used as tread.

These are popular swaps, and factories take advantage of the math involved when producing cars with diff rim and tire sizes.

17

Re: speedo accuracy, manufacturers tend to err on the side of caution (i.e. the speedo will over-read). Obviously it is better to have it tell you you are doing 70 wehn you are actually doing 65 then when you are actually doing 75, in terms of avoiding the speed cameras.

Another way of checking the speedo is against a GPS system. AFAIK my car speedo (Mazda RX-8) reads 74mph at a true 70mph, and 33mph at a true 30mph based on other RX-8 owners that have satnav fitted. So the error is not necessarily linear.

18

As a Mustang enthusiast, I can help you out with this one. The computer is programed for a specific tire circumference.

A quick explanantion of how the car knows your speed: The computer knows your rpms. It also knows what gear you’re in and the gearbox ratios of the tranny. This lets the computer calculate how fast the driveshaft is spinning. Thanks to the speedo gear (not related to the speedometer) the computer also knows what rear end gear ratio you have. Now the computer can calulate how many rpms the tire is turning at. Plug in the tire circumference and you get your speed.

Going from 2005, you can see that the standard tires on the Mustang are 215/65 on 16"s while the upgraded wheels are 235/55 17"s. Going through the math you’ll find that the 16" rim has a 84.83" circumference and the 17" rim has a 85.35" circumference, less than 1% difference. Ford probably programmed the computer with a 85" tire to average it out. You can slap any rim/tire you want on your car as long as you keep close to the original circumference. Go too big and the speedo reads low. Too low of a tire and the speedo reads high.

19

Since you’re a Mustang enthusiast:

What is your opinion of the new tires and the new rims I got. I went from 235 55R16 with 16" factory alloy rims to 275 40R17 with 17" factory alloy rims.

Which would you rather have? Why? Also, what would you consider a decent deal on 4 used factory 17" alloy rims and four-month old 27540r17 tires?

Also, know anyone looking for two 23555R16 tires on factory rims?

20

Please tell me you’re joking? You took a spare donut to 75 mph? Sorry, 2 spares to 75?
It’s like snow shoes. A wide tire can travel on top of snow where as a thin tire won’t. Traveling over snow (and not gripping the ground) is a bad thing. Same goes with standing puddles of water.

A wide tire is good when it actually makes contact with the ground. That ground can be wet, dry or ice (don’t expect much on ice but my Blizzaks do actually grip it). Your tires are almost certainly not winter tires (real snow tires come with a snowflake/mountain logo and not M+S) so don’t expect any grip on ice regardless of how wide your tires are.

I was going to ask how you were able to mount 40 mm wider tires on your car but then I realized that you’re talking about the rear tires only. Since I don’t think stock set ups on mustangs come with offset front/rear tire set ups (do they?) you should be careful as your steering dynamics are going to be very different. With super wide and grippy rear tires on a RWD car, expect understeer.