Author Topic: Tyre size calculations  (Read 34738 times)

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Offline deFuzster

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Tyre size calculations
« on: May 01, 2012, 04:11:18 PM »
Just found these useful tools for comparing tyre sizes and then the appropriate effect on your speedometer.

http://www.inawise.com/tyre-calculator/tyre-size-calculator.html
http://www.miata.net/garage/tirecalc.html
http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx
http://www.kouki.co.uk/utilities/visual-tyre-size-calculator

For those that don't know about metric Tyre sizing, I 'll add a quick rundown.

There are 3 numbers to a metric tyre size.

For an example:

285/75R16LT

285 refers to the nominal width of the tread in millimeters

75 is the profile of the tyre. It is expressed as a percentage of the nominal width of the tyre and represents the distance from the rim to the road (or outside of the tyre).

16 is the diameter of the rim expressed in Inches. Yes I know this is a metric size but the rim diameter is still expressed in inches.

LT simply refers to the tyre being a Light Truck construction.

There is appoximatley 25.4 millimeters in 1 Inch

So, to work out the diameter of this tyre, we have to add the bits together.

So if you picture a tyre side on and imagine we are going to measuure the diameter horizontally from left to right.

First you have the left sidewall of the tyre, then the rim in the middle and then the right sidewall on the right hand side.

The size of each side wall is the profile of the tyre:
285 x 75% = 213.75mm

The rim is a 16" rim so the diameter is:
16 x 25.4 = 406.4mm

So the diameter is:
 213.75 + 406.4 + 213.75 = 833.9mm

Or represented in the old money:
833.9 / 25.4 = 32.83 inches

Hence we normally refer to a 285/75R16LT as a 33" tyre.
« Last Edit: May 01, 2012, 04:15:09 PM by deFuzster »
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Offline Chuckanoo

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Re: Tyre size calculations
« Reply #1 on: May 01, 2012, 06:33:13 PM »
Interesting calculators, but how do you find out the equivalent size for a 205R16?
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Offline deFuzster

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Re: Tyre size calculations
« Reply #2 on: May 01, 2012, 07:00:12 PM »
205R16's are a very common size being standard on many middle size 4WD's including the Hilux.

It's a bit of an odd ball as the tyre doesn't rigorously conform to the standards. It's a hangover from the transition form imperial to metric sizing (remember the good old 10.5R15!)

So essentially it's a metric width using an imperial numbering standard.

When this numbering system is employed on a metric tyre the profile is usually 80 to 82.

In your case, the 205R16 is a 30" tyre.

They common replacement for these are the 225/75R16 and the 235/70R16 all being considered a 30" tyre on a 16" rim.

Hope that helps
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Offline Chuckanoo

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Re: Tyre size calculations
« Reply #3 on: September 23, 2012, 08:08:02 AM »
205R16's are a very common size being standard on many middle size 4WD's including the Hilux.



In your case, the 205R16 is a 30" tyre.

They common replacement for these are the 225/75R16 and the 235/70R16 all being considered a 30" tyre on a 16" rim.

Hope that helps

Sorry it doesn't

The 225/75R16's & 235/70R16 are actually a 29" tyre size (very close anyway, though the 235's are smaller)

225 / 75% = 168.75mm (side wall)
168.75 x 2 (side wall height)= 337.5mm
337.5 / 25.4 (1") = 13.28"
13.28" + 16" (rim) = 29.28" or 29 Inches

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Offline Chuckanoo

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Re: Tyre size calculations
« Reply #4 on: March 15, 2013, 11:11:24 PM »
I just found this Tyre Calculator on the ETyreStore website.
It can calculate Metric, Imperial & Truck tyre size.

http://www.etyrestore.com.au/tyre_calculator.php 

Now I can finally find out what the equivalent size the 205R16's will be  :)
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Offline deFuzster

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Re: Tyre size calculations
« Reply #5 on: March 16, 2013, 07:20:38 AM »
Nice tool Wayne

And as you can see, a 205R16 is equivalent to a 225/75R16 in diameter. (225 is 9mm larger in diameter) other than the width of course.

Now you don't have to take my word for it, you can check it yourself on the calculator you found.
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Offline Chuckanoo

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Re: Tyre size calculations
« Reply #6 on: March 16, 2013, 09:26:50 AM »
Using this calculator (and based upon the info in the Hilux's Gregory's repair book)

A 205R16 (7.00R16) is larger than the 225/75R16 by 2.45% or with the circumference difference by 54.45mm with the Actual Speed at Indicated 100 Klms/H: 97.63 Klms/H  :-X
No wonder why the speedo reads faster than the GPS ( by up to 10K's)  :o

This tool will be very valuable for when it's time to replace the Tyres on the Hilux  :)
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Offline deFuzster

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Re: Tyre size calculations
« Reply #7 on: March 18, 2013, 05:33:41 PM »
Hi again Wayne

using your tool that you posted at http://www.etyrestore.com.au/tyre_calculator.php
you will find some different numbers.

Firstly a 205R16 has the following specs from the tool:
Overall Diameter: 705.02mm
Rolling Circumference: 2214.9mm
Tyre Width: 205mm
Profile Height: 164mm

Following on from this, it would indicate that the 205R16 is in fact a 205/80R16 in metric long hand.

Try it yourself and plug in the 2 sizes into the tool and you will find that they are the same size in every dimension as I stated in my original reply to your question.

Further more, to compare it to other sizes:

Spec205R16225/75R16235/70R16
Overall Diameter:705.02mm714.14mm705.98mm
Rolling Circumference:2214.9mm2243.55mm2217.91mm
Tyre Width: 205mm225mm235mm
Tyre Profile: 164mm168.75mm164.5mm

There is also a 225/70R16 although it is not a common size. It is:
Overall Diameter: 692.542mm
Rolling Circumference: 2175.69mm
Tyre Width: 225mm
Profile Height: 157.5mm

This tyre is smaller in rolling circumference ( by 39.21mm) and as such is probably going in the wrong direction for what we do.

Now, as for spedo values; firstly I'll point out that most factory spedo's are not accurate. In fact, most are out by between 7 to 12km/hr at 100km/hr. Check it with a GPS and you'll see what I mean. That is, the spedo reads 100km/hr and the GPS will show 93km/hr for example with all the factory specs in place (including original tyre size).

So, let's pretend that the spedo is accurate at 100km/hr with the 205R16 tyres, below is what the actual speed of the car will be when the spedo reads 100km/hr disregarding the factory tolerances and inaccuracies:

Spec205R16225/70R16225/75R16235/70R16
Real Speed:100km/hr98.22km/hr101.29km/hr100.14km/hr

If however, when the spedo reads 100km/hr but the vehicle is only going say 95km/hr (as is usually the case), then this is what the values would be:

Spec205R16225/70R16225/75R16235/70R16
Real Speed is 95
when spedo is 100:
95km/hr93.32km/hr96.23km/hr95.13km/hr

So, if you're still with me, to answer your original questions.

Using your tool that you found, a 205R16 tyre is a 205/80R16 (from the tool)

Best choice for a equivalent tyre is either a 225/75R16 or a 235/70R16 (both popular sizes).


All these values I have used are straight from that tool.

Hope this helps you and anyone else that may have this question.

Oh, and I have just replaced the tyres on our 4Runner. It originally had 205R16's as well. We are now running 225/75R16's and are very happy with them. They fit on the same rim and the spedo has not significantly changed as shown above.
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Offline Kris

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Re: Tyre size calculations
« Reply #8 on: March 18, 2013, 06:12:40 PM »

Now, as for spedo values; firstly I'll point out that most factory spedo's are not accurate. In fact, most are out by between 7 to 12km/hr at 100km/hr. Check it with a GPS and you'll see what I mean. That is, the spedo reads 100km/hr and the GPS will show 93km/hr for example with all the factory specs in place (including original tyre size).


I am glad you pointed this out Mautrits. It is normally on the safe side of things (7 to 12km/h less). I asked the question to a friend who works at Ford as to why they do this. The response was that it is due to the variation in real sizes of tyres. This variation comes from a two main factors:-

1.\ Tolerances in manufacturing (+/- x%)
2.\ Where the tyre manufacturer measures their tyre size.
             - Do they measure the tyre diameter to the wear markers? The caucus? the new tread?

To me I found this a bit strange at first until I started standing different brands of tyres in the same size side by side and end to end and looked at the variance - it is a very interesting and surprising exercise!
 
« Last Edit: March 18, 2013, 06:15:41 PM by Kris »

Offline Poncho

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Re: Tyre size calculations
« Reply #9 on: March 18, 2013, 11:54:44 PM »
Manufacturing tolerances isn't going to give you that sort of differences, tyre variations of the same size isn't going to read 7 - 12KM/h out. Going to a different tyre size completely will give those sorts of differences.

The real answer is a lot of manufacturers are coving their ass from getting sued for fines. Dads Nissan Navara is factory 9-10KM/h under the speed limit while the Odometer is accurate according to On Track 4WD.

You can't have an accurate Odo with and inaccurate speedo without them specifically being set differently.

Test your speedo and Odo  and see if they are different, that will confirm they are set out by the factory.

Offline Kris

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Re: Tyre size calculations
« Reply #10 on: March 19, 2013, 07:55:37 AM »
Manufacturing tolerances isn't going to give you that sort of differences, tyre variations of the same size isn't going to read 7 - 12KM/h out. Going to a different tyre size completely will give those sorts of differences.

The real answer is a lot of manufacturers are coving their ass from getting sued for fines. Dads Nissan Navara is factory 9-10KM/h under the speed limit while the Odometer is accurate according to On Track 4WD.

You can't have an accurate Odo with and inaccurate speedo without them specifically being set differently.

Test your speedo and Odo  and see if they are different, that will confirm they are set out by the factory.

Poncho, you a dead right, manufacturing variations will not account for the full variation in all speedo's. As for ass covering - you are probably onto something there as well however I doubt you will ever get a manufacturer to admit to that.

As for variations between speedo and odometer there is a real possibility that there could be a variation between the two on modern cars whit digital odometers - it would be an interesting comparison.

Offline deFuzster

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Re: Tyre size calculations
« Reply #11 on: March 19, 2013, 09:16:54 AM »
By the numbers a 285/75R16 tyre should have a diameter of 833.9mm ( (0.75x285x2)+(16x25.4) )

Just had a look on the net and found the following from their respective manufacturer websites. I have also compared their respective spedo readings as compared to a spedo reading 100km/hr on my calculated tyre (833.9mm OD)
TyreStated SizeStated ODSpedo
Mickey Thompson Baja Radial MTZLT285/75R16 (33/11.50R16LT)841100.85km/hr
Mickey Thompson Baja Radial CLAW TTC285/75R16LT841100.85km/hr
BF Goodrich T/A KM2LT285/75R16841100.85km/hr
Cooper STT Armor-Tek3285/75R16841100.85km/hr
S/T MAXX Heavy Duty All-Terrain285/75R16841100.85km/hr
Mickey Thompson Baja Radial ATZ 5-RibLT285/75R16836100.25km/hr
Federal Couragia M/TLT285/75R16835100.13km/hr
Maxxis MT-762 Bighorn285/75R16834100.01km/hr
Nankang N-889 MudstarLT285/75R16834100.01km/hr

So, they are all pretty close.

Now, as Poncho pointed out, a 10km/hr difference would be from a significant tyre size change.

Lets have a look at that

Looking up the vehicle manufacturer descriptors I have found that (OD in brackets):
80 Series GXL was originally fitted with 275/70R16 (791.4mm or 31.2")
GQ Patrol was originally fitted with 215/80R16 (750.4mm or 29.5")
GU (early) Partol was originally fitted with either 235/80/16 (782.4mm or 30.8") or 265/70R16 (777.4mm or 30.6")

If the spedo was accurate whith those factory tyres, then if we fitted a 285/75R16 (using the Mickey and BF OD of 841mm) then when the spedo reads 100km/hr, the car would actually be going:

80 Series 106.27km/hr
GQ Patrol 112.07km/hr
GU (early) with 235's 107.49km/hr
GU (early) with 265's 108.18km/hr

Interesting...
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