Tyres and speedo accuracy

[BrianJ]

When I got my kombi beach it came with Michelin Agillis 51 215-65-R16C tyres and the speedo reads high about 2.5km/100km

I have noticed that 65 profile tyres are very hard to find in this width and diameter, so I started wondering if I should fit 215-70-R16C (whatever C means) as these are easier to find at a good price. I worked out the circumference difference and it came to 2.3%, about the right amount to correct the error in my speedo reading.

Its almost as if this is the tyre size the car had orriginally been set up to use, but they fitted slightly different ones at the time of manufacture.

Has anyone considered this aspect of tyre choice.

Brian

[Silver Caravelle]

Brian,

My Multivan came with Dunlops as OEM. The exact OEM code SP LT30-6 215 65 R16C appears not to be available.

Regards,

Scott

[The_Hawk]

A while back I did some calculations for the MkV Golf (I think) and all the "standard" wheel/tyre combos that were available. The basic theory is that vehicle speed is measured by the number of rotations at the XXX (wheel or gear box, or whatever, the ratio will always be the same). In short the largest circumference tyre will roll further per rotation and the smaller will roll less.
Since ADR requires the speedo NEVER read slower than your really travelling, but has an allowance for 10% over the correct speed reading you would have to calibrate to the largest wheel (and therefore fewest rotations), which means the smaller wheels would be travelling slightly less distance at the same revs and therefore the speed shown on your speedo will be higher than reality (i.e. you will really be going slower). So no matter what you should NEVER be able to claim "but the speedo said 90 Officer…" since it can only ever be showing over, not under.

So to do 100Km/H in the real world your speedo needs to be showing slightly higher for the smaller wheel/tyre combo, which looks something like this on shiny new tyres:



So the difference in this set of tyres is less than 1% error. If you factor in up to 10mm of tyre wear (which is pretty much MAX for 4WD tyres, road tyres are less, but for the calculation lets go with that) the error then gets to a MAXIMUM of 4% error.


Remember you have to go "faster" on the speedo to actually be doing 100Km/H in real life.

So assuming a real tread wear of say, 6-7mm, your speedo should be less than 1% inaccurate with new tyres and a maximum of 3.1% out with completely worn tyres…. yet people can get 5%+ on new tyres… Why can't the manufacturers get it right?


But anyway, here is a link to my spreadsheet if you want to try plugging in some combo's to see what the difference are:
http://mooshen.com/vwwc/Tyre%20Wear%20V2.0.xls

Or there is a whole page with calculators here:
http://www.carbibles.com/tyre_bible_pg2.html

[Silver Caravelle]

I also meant to add the GoodYear G26 CARGO come in many sizes including 215/65R16C 106T

See Beaurepaires site.

http://www.beaurepaires.com.au/TyreD...&from=nPerPage


Regards,

Scott

[gregozedobe]

You may correct the speedo error with tyres that have a larger rolling diameter/circumference, but your odometer will then be wrong (less than the actual distance travelled).

Note that different brands of tyres that are nominally the same size may well be significantly different. The exact details for each tyre are usually available from the manufacturers.

[gareth_oau]

dont forget that a tyre has a legal tread depth of at least 10mm, thats 20mm in diameter variance from legal to illegal.

This means from new to old, your speedo will change accuracy of over 3%.

Therefore, assuming your speedo is calibrated to the new tyres, then by the time they are old (but still legal) it will be out by 3kph at 100kph.

fortunately, it runs safe, so your speedo will show 100, but you're actually doing 97

[BrianJ]

Good info eveyone, thanks for the input.

Brian

[Silver Caravelle]

I also meant to add the GoodYear G26 CARGO come in many sizes including 215/65R16C 106T

See Beaurepaires site.

http://www.beaurepaires.com.au/TyreD...&from=nPerPage


Regards,

Scott

Hi there,

I had a quick snoop at tyres on vehicles in the Burwin van centre the other day. The current crop of 16 inch 215 65 106/104T tyres are Hankooks ( made in Hungary) and Bridgestones (made in Japan). Can't recall the model numbers on the tyres. Hope this helps with some other options. The Goodyears are also good tyres.

Cheers,

Scott

[Tornado T5]

Hi there,

I had a quick snoop at tyres on vehicles in the Burwin van centre the other day. The current crop of 16 inch 215 65 106/104T tyres are Hankooks ( made in Hungary) and Bridgestones (made in Japan). Can't recall the model numbers on the tyres. Hope this helps with some other options. The Goodyears are also good tyres.

Cheers,

Scott

Lol, I did the same today. And the damn note I took somehow has disappeared on the drive home from Burwins. So even VW are finding the Michelins too expensive. After my service today I need to replace at least 2 tyres which are the original Michelin Agilis 51's after 75,500km.

[BrianJ]

Lol, I did the same today. And the damn note I took somehow has disappeared on the drive home from Burwins. So even VW are finding the Michelins too expensive. After my service today I need to replace at least 2 tyres which are the original Michelin Agilis 51's after 75,500km.

That's good usage, I have done 65,000 on 4 of the same agilis and should get another 10k at least. They have been a good tyre, but the rubber is a bit soft in the outback dirt roads of QLD, and has caused a lot of cuts and shredding mainly on the edges of the front ones.

I would like to get a slightly harder rubber in my next ones.

Brian