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/TyreDetailAction.do?website=BFT&websegmentcode=LIGHT%20COMMERCIAL&mtpcode=00133&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/TyreDetailAction.do?website=BFT&websegmentcode=LIGHT%20COMMERCIAL&mtpcode=00133&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

[Deeceer]

"Since ADR requires the speedo NEVER read slower than your really travelling, but has an allowance for 10% over the correct speed reading"

Indeed, just today, this:

RACQ vehicle technologies manager Steve Spalding said speedo errors were usually on the safe side. "The actual speed is less than the indicated speed in nearly every instance. It's very rare to see a speedometer where it's inaccurate the other way," Mr Spalding said.

from this:

Digital speed cameras to target Queensland motorists

as virtually bankrupt Queensland moves to join the Victorian cash grab.

[Tornado T5]

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

I've experienced the same due to similar roads in rural Vic while towing heavy loads. I don't plan on having the van for another 75,000Km so might go for another brand although the longevity of the Michelins might balance out their initial higher cost.

[Seano]

My T4 speedo under reads by about 3 to 8 km/h between 60 and 110km/h so the T5 is not unique....

With regard to the 215/65R16C tyre......I can vouch for the GT Radial Maxmiler CX. In this size, it is an 8 ply tyre with a load rating of 109/107T so that's more than enough. Personally I run a 15 inch version in my T4 - cost me about $150 fitted, three years ago. Good tyre both on road and on the track. The C incidentally indicates a high load capacity tyre



There's also the Bridgestone R410 which has a 106 load rating....




Both are distributed by www.tyres4u.com.au

All of the van tyres will chip on rocky dirt roads.....every one of mine from the OEM Kleber to Hankooks to Goodyear through to the GT Radials has been subject to chop out. However, I've never had a rough road puncture. Only flat was due to a screw I picked up in a Cargo tyre in my own driveway...

I'll be going up a size slightly next time from the 205/65R15C to a 205/70R15C and it'll be the GT Radial Maxway tyre.......but probably not for another year or two the way the CX's are wearing