New Tyres v Speedometer !

[Searcaigh]

Last week I changed the rubber on one of my Discos from 235x70 R16 to 265x70 R16.

On Friday driving back on a highway from our off road trip and still having the GPS on (Garmin 128) I noticed that at an indicated 120kph I was actually doing 127kph as per the GPS. Previously the speedo was reading over by 2 kph at 120.

I was speaking to one of the mechanics who services Land Rovers this morning and he told me that despite the tyres all coming from one manufacturer (in my case Michelin) this anomoly frequently occurs between different tyre sizes from the same manufacturer.

He also mentioned that on Pre 95 Classic Range Rovers it was necessary to make sure the front and rear tyres are not only of the same size but of the same manufacturer and has seen serious mechanical damage to the Transfer Case when different tyre makes were used over a long period

[Disco]

Since you have changed the rolling diameter of you tires from 29" to 30.6", it's the reason your speedo reading has changed. I'm not sure I agree fully in what your mechanic has said about different tire brands (having the same rated size) causing transfer case problems. I rotate my 5 tires every 10,000km or so, which means that the spare is going to have a slightly larger rolling diameter than the other 4, and I've never experienced any problems.

[Rovin4life]

larger tires affect the speedometer. Works on any vehicle.

and having different tires and tire manufacturers will burn up a transfer case.

Any 4WD that uses different tires will burn up a transfer case.

Its what is called rolling circumference. if the RC is different it will burn up the TC.

Had one the other day on a Freelander.

[frickjp]

Different sized tires on the same car, not 4 matching "different" tires. What died on the freelander?

[TerryS]

Searcaigh,
I never saw a Land Rover speedometer that read correctly, even out of the box. My 110 came through with 7.50x16 and I'm running 235x75R16s, which are supposed to be the same as a 7.50x16. My speedo reads a perfect 10%high.
I don;'t believe the 'different manufacturer' line. as long as the diameter (hence, the circumference) is the same you should cause any TC grief.
The only rotation I do is front to rear, keeping them on the same side. I was always told that radials shouldn't be rotation reversed after install.

[Rovin4life]

same size tires, but different rolling circumference. Not to mention the tires were different treads and different manufacturers.

Vehicle threw codes and wouldnt shift very well.

the car had original michelins on front and cheap tires on the rear.


And no manufacturer or dealer will warrantee a tc for problems stemming from different tires. Seen it a few times at Ford as well.
'

[me.guevara]

Check my post here And the wheel goes 'round about tire size and speedo reading - let me know if you need more math details (I don't want to cause any non-alcohol related hangovers )

If you're running different brand tires, even if they have the same nominal RC, the differences in construction may cause different flex, road slip, and even RCs as the tires heat up - this would be the "excuse" the warranty folk would quote to refuse to service a bad TC.

I don't buy it though, this will happen to some extent with the same brand/size tires and we're not talking about a wimpy TC...

[frickjp]

Originally Posted by beezel

Vehicle threw codes and wouldnt shift very well.

What codes? Just curious. Freelanders don't have a transfer case, wondering what actually failed. We have some financially retentive folks driving these, nice to know what to watch out for.

[Disco]

As I mentioned in my earlier post on this matter, I've never had a problem with rotating my tires every 10,000km. Why do I do this you ask? As I like to experiment with different sized tires each time they come up for renewal, I don't like having a brand new spare, which no longer is compatable with the new additions, and, although I don't have a problem with finances, I don't like having a garage filling up with one-off spares, that even if I tried to sell them, I'd get peanuts for.

[Searcaigh]

Thanks for the replies.

I thought I had only changed the width not the circumference

[rmuller]

I'm running 32"s on my DII (which had 29s to start), and my speed is accurate +/- 1mph .. i checked with GPS, and radar. I didnt change anything.. go figure.

[Searcaigh]

My Speedo was near damn accurate before with the 235s, however the Defender is way out by a minimum 10kph at 80 and by the time I have wound it up to 120 the needle is like a windscreen wiper

[me.guevara]

You did, in fact, change the width: from 235 mm (9.25") to 265 mm (10.4").

In tire dimensions, the second number (in your case, 70) is an Aspect Ratio of sidewall width to tire width. Thus, when you increased the width, the sidewall also went up proportionally (from 16.4 cm to 18.5 cm), hence the increased diameter (from 73.5 cm to 77.4 cm, assuming a 16" wheel).

Curiously, the numbers only justify an increase in speedo indication of 5.73%...

[Searcaigh]

Thanks for doing the math Mr Guevara, never understood these figures so now it is clear, much appreciated

[Private Iron]

I know I am too late but here is a nice tire measurment tool that you might wanna bookmark for the future. Tells you differences in diameter and speedo changes. Really helpful.