Odometer Accuracy vs Overall Rolling Circumference

StuartW

StuartW

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Can anyone tell me, does the overall rolling wheel circumference of a car's wheel (tyre) still have a direct correlation to the accuracy of a car's odometer? I know it used to in the 'old days', but I have no idea how the odo and speedo pick up their readings now.

It looks like I need to change the profile of my rear tyres on the Trofeo due to a lack of availability of the size they are currently wearing and I am curious to know if these readings will still be correct. It's not hugely important as it won't be doing a great many miles and I would probably use the speedo from a GPS rather than the car's own one but it would be good to know.

It looks like the rears are going to have to go down from a 35 profile to a 30 - as if I needed the ride to get even more firm ...
 
Zep

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There are a few calculators on t’internet that calculate the difference. It will affect the reading as the speedo and odo take their readings from the ABS ECU I believe.
 
Oneball

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You might find it makes it more accurate, just measure with a gps and compare after you’ve changed them.

Worth doing anyway, I wonder if they calibrated it in the change from the standard to race.

Has it got a pit lane limiter?
 
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CatmanV2

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I really doubt a typical GPS is going to have a lower margin of error. Just looking at some of the tracks that get plotted when we go walking / running.

C
 
StuartW

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Thanks guys & Zep, I have taken a look at the calculator and I agree, it doesn't make a huge difference.

Zep said:
There are a few calculators on t’internet that calculate the difference. It will affect the reading as the speedo and odo take their readings from the ABS ECU I believe.
Click to expand...

The car does not have ABS however so I'm not sure where it's readings are taken from?

Oneball said:
You might find it makes it more accurate, just measure with a gps and compare after you’ve changed them.

Worth doing anyway, I wonder if they calibrated it in the change from the standard to race.

Has it got a pit lane limiter?
Click to expand...

And yes, it does have a pit lane limiter. I haven't tried it though as we went into lockdown just after the car arrived at home so I have barely driven it in anger as yet
 
Oneball

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It may still have the pick ups in the hubs even if it’s not got ABS. Is there any traction control? That’ll use the same sensors as ABS.
 
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Sam McGoo

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Dropping from 35 to 30 will make it over-read a bit. But, if you can, go a bit wider at the same time and that will put it back in the right direction.
Have a play with the calculators and see what works best.
 
StuartW

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Oneball said:
It may still have the pick ups in the hubs even if it’s not got ABS. Is there any traction control? That’ll use the same sensors as ABS.
Click to expand...

No, there's no traction control either but I guess it must be from the hubs

Sam McGoo said:
Dropping from 35 to 30 will make it over-read a bit. But, if you can, go a bit wider at the same time and that will put it back in the right direction.
Have a play with the calculators and see what works best.
Click to expand...

I guess it will be a case of seeing what will fit the rims and then getting what I can as close as possible to what should be on there. I don't think there's any chance of going wider though as the wheel is a huge 13" rim on a comparatively small 18" diameter, so it currently has 345 35 18 and the best I can find right now is 335 30 18. There is a Mickey Thompson street tyre available in my size of 345 35 18 but there are question marks over it's legality in the UK
 
Zep

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The difference at that size is 6% which is a fair chunk. Have you tried Longstone Tyres, they do the weird sizes for 80s supercars so might know where to get some.
 
StuartW

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Zep said:
The difference at that size is 6% which is a fair chunk. Have you tried Longstone Tyres, they do the weird sizes for 80s supercars so might know where to get some.
Click to expand...

I have spoken to Tom over there, but he was struggling with the 13" unfortunately

The frustration is that both Bridgestone & Pirelli have the moulds for this size but they make them exclusively for the XJ220 so IF you can persuade them to sell you a pair of rears, they are charging around £3k for a pair which I could well ruin after a couple of spirited weekends at a track.
No one ever said that motor sport was a cheap hobby!
 
Oneball

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StuartW said:
It's their ET Street s/s which is a DOT street tyre in the US but yes, being a MT it is more geared to the drag strip so not ideal I agree
Click to expand...

They’re a drag tyre. They’ll be a really soft compound, you’ll probably cook them in a lap.
 
Zep

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What about a an intermediate race tyre? I don’t think the construction and use regs require an e-Mark.
 
Zep

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StuartW said:
I need it to be road legal though
Click to expand...

I had a quick look at the construction and use regs, it doesn’t mention any requirement for an e mark to be road legal. I assume (it would be worth checking) that provided it has tread, it would be legal.
 
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