Different Tyre Diameters

[Dougal]

How much does a RRS care if your spare is a little smaller than your other tyres?

Talking 275/40R20 spare and 275/45R20 main set. I'm thinking at a minimum the tyre pressure monitoring (working off abs sensors) will throw up a flag.
It calculates about 14mm OD difference, but the actual tyre diameters could be different either way from the sidewall numbers.

[Redback]

That's a big difference, 28mm in diameter, personally I wouldn't.

Baz.

[Dougal]

That's a big difference, 28mm in diameter, personally I wouldn't.

Baz.

756mm vs 728mm is under 4% difference. That is also the OD, not the diameter of the casing which determines your rolling radius. I would expect the casing diameters to be even closer than that as the 45's have a lot more tread.

Has anyone done it?

[~Rich~]

It's the circumference difference that will take it's toll on your driveline. The diff is 86.39mm per revolution or 36.398m per kilometer, that is a hell of a lot of difference.


Tyre Size Difference Comparison

Select Wheel/Tyre 1 / 275 / 40 R20
Select Wheel/Tyre 2 / 275 / 45 R20

Results for your selected Wheel/Tyre 1 Rolling Radius 364mm Circumference 2287.08mm Diameter 728mm

Results for your selected Wheel/Tyre 2 Rolling Radius 377.75mm Circumference 2373.47mm Diameter 755.5mm

Differences between Wheel/Tyre 1 & 2 Rolling Radius 13.75mm Circumference 86.39mm Diameter 27.5mm When your speedo reads 100km/hr you're actually at 103.64 km/hr


(http://www.exploroz.com/Shop/ExplorOz+Spare_Wheel_Cover_-_Size_005_-_760mm.aspx)


(http://www.exploroz.com/Shop/ExplorOz+Spare_Wheel_Cover_-_Size_005_-_760mm.aspx)

[101RRS]

I have 255/60 R 18s on mine (3 worn, 1 only a quarter worn) - no issues.

The spare is a new 255/55 R18 and one of the worn tyres on the car got a flat and I had to put the spare on - opposite side to the good 255/60. Again no issues but I did keep it on for the shortest possible time.

Difference in rolling diameter is 26mm.

Ok for emergencies and short periods only I think - until you can get some plugs into the puncture.

Garry

[Scouse]

It will log a fault eventually & you'll get various suspension/transfer case/ABS /TC fauls logged due to the different wheel speeds throwing the car out of whack. The suspension will drop to the ground eventually too.

We chased these faults for days on one car before finding a different size tyre on one corner. It wasn't much difference, it could have been like Garry's tyres from memory.

[Redback]

756mm vs 728mm is under 4% difference. That is also the OD, not the diameter of the casing which determines your rolling radius. I would expect the casing diameters to be even closer than that as the 45's have a lot more tread.

Has anyone done it?

Why ask if you don't see a problem:confused:

Like I said I wouldn't do it.

Baz.

[Dougal]

Why ask if you don't see a problem:confused:

Like I said I wouldn't do it.

Baz.

I ask to see if anyone has done it and either experienced a problem or not.
Garrycol's experience suggests there won't be a problem for the use it will get. The smaller tyre would be a spare wheel which may never be used (punctures aren't that common). If it is used it would only be until the usual tyre can be repaired or replaced.

Rich, 3% is 3%. It doesn't matter if you want to call it 30km in 1000km or 30,000km in 1,000,000km.
The speedo won't read 3% out if only one wheel is 3% different in size, the speed difference would be halfed by the differential on that axle and halfed again by the centre differential. Total difference in measured speed would be 1/4 of 3% which is 0.75%.

Tyres don't run on the outer circumference for turns per km, they run on the radius of the steel belts in the outer casing. The tread blocks flex when entering and leaving the contact patch to make up this small difference in rolling diameter.
Just like V belts and timing belts.
If you were to pump a tyre to around 200psi to make deflection minimal then the difference in rolling circumference to actual circumference also becomes minimal.

[101RRS]

I travelled 400km with the smaller diameter on the back left BUT as said it was a new tyre and the one on the same side was a little worn larger tyre so while in theory there was a big difference in reality it may have been marginal. The only reason I have the difference is because I need new tyres soon an will do the lot at the same time.

[gghaggis]

The car measures individual wheel speeds and compares that to the known attitude and trajectory of the car. If they don't match, the DSC will decide whether to activate - if it does, you'll feel a grinding sensation as it tries to match the wheel speeds. Continual running like this will escalate the priority of the logged fault. Eventually the EAS will throw a hissy fit. Mine also disabled TR, HDC and may or may not have contributed to a temporary transmission fault.

Unless you absolutely have to (the tsunami is lapping at your heels), don't do it. If you absolutely have to, stop every 40 k's or so and reset the car, drive in Sand mode to reduce ABS interference, with the DSC off.

Cheers,

Gordon

[Dougal]

The car measures individual wheel speeds and compares that to the known attitude and trajectory of the car. If they don't match, the DSC will decide whether to activate - if it does, you'll feel a grinding sensation as it tries to match the wheel speeds. Continual running like this will escalate the priority of the logged fault. Eventually the EAS will throw a hissy fit. Mine also disabled TR, HDC and may or may not have contributed to a temporary transmission fault.

Unless you absolutely have to (the tsunami is lapping at your heels), don't do it. If you absolutely have to, stop every 40 k's or so and reset the car, drive in Sand mode to reduce ABS interference, with the DSC off.

Cheers,

Gordon

I'd expect the car to trigger a tyre inflation error long before traction/stability control will kick in.
What are the thresholds for each in the RRS? In other vheicles I have heard 1/8th of a wheel revolution (12%) is needed for action to be taken.

[Scouse]

D3s & early Sports didn't have the tyre pressure warning system, out here at least.

[jonesy63]

Out of interest... what difference in tyre diameter is required to trigger DSC?

For instance, I have 4 worn tyres and put on a spare which has 5mm more tread depth (10mm bigger diameter)... would that be enough?

Cheers.

[Dougal]

D3s & early Sports didn't have the tyre pressure warning system, out here at least.

A quick read of the owners manual shows the TPM system if fitted to these usese sensors on the valve stems, you get some quite different looking valves poking out of your rims if you have it.
This is different to the system used by some cars to simply monitor the different abs sensors and flag if one wheel consistently turns faster or slower than the rest.

Jonesy, I'm quite confident stability and traction control aren't going to trigger based off a constant wheel speed difference of that little. Stability control is tied into steering wheel sensors and yaw sensors, exactly how it's coded will be a well kept secret.
Traction control is based on wheel spin/slip, not a minor difference in speed.

[101RRS]

Out of interest... what difference in tyre diameter is required to trigger DSC?

For instance, I have 4 worn tyres and put on a spare which has 5mm more tread depth (10mm bigger diameter)... would that be enough?

Cheers.

Based on my experience I doubt it - however a tyre with about 5 psi in it compared to all the others with about 40psi in them will set DSC off - not sure what the different in tyre dia is between a deflating tyre and an inflated tyre is but was enough to set DSC off - I had not other indication that the tyre was going down.

Garry

[Redback]

Out of interest... what difference in tyre diameter is required to trigger DSC?

For instance, I have 4 worn tyres and put on a spare which has 5mm more tread depth (10mm bigger diameter)... would that be enough?

Cheers.

I don't think 5mm would be an issue Rob, 10mm + might be though, if fitted to the front not as much of an issue, given the 60/40 ratio on AWD systems, probable not a problem on a gravel road, I wouldn't do it on the tar, it's a no no on the D2 because of diff/transfer case wind up, I suspect maybe the same on the D3/D4/RRS, but not to the same extent as long as the difference wasn't to great.

Just my 2 cents

Baz.

[Dougal]

I don't think 5mm would be an issue Rob, 10mm + might be though, if fitted to the front not as much of an issue, given the 60/40 ratio on AWD systems, probable not a problem on a gravel road, I wouldn't do it on the tar, it's a no no on the D2 because of diff/transfer case wind up, I suspect maybe the same on the D3/D4/RRS, but not to the same extent as long as the difference wasn't to great.

Just my 2 cents

Baz.

D2's have three open diffs. There is no diff/TC windup unless you drive around on tarmac with the CDL in. There are many reasons to not drive around with the CDL in on tarmac even with four equal tyres.

[jonesy63]

I don't think 5mm would be an issue Rob, 10mm + might be though, if fitted to the front not as much of an issue, given the 60/40 ratio on AWD systems, probable not a problem on a gravel road, I wouldn't do it on the tar, it's a no no on the D2 because of diff/transfer case wind up, I suspect maybe the same on the D3/D4/RRS, but not to the same extent as long as the difference wasn't to great.

Just my 2 cents

Baz.

Thanks Baz. In future, I guess I will need to do 5 wheel rotations on the MT tyres. :angel:

[Redback]

Thanks Baz. In future, I guess I will need to do 5 wheel rotations on the MT tyres. :angel:

It is recommended on AWD vehicles to include the spare in rotations.

Baz.

[Learner]

[QUOTE=Redback;1695 Snip
given the 60/40 ratio on AWD systems,
SNIP z.[/QUOTE]

Hi Baz,
Would you please explain what you mean by this and its relevance to D3/D4/RRS

[Redback]

Hi Baz,
Would you please explain what you mean by this and its relevance to D3/D4/RRS

Differant but similar systems