What is an allowable rolling radius difference between tyres for ABS/Anti-skid systems to cope?

[Droverunner]

I am looking to fit a spacesaver to my Viano and I've asked about their availability in another thread without much luck.

It would be so much simpler if I could just choose one of the more common Mercedes spacesavers from another model available used but while they will bolt on OK they are mostly too small or much too large in terms of height and therefore rolling radius/circumference.

I've read generic comments that cars will cope with 3% difference... and other places say10% difference is OK.

Obviously the ABS/Anti-skid and traction systems will have to be happy with any difference... as will the diff if running the spacesaveron the rear.

Anyone have any ideas on this?

 

[LostKiwi]

Your biggest issue on a van will be load rating. You'll almost certainly need to use a spare rated for a van.
For diffs I wouldn't want to run more than 5% difference to prevent excess heat build up.

 

[Droverunner]

I don't know much about van tyres but as my Viano is a people carrier type many of the space savers were quite close in load capacity to my Viano's (correct fit) current tyres. The real devil was the circumference because I couldn't find any reliable source for a definite guide on an acceptable percentage variance.... and I just didn't want to risk ABS/Anti-slip glitches or excess diff workload so I had to discount the huge majority of cheap used wheels on Ebay which were all 5% plus different.

Anyway after two hours online I tracked down and bought a Mercedes space saver with the correct PCD and a tyre with a load rating 75kg higher than the ones on my vehicle plus a circumference that was just 0.6% different.

This tyre...

Buy Continental Tyres | Buy Online, Fit Locally | Bush Tyres

Shop Continental tyres online today! Bush Tyres are a leading stockist of tyres for car, commercial vehicles and the agricultural sector.

www.bushtyres.co.uk

To work out the suitable sizes I used this great online calculator...

Alloy Wheel & Tyre Fitment Calculator. Offset, Tyre Stretch and Speedo Error | Will They Fit

Alloy Wheel & tyre fitment calculator - it calculates inset, poke, speedo error, tyre stretch and loads more.

www.willtheyfit.com

So after a bit of effort it's worked out but at double the used cost of the common ones for Mercedes cars.