Wheels/Tyres: DSTC & wheel diameter

[balado]

S60 2004 auto 2.0T
After problems with leaking rims that have not been resolved reconditioning and bead sealing, i am at the stage where two wheels can't be relied on. Am getting too old to be checking tyres before every journey.
Since my tyres are approaching the legal limit, I intend buying new tyres and wheels. Am currently running 225 45 17 91w and intend going down to 205 55 16 091w, which I believe will give a smoother ride and are less expensive.
My question is that the handbook states that "deactivate the DTSC if you must use a wheel with different dimensions". How significant / important is this? Like my car I am rather old and the car is never driven hard or in extreme conditions. In all 13 years I have had the vehicle i have never been aware of the active presence of the DSTC. Since the DSTC always comes on at startup MUST I always switch it off if I go down this route?

Thanks in advance for any input

Balado

[DieselDoNicely]

The cars were design to use different wheel sizes. a 16 inch wheel with cause a speedo error. An error you can correct by updating the setting in the cars ECU.

When I ran 16 inchers I didnt bother, and the ABS worked.

DDN

[oragex]

It's replacing a tire with overall diameter of 24.97in with a tire that has 24.88in

It's almost the same, shouldn't be a problem. I'd just make sure the 16in wheels clear the front calipers.

[balado]

thanks oragex...not sure what the front caliper means (excuse my ignorance) Itend to have the new wheels delivered to my local indie garage who know my car well, and have just renewed my front discs at 55,000 miles. Dont want to land them in a situation they can't resond to.

Balado

[4x4]

Looks close to original to me, nothing to concern you

[old onions]

I swap between summer alloys with 225/45 x 17 , and winter steels with 205/55 x 16 every winter.

Everything works fine , I have no problems with clearance on the smaller wheels, and the ride quality is vastly improved on the 16s.

I do declare it to my insurance company and they "allow" it as "a deviation from standard specification" with no charge (and so they should it makes the car safer during the winter months)

[sparkeysteve]

Originally Posted by DieselDoNicely

The cars were design to use different wheel sizes. a 16 inch wheel with cause a speedo error. An error you can correct by updating the setting in the cars ECU.

When I ran 16 inchers I didnt bother, and the ABS worked.

DDN

how do you do this?
i have VIDA but can't find where to change this setting

[Simon Jones]

Originally Posted by balado

My question is that the handbook states that "deactivate the DTSC if you must use a wheel with different dimensions". How significant / important is this? Like my car I am rather old and the car is never driven hard or in extreme conditions. In all 13 years I have had the vehicle i have never been aware of the active presence of the DSTC. Since the DSTC always comes on at startup MUST I always switch it off if I go down this route?

The key word there is 'a' wheel, not 'the' wheels. DTSC and ABS works by monitoring the relative speed of each wheel and if one starts to speed up it will apply the brake to prevent it spinning. If you fit a skinny spare wheel which may not have the same rolling circumference as the other wheel then it could cause the ABS to try to slow it down. If you replace all 4 tyres/wheels than they will all be going round at the same speed so there will be no issues.

[DieselDoNicely]

Originally Posted by sparkeysteve

how do you do this?
i have VIDA but can't find where to change this setting

Never tried to do it. But the cars had 16 wheel options so must be there somewhere. The 16 inch wheel adds about 4 or 5 mph to the actual speed.

DDN

[Brendan W]

I'm with Simon on this. The operative part of the sentence is 'a wheel' and ABS compares speeds.
There are other times you are better off with STC switched off like negotiating snow and ice.

Originally Posted by DieselDoNicely

Never tried to do it. But the cars had 16 wheel options so must be there somewhere. The 16 inch wheel adds about 4 or 5 mph to the actual speed.

Presumably it adds a proportional amount to the odometer as well and messes with service intervals and mpg ?