Allowed Tyre dia difference front to rear

[FraserWaters]

I swopped my Vito w639 tyres on the rear for a pair of beefy 225/75/16 on steel rims but didn't do the front yet, they're 225/55/17 on alloys. I know it looks weird but i can't afford the new front's until payday.

Van drives spot on but after about 3 minutes of driving the speedo stopped working and the ABS lights came on, switch off and the same again after 3 minutes of driving. It doesn't go into limp mode as such, it still drives fine but no speedo, luckily i have a GPS speedo too.

My question is what is the allowed difference in rolling dia to stop this happening, could i put 225/70/16 up front and 225/75/16 out back? I'm a bit worried about clearance on the front when going lock to lock...

Cheers...

 

[400ixl]

Legally they have to be the same size on the same axle, so what you have done is technically legal. Now whether it will throw sensors as many now use the ABS system to monitor all wheel rotations independently and throw issues if they down all match. If you go to Tyre Size Calculator and put in the sizes you will see what are the closest diameter matches which will mean the sensors are least effected.

 

[FraserWaters]

Thanks, I'd actually worked out the dia on a napkin over lunch. Wish i'd asked google tbh ;-)

I'm going to run 225/70/16 up front to just make sure there's no clearance issues and stay within about 3% of the bigger rear wheels, hopefully that doesn't upset the ABS ring sensors.

Cheers

 

[BTB 500]

Can't see why the system would allow for much more than about 10-12mm difference in overall diameter between front and back ... i.e. brand new vs bald tyres.

Obviously if you've changed the rolling radius of the rear wheels from stock the speed and distance shown on the cluster will both be out.

 

[FraserWaters]

Quick update:

I fitted 225/75/16 rear and 225/70/16 front Michelin latitude cross tyres to my w639. No clearance issues at all, tyres fit inside arches with no rubbing even lock to lock. Speedo and warning lights are all behaving so there must be a tolerance built into the sensors of about 12mm.
Van drives great on our sh## roads.