Just changed over a shagged set of 235x45x17's to the spare set of wheels/tyres that came with the car (lucky me ), so now have 2 brand new on the front, and 2 part worn on the rear. The 2 rears have 6mm and 4mm tread left respectively - does the 2mm difference matter?

(also treated the spare set to a full clean and poorboys wheel wax - heck, I'm getting sad in my old age

 

I don't remember what the cutoff size deference is but what I'd do is run a tape measure (preferably a flexible one like used for sewing) around the most worn tire and a front tire. If it's within half and inch to an inch I'd say don't worry too much, if it's more I'd at least get a second opinion.

I've had people burn up their center diffs by running new tires on only 2 wheels. One guy ran his wrong size spare too long and his rear diff actually caught on fire somehow, good thing there was a snow bank to back into to put it out.

 

Thanks for your reply.

Rather than take a tape measure to it, I dug the calculator out. A 235x45x17 tyre has a rolling circumference of 2021mm. If the tyre is 2mm worn this reduces it to 2007mm - a 14mm difference.

This is a little over ½" so, from what you say, this shouldn't matter?

The difference is between rear left (sightly worn - 6mm tread) to rear right (more worn - 4mm tread).

The fronts are brand new (7mm, from the web) so the worst case difference is 7mm vs 4mm, so 18mm/¾" difference in rolling circumference - still OK you say?

 

I make it 12.6mm difference - how sad am I to do the calcs?

 

Agreed - I slipped up somewhere!

As sad as *I* am

 

Think I read somewhere (re Volvo 4wd) that tread depth differences more than 2 mm should be avoided, also tyres should all be same make/type.

Suspect it would be reasonable to apply same to Scoob. I always buy 4 tyres at a time and swap fronts to back at 10-12k to even out differences over my usual 20-24k tyre life.

Also I keep a part worn spare (one of my own old tyres) that would be a better replacement for a worn tyre in the event of an unrepairable puncture.

 

Done a bit of digging and etyres, for example, say 3-4mm is what many manufacturers recommend, but that tyres should match across the axle.

I think that I will keep the tyres on for the next few weeks, so they are scrubbed in, ready for a Ben Elliott driver training day on the 9th Feb. I'll then probably take them off again and keep them for track days, but may have to bite on the bullet and buy a new one to even up the axles. Seems a shame, but cheaper than replacing the diffs!!

The tyres in question are RE070's and, since these are a bit noisy, in deference to the family I will probably go for T1-Rs or F1's for day-to-day driving on the other set of wheels.

 

At least you had reason to do it! I just did it cos I'm sad and bored at work...

 

Hang on fellas! Whilst the maths is very interesting, i would just question whether you do all your driving on arrow straight roads, or perhaps use the roads I use - curving here and there, and consequently the diff being active all the time. Surely the diff(s) doesn't know whether you have uneven wear on the tyres or are driving round a corner.

The guidance given by manufactures probably relates to safety in the event of a sudden direction change or hitting standing water - this sort of thing could unsettle a car with uneven wear.

Just a thought!

Howard

 

You may have a point, but I wonder what percentage of driving is around corners rather than straight? The problem with uneven rolling circumference will be a constant slip in the diff rather than occasional/periodic - and when you do go around a corner, the amount of slip will be even more.

As has been hinted at, it possibly results in overheating.

Bottom line - I don't really know, so I either play safe, or I throw caution to the wind and see what happens.

That makes a lot of sense - stability under heavy braking might be an issue too.

Still unsure though!

 

I've been thinking about this issue re uneven tyre tread depths lately and I keep coming back to the fact that a diffs sole purpose and design is to allow the wheels to each travel at varying speeds. As howard says this happens every day on all twistys and roundabouts and I believe this would be more so on track days as you are consatntly going one way around a track lap after lap. Perhaps this prop wind up is an urban myth when it comes to small differences in tyre tread depth (obviously its an issue when towing on two wheels of course). Anyway thats my sad thought of the day