I thought I would share with the community my experiences of a recent change of tyres that led to issues with my car, a MY05 JDM. Since none of the experts I discussed this with had come across this specific problem before, it might help someone else out there who comes across the same issue.
I had fitted a new set of Toyo T1Rs to my new wheels back in November 2006, replacing the original RE070s which by then had done around 19,000 miles. They have been an OK tyre, at least I no longer had the tramlining of the RE070s, but neither did I have the grip. Still, after another 16,000 miles or so, by February this year I decided to replace the front tyres as the inner edges had worn, and the rears still had 5mm of tread left.
After looking around for replacements, I ended up going for the Vredestein Ultrac Sessantas, in the same size, 235/40ZR18. These were fitted, and I went about my minimal mileage use of the car. However, I started to notice an odd sound coming from the transmission, together with a shunting motion. It seemed to happen when feathering the throttle, maintaining a constant speed on a slight downward slope, and at any speed.
I mentioned this when I had the car serviced, but it seems this is an unusual happening, though we started to discuss having the centre diff rebuilt, as, on the rolling road it could be seen to be alternating the drive from the front wheels to the rear, leading to the shunting motion. So, I came back from the service thinking about getting the car booked in for a centre diff rebuild (no expense spared!).
I then started digging for information, and contacted a couple of people who I respect for their knowledge of Subarus. Firstly, I have exchanged a number of emails with John Felstead, and, having logically eliminated potential problems, he gradually got me to think about the tyres, and went off to see if he could find the rolling circumference of both the T1Rs and the Vredesteins. Around this time I spoke to Dave Hollamby at Gatwick Subaru. He hadn’t come across this particular problem before, but suggested that I try a different set of wheels and tyres to see if the problem persisted. So, as I had ignored my wife, and still had my original wheels and tyres stacked up in the garage, I went back to my semi slicks.
All the symptoms have disappeared with the old setup, and my gear-changing ability is back – another by-product of the shunting issue was that the car snatched slightly with every gear-change – I assumed I was just losing it
I took a couple of measurements with the wheels off. The new tyres have 7mm of tread (the Toyos have 5mm) so not much difference there. However, the circumference of the Vredesteins is around 3.5mm greater, so the centre diff was having to compensate, leading to the shunting.
Needless to say, another 2 new Vredesteins are on order, and unless they are just totally unsuited to the car (which I don’t believe) it should be problem solved.
Many thanks in particular to John Felstead and Dave Hollamby, I guess we can add this experience to the pool of Subaru-related knowledge out there – don’t mix old and new tyres, even on different axles.