[svxistentialist]

Originally Posted by m.messenger

I currently run 18" wheels with 245 x 45 x 18 Vredestein Ultrac tyres - they fill the arches to the max but just OK, except on full bad bumps when they sometimes tickle the plastic shrouds at the back of the front arches and the outer sides of the rear arch shrouds - otherwise great. Before that I had Bridgestone run-flat 225 x 40 x 18 - too harsh because of the reinforced run-flat sidewalls.
The point is thet I have an Origin B2 GPRS radar thingy which gives a constant speed read: with the standard 16" wheels and 225 x 50 tyres the diff between the dash speedo and the GPRS speed ( once it catches up) was about 4-5 mph - with the bigger radius 225 x 40 Bridgestones it was about 3 mph - with the Ultracs I am spot on - 0 mph difference.
So there we go!

MM

Hi Michael.

Thanks for the comparatives. Here's one that may interest you. It is interesting you show about 4-5 mph difference between true speed and speedometer reading with stock tyres and rims. I keep 5 rims shod because I won't go a long run, such as the UK to JAE, with that piddling get-u-home space saver.

With my last SVX, I changed from a set of [4] Savas, which I liked because they were quiet, to a set of Michelin. Naturally, because I would only use the fifth rim for long runs, and only as a spare, I did not bother to buy a fifth tyre at $165 or so each. Just to check on rolling radius, I stood both wheels together, part worn Sava alongside brand new Michelin. Imagine my surprise when the Michelin shod wheel with full tyre tread depth stood 1.5 cms shorter than the half worn Sava! Both with regulation tyre pressure!

To me, this means that your rolling radius is going to vary between makes. This also partly explains why Subaru insists that we change all four tyres together. Buy only two at a time, and a bigger or smaller set on one axle will very soon put loads of pressure on the differentials.


Just one to watch.

Joe