bluespanner Posted March 3, 2012 Share Posted March 3, 2012 ...if I put 255/65R16s on the front and 255/70R16s on the back? It's a p38 v8 auto. Roland Quote Link to comment Share on other sites More sharing options...
bluespanner Posted March 3, 2012 Author Share Posted March 3, 2012 Tyre calculator jobby says there is a difference of 3.5%. Quote Link to comment Share on other sites More sharing options...
stuck Posted March 3, 2012 Share Posted March 3, 2012 I can't imagine it would be very happy as you are slightly changing the ratios between front and rear as I'm sure you know. More to the point why would you want to? Quote Link to comment Share on other sites More sharing options...
bluespanner Posted March 3, 2012 Author Share Posted March 3, 2012 Because I have two of the bigger tyres, that's all. I'd better be on the look out for two more Quote Link to comment Share on other sites More sharing options...
Cynic-al Posted March 5, 2012 Share Posted March 5, 2012 You could underinflate your bigger tyres to keep the rolling diameter the same ... Quote Link to comment Share on other sites More sharing options...
sean f Posted March 5, 2012 Share Posted March 5, 2012 The critical number is the rolling circumference i.e. how far the vehicle travels in one rotation of the wheel, since this is directly proportional to the diameter it is easier to just look at diameters. I haven't got a calculator out but have no reason to doubt the 3.5% difference quoted above, it sounds about right. The only addition is what the tyre ACCUTALLY measures, the variation in tyre sizes from different manufactures for the same listed size is pretty big and it is not unknown to have 2 tyres from the same manufacture with different quoted sizes but to actually find in practice they are the same physical size (particularly with remoulds). For this reason you need to be a bit carefull mixing tyres from different tyre manufactures on the same vehicle, most vehicle manufactures recomend against it but in reality not many people are going to change all 4 tyres when only 1 is damaged just try not to go to extremes. If you measure diameters to compare them make sure you are measuring like with like, if the tyres are mounted then the width of the rim makes a difference as does obviously the pressure in the tyre. Ideally the measurement which should be used is with the tyre on the vehicle at the correct pressure and the vehicle loaded as normal, the distance needed is from the centre of the hub to the ground, twice this is the diameter. This takes account of the amount the side walls flex. In practice standing the unmounted tyres by side is generally close enough for a comparision. Quote Link to comment Share on other sites More sharing options...
g&t Posted March 5, 2012 Share Posted March 5, 2012 Would it not also effect the VCU? Quote Link to comment Share on other sites More sharing options...
bishbosh Posted March 6, 2012 Share Posted March 6, 2012 You could underinflate your bigger tyres to keep the rolling diameter the same ... I am sure this was said tongue in cheek, but just to be sure, definitely DO NOT do this - under inflated tyres overheat and burst. A blowout in a rear tyre on a 2 tonne truck is not a good idea.... Quote Link to comment Share on other sites More sharing options...
Snagger Posted March 6, 2012 Share Posted March 6, 2012 I regard it as extremely important to match not only the dimensions of the tyres ona 4wd vehicle, but also the tread pattern and compound. I only ever run a different tyre if it's a spare wheel, and even then it's only as a very short term measure with the damaged tyre being replaced and fitted as a matter of urgency. It's not just a matter of avoiding damage to the 4wd mechanism, but also of ensuring proper handling and appropriate levels of grip for correct brake operation. Quote Link to comment Share on other sites More sharing options...
sean f Posted March 7, 2012 Share Posted March 7, 2012 I regard it as extremely important to match not only the dimensions of the tyres ona 4wd vehicle, but also the tread pattern and compound. I only ever run a different tyre if it's a spare wheel, and even then it's only as a very short term measure with the damaged tyre being replaced and fitted as a matter of urgency. It's not just a matter of avoiding damage to the 4wd mechanism, but also of ensuring proper handling and appropriate levels of grip for correct brake operation. I was looking at this only from the mechanical aspect with different tread patterns, even of the same make of tyre often having different diameters. For handling and braking safety you are of cause totally correct. Quote Link to comment Share on other sites More sharing options...
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