Diffs V tyres ??

Series Diff ratio is 4.7, RRC 3.54 so your engine will be spinning 32% faster for any given road speed. To regain that gearing you will need

32% larger tyres - if your present tyres are 780mm tall then you'll need to got to 1030mm tyres (which ain't going to fit).

You could look at 4.1 ratio diffs (Rover car) which gives a 15% increase in engine speed, and with the above example would mean you needed 895mm tyres which would fit with a body and suspension lift if you were really lucky.

On or around Mon, 13 Sep 2004 09:44:16 +1200, EMB enlightened us thusly:

that's much more informative than my reply was going to be.

along the lines of "fecking enormous".

IT IS THE CIRCUMFERENCE OF THE NEW TYRE and not the diameter which needs to be increased by 32% to bring the ratios back into line!! If you were to fit 1030mm tyres ( God knows what tyres or how they would fit you would raise the gearing by far more than the 32% I will post again in a while when I have worked out the sums as to what diameter tyres you would need

David LLAMA 4x4

Just done some very quick sums ( in my head so I may be wrong1) and to put the ratios right you would need tyres in the 38" tall bracket which give or take 7" or 8" will not fit in the wheelarches of a RRClassic... IIRC an overdrive unit is about a 30% ratio change - what about fitting an overdrive as well so on the road you could run with the overdrive putting you about back to normal and offroad you could run without the overdrive and have the much lower gearing??

David LLAMA 4x4

Yes, but as circumference = pi * diameter, the diameter will also need to be 32% bigger (as pi is fixed). You can use either in your maths and the answer will be correct.

Paul

Ah yes I thought they would be quite big, but 38" is quite tall. The body clearance is not an issue as the body will not be a Range Rover :) And it does have an overdrive already did not think of using that in the equasion !!!!!!!!!

Thanks all for the quick advice Rich

The diameter and circumference have a linear relationship (circumference = diameter*pi), so increasing one by 32% will increase the other by the same amount. 1030mm is about 40" - suitable for those Icelandic jobbies, but a bit OTT for the UK.

Good thinking, fidm't bother working that through... either way you are going to struggle with 38" plus tyres! It isn't just the clearances think of how much inertia you will have to break halfshafts with!!

David LLAMA 4x4

Last time I checked circumference and diameter were related by a constant .. Pi !!!

David, any percentage increase in diameter has exactly the same effect on the circumference, and vice-versa.

On or around Tue, 14 Sep 2004 08:17:16 +1200, EMB enlightened us thusly:

alright, enough already, he's quite red enough now... :-)

and we've all at times posted equivalent bollocks.

I admitted earlier in the thread that my posting was not thought through to its ultimate conclusion but at least the info was still accurate amd not of the bollocks standards that some info posted stoops to!!. A-level maths was a lot of years ago now!! I am still waiting to see a RRClassic with 40" tyres that is actually useable, safe and worth the effort of getting them to fit!

David LLAMA 4x4

Sorry David....despite having several hours to do so, the other postings hadn't managed to hit our newsserver when I replied, but they all arrived about 10 minutes after I hit 'Send'. :-)