How critical are tyre diameters ?

GrumpyGel said:

...So with the older ratio and driving in a straight line, slightly larger tyres would be a benefit. However, when cornering, it would be advantageous to have the rear axel turning slower, so slightly larger on the rear is better on the later 1:1 ratio IRDs (as well as older ones)...

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There is no option for a 1:1 ratio ird, assuming you mean 1:1 ratio of front/rear wheels turning?

Hippo said:

There is no option for a 1:1 ratio ird, assuming you mean 1:1 ratio of front/rear wheels turning?

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My understanding was that the early cars had a small under gearing in the IRD to the front prop, but the later ones were 1:1. It now appears that the early under gearing may be 0.8% and the later ones were actually 0.2%. But that may not be true and all were 0.8% but maybe the 2.5's were 0.2%.

So the front prop may or may not be under geared to the front axle, so if there are 2 new/larger tyres putting them on the front can only be 'bad', whereas on the rear, on driving straight, they can possibly be of benefit but not worse than on the front - and on cornering they can only be of benefit (however small or large) on the rear.

I bought the car had different front- back tires..... a week ago we measured the difference in diameter.
The front tires were 3 cm larger in diameter...... but did not produce damage to the transmission....VCU has been tested and is not locked
I immediately changed the whole set of tires...
....anyway it was probably lucky

O the joys of having a loose VCU [emoji4] [emoji11] [emoji41]

Hi All,

A quick update on what I finally decided. I managed to get a set of 4 Bridgestone Dueller 689 tyres fitted at less than £70 each.

I had 3 sets of these on my Hilux before and got 60k miles+ per set and did a lot of heavy towing and i never lost the tail in the wet no matter how hard i tried ! These tyres with about 3000 miles on them now are proving to be much quieter and smoother than the Pirelli Scorpions STRs fitted before and the Pirellis on the front were goosed after about 20k miles.

The Bridgestone tread pattern is a bit more aggressive than the Pirellis and used to work well on the Hilux pulling a 3.5 tonne trailer off a muddy field - and no, I'm not going to try it with the Freelander !!

Hope this might help someone else looking for tyre options........

thanks for the update