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tyre size effect on towing


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hi guys.

I purposefully have not put this in the discovery section because I feel it will apply to drivers of discoverys, range rovers and probably moreso defenders.

It has recently come to my attention, through a few calculations that the transfer box in my discovery, although serial numbered a 28DxxxxxxxxxxxE box, is infact 1.41:1 ratio, which is the cause of my high revs per given road speed.

I shall soon be looking for a set of tyres, hopefully to be used both on and off road for work purposes Involving a lot of agricultural and woodland locations.

When I thought that I had a fault somewhere else and had a 1.222 box, I was looking at fitting 265/75s. No lift, Im not a fan, but perhaps a set of modest arch flares with some trimming. It was supposed that I wouldnt notice much difference.

To get my road speed ratio back to where it should be, It would seem that fitting 33s would be a solution. But is it as simple as that? is the force required by the engine to turn a small wheel faster (1.222box) the same as turning a bigger wheel more slowly? (1.41box)

essentially, 33s would put my road speed per 1000rpm the same as my father's 300tdi on 235/75s, but would it tow as well? currently geared like a defender it tows better, yet returns very pleasing fuel economy. I don't tow huge loads often, but id like to know if it's going to fall on its arse when I do. 33s would also give me a bit better tow height to my sankey (already tows at bumper height)

I think you can probably tell what Im getting at. from a cost point of view, Im also considering going to a 15inch wheel as the tyres seem infinitely cheaper.

Any advice greatfully received.


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Regarding engine power, gearing and tyresize, there are 2 things to consider:

1. Bigger (taller) tyres have a larger circumference, so the engine will do less revs for a given speed. This is exactly the same effect as changing the gearing, and as you calculated,to compensate for you TC you need tyres of 32-33" diameter.

2. Bigger tyres will increase the height of your Disco and thus (air)drag. Wider tyres could also increase road drag/rolling resistance.

In your case, towing should remain better than a standard Disco if you don't go over 32", with perhaps a little loss of topspeed. So 265/75R16 seems like a good plan.

However, fitting them without a lift might require a decent amount of trimming. Even more so should you look for even larger tyres.

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My dad used to do alot of towing around the uk in a 110 he would go up one side of the country at the statrt of the week and come back aqgain down the other side of the country. He did 450,000 miles in a 200 Tdi 110. But he used to use Disco 1 size tyres as they made the 110 tow alot better . Did that with the two TD5 110's he had after that one doing the same sort of mileage .

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