There is no such thing as a 19.5 x8 wheel
The 225 and 245 will will take a 6.00 or 6.750 rim
the 265 and 285 will go on a 7.500 rim.
There is a 19.5x8.25 rim for 305/70R19.5

Charlie

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Unimog U500 w/Unicat

Before you get too carried away with the idea that bigger tyres will fix your speed problem , these were never rocket ships! Is your speed limited because you are running out of revs, if not then fitting taller tyres wont help
and you will just have another problem , not being able to make normal use of the gearbox , and having to use lower gears climbing and slightly slower acceleration . What sort of cruise speed are you looking for?

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Current : 2007 Mowag Bucher Duro 6x6 Motorhome , 2006 Sedici 4x4, 2007 Range Rover supercharged

I have no idea (or interest!) in whats available, just saying what kind of size you need to fit the ops preferred width (265) on. After that it's upto to the OP to find out or others to say whats actually available, you would have to get the nearest equivalent. there is some degree of flexibilty in terms of what tyres will fit what rims. I'm currently running 235s on 8 inch rims, but they were apparently a bitch to fit! But that was what I happened to have left over in my garage and at £600 a set I wasn't about to buy a new set! Also I went for an 8inch rim, so I could get a -33 offset (not possible on 7inch rims) and not lose too much on the steering lock, knowing there was room in the wheelarches (just!). You can get any wheel you like made (assuming it's physically possible), at a cost. 19.5x6.5, 7.5, 8, 8.25 or 10, whatever you like!

My main points are:
a. I don't think this will work without a great deal of modification
b. It will have undesirable consequences you don't appear to have considered. If as tacrman has suggested, you aren't just running out of revs you may end up with an even lower top speed because the engine won't pull the gearing!
Then there is the acceleration I have already mentioned, you may find hill climbing a chore. then there is physically fitting the wheels, you need to consider steering lock, fouling on bodywork and suspension components etc and in with that as well as the size of the wheels you need to think about the offset, which you haven't mentioned. you may need a big -ve offset for the tyres you want to clear components on lock, which may require a custom wheel. After all that you need to think about the additional strain all this will put on other components such as steering, bearings, transmission etc. You may need a suspension lift, which will have an impact on steering geometry so steering may require modification. You may find you need to extend propshafts, anti-rollbars don't fit etc etc. Of course, all this can be overcome, but there are a multitude of things to consider before undertaking such a mod. I'm not saying it can't be done, just that it's not as simple as just chucking on a bigger set of wheels and tyres.

Your opening up a can of worms making such a big mod I reckon without some serious investigation first.

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1990 Landcruiser H60. Full rebuild completed 2014

Thank you all for your thoughts, that's the sort of thing I need.

I am not running out of revs, it's just running at 2200 rpm at 50 and I would like to get this down to 1700, so it's still on boost. I don't particularly want wider tyres as this will increase rolling resistance.

As Moggy says, depth is a sum of the width, but maybe I should lower my sights a bit and go for 245

I have to get a new speedo anyway for the MOT as this is in Km, so I will get it re-calibrated. As you can see from the picture there is loads of room in the wheel arch, so I don't have to mess with the suspension (there is one in iceland with R20 unimog wheels)

Any more ideas welcome

Bruce.

it was after looking at the pics I thought you might struggle! At least some trimming on the bumpers and wheel arches, not forgetting it has to not foul on full articulation and full lock.

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1990 Landcruiser H60. Full rebuild completed 2014