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I have fitted larger diameter tyres to most vehicles i have owned, including 2wd's and 4wd's. Once the speedo, tripmeter, odometer, and fuel consumption readouts are corrected, whether that be by simple calculation or an adapter or reprogramming, i found that fuel consumption was about the same or better. If the vehicle's engine revs a lot at highway speeds there is a decent chance fuel economy will get better.
Whether you have an auto or manual, during most driving the revs tend to be kept within a certain band, so changing the gearing with different tyres just means that each gear will get used within a slightly different speed range.
There are a lot of misconceptions about the effects of larger diameter and larger width tyres.
A good way to look at it is to compare a typical bicycle and a monster truck. A monster truck has large diameter and wide tyres and they are brilliant off road, whether in mud or sand, whereas a typical bicycle even though it has large diameter tyres is terrible off road, sinking and losing traction. Of course we can all readily acknowledge that small diameter and narrow tyres are the worst, try taking a shopping trolley off road.
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I went down the larger tyres route with the D1, but decided the disadvantages outweighed the advantages in most situations. I have decided the Defender's standard 235/85/16s are more than adequate for my needs, as I have no plans to climb cliffs, so do not intend to lift it or go for bigger tyres.
I guess it depends on each person's needs. If I was building a truck to spend most of its time off road I might think differently. I do like the look of a D1 with 245/70/16 MTs, but its noisy and doesn't brake as well on road.
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Am finding 255/85R16 rubber on stock Boost Alloy rims on my 2013 Defender, with a BAS tune, are pretty much perfect size.