Both those sizes are pretty common. 265/70/16 is about a 30.5 inch tyre, 265/75/16 is 31-31.5 inch.
Each tyre manufacturer will have slightly different exact tyre dimensions for a given size. So a 31 inch tyre maybe slightly more or less than 31 inch.
You probably won't notice a difference, the ute won't really notice a difference if you go up or down less than an inch. Other than a slight change in gearing and your speedo will read a bit different. Maybe 1-3kph difference.
Changing sizes you may notice a negligible change in fuel consumption, if the new tyres are lighter than your current. Heavier tyre, more effort to turn them. You can compare tyre weights easily by googling the specs.
If you do a bit of off road, maintaining the 31 inch will keep a bit of clearance between your diff and the dirt. An inch doesn't sound like much but it can be...
Your could also look at 275/70/16 - slightly wider and slightly smaller - reasonably common tyre size with a few a/t options.
If you do a bit of paddock driving, wet/muddy areas driving, up ski fields etc, I'd go down AT route instead of HT every time.
Look at some of these as options, all get very good reviews, especially the Wildpeak and Yokohamas.
https://www.trademe.co.nz/a/motors/c...1?bof=Fep1wo5g
https://www.trademe.co.nz/a/motors/c...1?bof=Fep1wo5g
https://www.trademe.co.nz/a/motors/c...9?bof=Fep1wo5g
https://www.trademe.co.nz/a/motors/c...1?bof=Fep1wo5g
https://www.trademe.co.nz/a/motors/c...1?bof=Fep1wo5g
I usually buy a new set of tyres every year, have tried so many options. At the moment running Mickey Thompson ATz P3's on my Landcruiser, they've done just over 50,000km and are less than half worn.
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