Super Singles question

[BigSkyBrad]

I'm looking to go to super singles on my FGB71, but have a question relating to the increase in diameter/circumference, and the subsequent decrease in rpm and shift away from the power sweet spot (though with a turbo-diesel it's pretty broad and torquey).

According to https://tiresize.com/comparison going from 215/75-17.5 (5.285 final drive) to 37x12.5-17 is an increase of 22.5%, or to 285/70-19.5 is an increase of 16.6%.
However, mine is a UK-spec with 205/75-17.5 (with 4.875 diffs) and going to 285/70-19.5 or 35x12.5-17 is an increase of 18.9 % and 18.2%.

These percentages assume the same diff ratios. I figure the difference in tyre size in the first three gears won't be as noticable as in the higher gears, my question is will my taller ratio cope with the taller tyres?.

 

[SkiFreak]

Is it manual or auto? Do you have low range?

 

[BigSkyBrad]

Manual, only high range. The UK importers seem to think it'll never leave the tarmac!

 

[SkiFreak]

My FG84 is much better suited to bigger tyres, but with a FGB71 with no low range I would be concerned that bigger tyres could reduce the truck's off-road capacity even further.

 

[BigSkyBrad]

The truck won't see serious offroad stuff - just snow, gravel roads and mild fire tracks, 1st-world travelling. I was more worried it would struggle on highway gradients with bigger tyres and the taller gearing.

 

[SkiFreak]

I cannot speak for the FGB71, but with my truck is was definitely beneficial for driving at highway speeds with the super singles.

 

[BigSkyBrad]

Using the Tremec gearbox website final drive calculator, I’ve worked out that at 56mph (90kph) which is the UK over-3.5ton speed limit, my truck on 285/70-19.5 will be doing 1900 rpm, which is just inside the hp/torque sweet spot. This is just 50rpm less than an OZ truck on 37’s according to the calculator, so I guess it will be ok.

Certainly, less cruising rpm should mean less fuel consumption, tyre wear and engine wear. I can always shift down for the steep bits.