Depends on the tire. Very simple. Look on manufacturers web site under specs.
It would depend on the vehicle.
the tread type matters very little in fuel economy. the difference in tread may or may not effect you mileage only a minimal amount. 1mpg maybe. what WOULD effect it is the weight of the tire. mudders tend to be heavier so deendign on the specific tire you get you may be adding weight and thereby adding rolling resistance. more resistance, less mileage. long story short, this question is much the same as asking what kind of mileage difference between a Tahoe and a Yukon. theyre the same, but the package is what changes things.
The 235/75/16 is larger.
75% of 315 = 236.25
245/75-16
285/75/16 will fit without rubbing. we have put as large as 315/70/16's on them many times and they will rub when the steering wheel if fully turned.
The size you have is about as cheap as you will get in a 16" tire.
225/75-16 or 255/65-16
245-75-16
That is quite a bit wider, make sure there is clearance when steering is on full lock.
75 [lots] of 420 = 31500.
It is written on the side of the tire.