Gen 3 - New tyres for my jimny

The point I am making is that, for the most part, a lighter rim does NOT offset the impacts of a heavier tire, and this is especially true for braking.

We're discussing a Jimny here, solid axle front & rear, the weight of the axles is considerably more than that of the tires so unless you're make a huge change in tire size, we can ignore the increase in unsprung weight, the impact on mileage is not going to come from the weight of the tires as much as it will the increased rolling resistance (if he goes wider) and the effective gearing change (if he goes taller).

The most significant impact of a heavier tire is on braking and that is because the weight is rotating, the assembled wheel might weigh the same because of a lighter rim, but that simply means a larger portion of the weight is now further away from the axis of rotation and that exacerbates the problem.

This is something that you literally have to experience to understand - some people don't get it until the switch back from a larger wheel to the stock size, if they ever do.

Since the OP is sticking with stock size tyres -- and has said nothing about changing rims at all -- this discussion is getting farther away from the topic.  Hopefully he has stopped reading by this point.  

fordem wrote:

The most significant impact of a heavier tire is on braking and that is because the weight is rotating, the assembled wheel might weigh the same because of a lighter rim, but that simply means a larger portion of the weight is now further away from the axis of rotation and that exacerbates the problem.

Here is a simple thought experiment to help you see what is being discussed.  A wheel rotating at a given speed has a certain amount of energy in it, call it the rotational energy.  (The rotational energy is turned into heat by the brakes.)  

The wheel's energy is composed of two parts: the energy of the tyre A and the energy of the rim B.  Total energy of the wheel is A+B.

Now let's say you reduce the weight of the rim.  Take the extreme case and reduce it to zero, meaning we have a rim made of weightless material on which the same tyre is mounted.  The total energy of the wheel is still A + B.  But if B = 0, then the total energy can be expressed as A + 0 or A.  

The energy of the tyre does not depend on the energy of the rim, in other words.  I think we can all agree on this.

The OP is going with a heavier tyre.  That is already a given, see his original post.  So he is going from

Wheel energy (stock tyre) W1 = A1 + B1
to 
Wheel energy (new, heavier tyre) W2 = A2 + B1 with A2 > A1 and thus W2 > W1

What I suggested, which you perhaps misread, is that if he does choose to go with A2, he could then look for a lighter rim (B2) such that B2 < B1.  This would create W3 = A2 + B2, with W3 < W2 and thus less additional load on the brakes i.e. better braking efficiency than just putting a heavier tyre on the same rims as before.