SirkoZ opened the ticket and wrote:

Version: 0.7.0-RC2 (and before) and nightlies since quite some time (I haven't used default economy to see it before now).

The two decreased productions/production levels (the lowest and one higher than the lowest) of default economy are currently calculated wrongly with this formula:
i->production_rate[0] = min(indspec->production_rate[0] * i->prod_level / PRODLEVEL_DEFAULT, 0xFF);

E.g. Coal Mine -> TTD calculation for decrease: (prod_rate >> 1) + 1 so we get 15/2 + 1 = 7 * 8 or 9 = production of 64 or 72 tonnes of coal per month. At the same time the lowest production would be 32 or 40 tonnes per month.

With this new formula, production is 56 or 63 and if it decreases once more, it's only 24 or 30 per month which is totally wrong/as low as the oil wells which start at 96/108.

It should be coded (as it's in TTD) that industries gain one unit of production when they decrease (if of course production rate isn't cleanly divisible with 2).

Reported version: 0.7.0-RC2
Operating system: All

This issue was imported from FlySpray: https://bugs.openttd.org/task/2772