Fridge/Freezer Tempurature Difference

If I have a 15 Cubic foot freezer-over-fridge for the garage (no basements in Texas), and if I can keep about 35F between the freezer and the fridge (from 4F to 39F), and if I can "fool" the built-in original thermostat into thinking the freezer's at 4F when it's really at 35F -
- then will the fridge be about 70F?
That way I could lager in the freezer and primary/secondary in the fridge, right?

Huh? Why would you want to get your fridge to 70 degrees? Am I misunderstanding your question.

IMO, you shouldn't try tricking your fridge to do anything. In fact, your refrigerator will be a good place to lager your beer post-fermentation. The lagering phase, remember, is that phase after the beer is done fermenting in which you store the beer at temps as near freezing as possible for about 4 weeks. You refrigerator should provide an environment suitable for this without trickery .

Of course, the fermentation will occur around 55 degrees, so your biggest challenge may be keeping your fermenting beer around this temp. Right now, my brew closet is a constant 65 degrees. I put my primary fermenter in a water-filled bucket and add a few bottles of ice once a day. The fermenting beers stays between 54 and 57.

IMO, 70 degrees is way to warm for a lager yeast to work properly.

I was hoping to be able to lager for weeks and months at about 35F. Haven't lagered yet. Figured that if the freezer would hold 35F and if the fridge then held about 70F, then I could at least primary and secondary an ale while the freezer is at about 35F.

I haven't built a fermentation chiller cabinet yet. But my garage gets to about 103F in the summer, and I am looking for a stable temp for lagering possibilities. Fermenting the lager at 55F would occur in the fridge, and that's if the freezer could be set at about 20F.

Trying to think and ask questions before messing up TOOOooo bad!

The thermostats don't measure differential temperature, so that won't work. Go to a homebrew store and drop $55 on a Johnson Controls unit that in effect replaces the original thermostat. (Unless you know enough electronics to build one of the several designs available on the web.)

Most fridge/freezer setups regulate the temperature of the fridge (because that is more important). The freezer's actual temperature depends on how often and how long the compressor runs, but you don't really have any control. It can be anywhere from 0F-20F

This method breaks down when you use a controller to run the fridge at fermentation or lagering temps, because the run time of the unit becomes much shorter.

You can tightly control freezer or fridge, but not both. My Ranco's instructions caution against trying to use the freezer section when controlling the fridge.