The GMT800 Subs & Tahoes are down the driver side, from under the 2nd row seat to just ahead of the rear axle.
I just took mine out <2wks ago to change my fuel pump. If one had big tires and/or a big lift, it would be pretty straightfoward to put a 5-6" tall rectangular space and drop or duplicate the whole framing support for the standard spare tire mount below the added tank and double the fuel capacity of the vehicle.
I've already seen some dual tank mods on these vehicles where two filler necks / caps are fit behind the factory door. Fill them both, rig the secondary as a transfer tank with a hard-plumbed pump on a dash switch. Run down the stock tank and flip the switch to dump the other into it.
hmm I didnt' take any pics of that big space while I had the spare and fuel tank out...
There's steel intermediate plumbing in the fuel filler and primary vapor vent lines running from tank to filler. Be easy to re-work or tap into that section for transfer plumbing. Any muffler shop could weld a 1/4"-5/16" barbette onto that metal section, after you dismount it. and 12v fuel pumps are cheap -
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38"x38"x4" is 5,776cu",~231cu"/gal, that's 25 gallons. Call it ~6gals per 1" of tank height. 5" equals / double your stock tank.
Cost a lot more than 5-6 20L jugs, but if you can fab much of it yourself you'd save a lot of the cost. Having a custom aluminum tank with some slosh baffles in it would be real nice, with a filler neck and a 5/16" out the bottom. You could literally hit the transfer pump while you are runnign down the road. And THAT would be worth the cost and inconvenience of dealing with 5-6 jugs. Too, that's another ~200# at the rear of the vehicle. Either way.
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one-stop-shop for spun aluminum gas tanks, custom aluminum gas tanks, polished stainless steel gas tanks and gas tank accessories.
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Too, I can envision a tank made to fill that spare tire area rectangle, with a (square?) 'donut hole' which fits around the spare tire winch mechanism, which could literalyl allow you to bolt in the tank to the existing vehicle framing and keep the spare right where it is at. A 3-4" tank at max width and length for the space would be at least 20gal. You'd have to change / modify the fuel filler location a little to fit two necks / caps. And plumb the transfer pump, which would be attached to the new tank. And a wiring kit