Toto Drake only flushing 1 gallon of water instead of 1.6

I just installed the highly recommended Toto Drake and I am pretty disappointed. I kept flushing it thinking "This is not flushing like it should" So I decided to test it. I turned off the water supply, and flushed the toilet. I then got a 1 gallon milk jug, filled it with water and then started filling the tank. I put the whole gallon in and the water level was about 1/8th an inch higher than the normal water height in the tank. So my Toto Drake is shorting me on over a half gallon of water per flush, no wonder it is not impressing! Please advise, I bought this toilet because my previous one was bad, but the Toto is no better right now. I am pressing the lever completely down, this shouldn't be rocket science right? Thanks for any help you can give.

Yes, it's not rocket science.
The black tube on the fill valve also refills the bowl. If you turn off the water supply, then the bowl won't refill, and with low water in the bowl, it won't be a good flush.

The CST744S is a 900 gram bowl. I think the next step, would be to "use" the toilet for a week, and then report back. I don't see how flushing water is going to tell you anything.

The other half a gallon plus is in the bowl. Filling the tank manually doesn't replenish the bowl, as Terry said.

If you previously had an old 5 gallon per flush toilet or something, then it is going to take a little adjustment from what you're used to. There is far less water flushing far more efficiently in today's toilets, and Toto does it better than anyone out there. If you have real problems, not "it looks funny" problems with the flush of this toilet, then there is probably something else wrong in your plumbing.

When first using a low flow toilet, it does require a change in one's mind as what a normal flush looks like. I'd bet the farm the toilet is working 100% as designed. As others have noted, the added 1/2 gallon of water is in the bowl and that is filled with the little tube that should be clipped to the fill valve tube. Don't feel dumb or stupid, you be surprised how many think the toilet isn't working right because it doesn't empty the tank when flushed. The problems many toilet manufacturers had in the early days of low flow was caused by their attempt to make their old designs work with less water. They failed of course, but this gave the public the idea that low flow toilets needed constant plunging. Sadly, some of these manufacturers are still trying to tweak their old water hogs.

To be fair(ish), modifying an old toilet to hold a lot more than 1.6 gallons and only flush 1.6 gallons of that, does increase the pressure with which if flushes, thereby increasing the performance of the bowl somewhat, relative to the amount of water being used. But it is basically just an adaptation of the old concept - high tank toilets.

But yes, the real solution that good manufacturers like Toto have done is to totally redesign the toilet trap, bowl shape, etc to efficiently flush with less water. Those who aren't doing that will never get satisfactory performance with 1/3 of the water.

Side note... I kinda like the old high tanks... maybe we should bring them back - the whole style of them, the pull chain... its all kinda fun Wonder if that would help or hurt today's toilets.

To determine the actual flush volume, use the water meter, not a manual fill (because the latter won't be accurate for the reasons already mentioned.) My water meter can measure fractional cubic feet to reasonable precision which is easy to convert to gallons (reading the needle indicator.) If one flush doesn't give enough precision, do three or more flushes. I can't recall how fine the readout was on my previous water meters, but this repeated flush method can give you an order of magnitude more accuracy fairly easily if you need it. It is possible that something isn't working right, but what you describe doesn't really indicate that yet.

Are you certain it is a 1.6 gallon tank/bowl and not one of the 1.28 gpf varieties? I have three of the lower volume EcoDrakes and they flush just fine. (And I've measured flush volumes on all three a few years back, they varied somewhat from the stated capacity but nothing like what you claim.) None are ADA height variety either and they are on three different levels. So if you are having trouble my first guess would be something wrong with the venting or other aspect of the house plumbing. There are a number of plumbers here who could quickly diagnose those sorts of problems if you gave them a proper description.