You can go to any hardware store and pick up a replacement flapper that will adjust down to the same thing for $4.99
The Mitch Greenflush is $19.95 Plus shipping and handling.
I've tried installing the quicker dropping flappers on old bowls.
Keep your plunger nearby.
As long as you enjoy plunging when someone else uses the bathroom, it can be fun to see "How Low" you can go.
After plunging for someone a few times, it ain't so fun.
Yes, you too can buy a $5 flapper for $20 plus SH and watch your toilet plug.
Measuring the tank does not let you know how much water is being used.
While the tank is refilling, the fill tube is putting water down the overflow tube into the bowl.
With some 3.5 gallon toilets, at least 1.5 of those gallons is just filling the bowl.
Toilets sold since 1992 have used 1.6 gallons.
If your home was built since 1992, you will notice that the flapper drops rather quickly.
Hardly any water leaves the tank.
You can order replacement flappers that can be adjusted to let out 1.28 gallons.
They carry those in the plumbing isle.
If you indeed do have an toilet from the 80's or earlier, then those would use 3.5 gallons or more.
The trapways on those weren't not designed to work real well with less water.
But even so, the 3.5 gallon toilets, using 3.5 gallons, will plug more often then a good 1.6 gallon toilet.
We remove a lot of the 3.5 bowls for better performing 1.6 bowls.