Good to know that you do know.
I still think that your pedal cadence assumptions are Too Optimistic.
I know it does sound like splitting hairs here but it is actually really relevant.
Here is why:
PRACTICALLY SPEAKING, A STRONG YOUNG ATHLETE WOULD HAVE A DIFFICULT TIME IN ACHIEVING 20 MPH ON YOUR 5 speed limited-edition 24" wheeled Breeze.
As one can plainly see, that the potential top speed that one could hope to achieve on this 24" wheeled Breeze is going to be significantly less than that of say the 5 speed SUBURBANS or the 5 speed COLLEGIATES, and way way way below what one could realistically achieve on a VARSITY/CONTINENTAL/10spSUBURBAN.
Now without getting into an in depth discussion on the merits of "spinning" versus "mashing" and vice-versa and how they contribute to achieving fairly the same goal in rider's speed only through different cadences (personal style of pedalling..).
You go out and try and find someone that can pedal a 24" wheeled BREEZE 5 speed at 20 mph on level ground. You won't!
If they can, they are likely someone who is a tri-athlon competitor.
I am saying that realistically you are looking at perhaps the 15.5 mph to 17.3 mph range TOPS depending on the fitness and athletic strength of the rider. One will certainly not be able to maintain an average speed of 12 mph to 13 mph with such a bicycle in its factory original form. Sure, with a big enough hill to go down, and yes, you will exceed 30 mph unless you judiciously apply some braking.
I'm simply saying that the online calculators are great for what they can easily do, but they are not perfect and are flawed, just as the age-old caveman approach of the simple math of calculating GEAR number (gear inches) is not perfect and flawed.
Assumptions are made. Garbage in and garbage out is the norm IF THOSE ASSUMPTIONS ARE OUTTA-WACK & UNREALISTIC.
I know that you do understand this fully but for those that perhaps now or in the future who stumble across this posting as a resource for seeking information on their project bicycle, may better understand that calculations and math are really relevant towards comparing potential changes. Certainly, someone with say a single speed, 55 pound beach cruiser, finds it perhaps easy enough to decide whether to change the rear gear cog --or-- the front crank gearwheel, based on relative experience with other bikes, or perhaps their buddies' bicycles. You do need to consider that a near 40 pound, 24" steel wheel, Breeze 5 speed is not going to be directly compareable to say even a heavy 28 pound to 30 pound derailleur equipped tourist type or road-bike from any of the past fifty years. I am saying that failure to take that into consideration really skews your online calculations because realistically that someone cannot maintain the pedal cadence that you assume for the rider of that old Schwinn. Theoretically, if one could, while pedalling such an old heavy Schwinn like yours, then the degree of accuracy from the online calculator would be great but because it is far too unrealistic, your calculator gives you garbage that is too optimistic and unachievable, and if you rely on said info, you'd be using perhaps numbers that are not the best. It would be far better to use a more conservative and realistic pedal cadence that is actually achievable on such an old heavy Schwinn bicycle, to provide more useful and accurate numbers. Don't care about what pedal cadence that you may be able to achieve on your 22 pound bicycle with very lightweight wheels. You will not be able to maintain such a high "spinning rate" of the pedals on such an old heavy Schwinn. That is not necessarily a bad thing or a good thing. It just showcases the real-world differences that one cannot simply overlook. They are very good bicycles, but distinctly different animals in how they perform. You do need to be realistic when comparing the different animals, because like the old saying of comparing apples to apples and oranges to oranges certainly applies with vastly different bicycles.
Hey, again on the GEAR number (gear inches) computations, previously mentioned:
For 700C (622mm) wheels, ONE SHOULD USE (27) FOR THEIR COMPUTATION AND TREAT THEM AS IF THEY WERE TWENTY-SEVEN INCH WHEELS. Why? Because (622mm) is close enough to 27" (630mm).
You can see that there are other assumptions that must be made to keep things Simple and easy:
Everything from (597mm) (590mm), 650, 650a, 650b, 650c sizes etc, and ( 559mm) ancient cruiser--mtn bike wheels ARE ALL CONSIDERED TWENTY-SIX (26) to keep things Simple and Easy, and it generally is entirely compareable when directly comparing the same, and not too outta-wack when comparing something that falls into that (26) category.
Heck, yes, now production tire circumference differences and shape of road contact tread patch of the tire can have a significant effect on a bicycle's potential top-speed too, but for practical and realistic purposes this really only becomes a significant factor on bicycles which are already very capable machines and less likely to impact such an old ordinary sedate bicycle.
The online gear calculators are great but do remember that they rely on realistic accurate input.
Calculate cycling speeds at a specified cadence given chainring, sprocket, tire and wheel sizes.
www.bikecalc.com
It is just one of many and all are fairly decent when fed reliable and applicable information.