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I'm a huge guy, I use my Sun Retro-7 (7 speed) to haul groceries & get some exercise doin' it.

So, to make it a little quicker, after some trial & error (I listed below, in case you're bored) I changed the tires. A Schwalbe Big Apple 26x2.15 (54mm actual) on the rear at 46-48 psi.
A Schwalbe Big Ben 26x2.0 (47mm actual) on the front at 45-48 psi with a RhinoDillo liner. It looks like a 1.75" tire because it almost is.
It's faster. The bike actually "rides" like it lost weight, but there is still a little cushion.
We're talkin' 325lbs plus with me on it. It's not fast, but it's faster & way more fun to ride.
I changed wheels which lets me get away with my fat butt riding at higher air pressure on more slim tires. New wheels are heavier, but it still feels faster.

Trial & error:
1) Tire change to Schwalbe HS361 ReTurner from Sun (Kenda 927). Faster with less great Kenda "cushion".
You can't depend on published size, you have to measure. Old were 57mm, new were 54mm wide, but they're both listed as 2.125"
2) Tried a Schwalbe Big Apple 26 x 2.35 on the rear, it was harder to pedal, even at max psi, poor cushion but the diameter is so big it actually changes your gear ratios, kinda cool.