Folding Bikes - Wheel size circumfrence for 20" Dahon D7?

I just got a cheapo $8.60 Schwinn computer from Amazon. I am amazed by all the cool functionality they have in this little thing! For the wheel size I used the suggested 1596 mm . I notice my distances seem around 2% higher than google maps. So I lowered it to 1560 . Anyone done any scientific study to confirm the correct size ? I assume when Shwinn suggests 1596 they are probably assuming it is a BMX bike with big tires, not road tires.

Do a roll-out measurement: mark the floor, roll out a whole front wheel revolution, mark again, measure distance between marks. Easier of you start with the valve down.

Yeah, I used the 'suggested' setting on my Downtube. Was out by a lot compared to GPS.

Ended up with 1519 when measured by Prof Jur's method. The skinny tires on my other bike needed another measurement.

Thanks, I did the rollout and the number came to 1520, which is almost exact now with the google maps distances! The tires are Kenda, which came withe the Dahon D7. I was so happy at first when I saw myself easily cruising at 25-30 mph! Now these numbers will all drop a few mph!

When I put the pooter on my Downtube, I mistakenly set it to KM/h. Imagine my delight at the speeds that bike made..