following on from this thread about el cheapo DV recorder:
http://www.singletrackworld.com/forum/topic/cheap-helmet-cam-has-this-been-done#post-923905

What is the average file size for a 720×480 jpeg?
If a digital video recorder is writing to an SD card at the rate of 30fps will a class 4 card (4MBps) be sufficiently fast for a smooth recording, or will a class 6, 6MBps, be necessary (or even class 8 )

An example of a 720×480 jpg resized in Irfanview comes up as 288kb, so at 30 fps that's 8,640 kbs is close to 1 MB isnt it? so it should be fine with a class 2 or 4 card? or have I got the theory all wrong?

It depends entirely on the level of compression used to be honest too? Paintshop saves a 720×480 jpeg, "10" compression level (0 min, 100 max compression) at 12kB. At 30fps that'd be approx 360 kB/s, well under 1 megabyte per second, but requiring 4 megabits per second transfer speed.

At 30fps that'd be approx 360 kB/s, well under 1 megabyte per second, but requiring 4 megabits per second transfer speed

sorry, CK, I dont get this bit ^

Sorry, my figures were converted wrong way but:

My file size (raw jpeg, saved from paintshop – not that I'm sure this is how mpeg is constructed so not entirely accurate sizing) is 12kB. That's
12000 bytes.
12000 bytes = 96000 bits.

30 of them a second:
360 000 bytes = 2 880 000 bits

So you'd need to be sure whether your card speed was quoted in bits or bytes per second – 8 bits in a byte.

Edit:
Card classes are quoted in bytes per sec so:
If this is the correct data save rate (i.e. if your system uses mjpeg rather than mpeg) then you can get away with a class 2 (2 megabytes per second) as you only use 1/3 of a megabyte per second at 30fps.

I think the system is using AVI which is jpeg based.

But I think Im now comfortable that a class 4 card will be fine and wont need to splash on 6 or 8 to get a smooth recording.

cheers CK

Cards are so cheap that you might as well just get the fastest one you can & have the reassurance that it's going to be able to cope.

Mymemory do a Lexar Professional 4GB SD card for £20. I use it in my Nikon D80 which generally saves jpegs around about 4MB. It will shoot 3fps continuous and the card copes with that no problems.
I bought a cheap card from mymemory a while ago as a back-up & the camera virtually stops after about 4 or 5 shots, continuous. It then shoots perhaps 1 every couple of seconds as the buffer clears.