Does anyone know about how to use the data logger?

I don't even now wht in the world you are talking about..... Could you please add a little more detail to your question?

Ok, I'm guessing that if you don't know what or where this is, you probably can't help.

From the main menu under replay theatre you can watch a saved replay, a demo, or use the data logger. I've read in reviews that you can see your replay in slow motion, see what corners you are taking too slow, compare with past performances, and lots of other things.

I just can't figure out how or what to save for it to be available.

I think the data logger has to do with your performance. If you look in the manual you will see some screenshots with all this data measuring car speed and traction etc... I think this can be viewed with the 'data logger' when watching a saved replay.

This is just a guess though....

I think the data logger is to compare two cars (saved replay) or one car for like aacelration, top speed, andsomething like that.

Yeah, that's what I was getting at. It's some sort of analysis, I know that much....

I couldn't get it to work though. I have a couple of replays saved, but when you select load, it just shows empty slots.

i believe you need 2 saves one on a memory card each , and you load them both and it will probably analize it somehow ill check in the morning , im usually good at the analysing stuff lol

Figured it out!

You can only save data for the data logger when you run in a non race mode, ie free run and time trials in arcade mode or machine test or run settings in GT mode. Doesn't matter how many laps you do when you go to save, it only saves your best lap.
The save option has a picture of a car instead of a movie camera.

hat's od, I think I can save it anywhere

When you go to the data logger get two cars which have raced on a certian circuit. Put them on and comparetheir times and when they are ahead. I had 2 MX-5's and the one with more power had a slower time and had the lead up to the first corner. the less powerful had the lead for the rest of the race