Looking on the internet deeply has found these results:

I am on a lorry. is the most popular phrase on the web.

I am in a lorry. 12 results on the web	I am on a lorry. 18 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
There you are, parked on a lorry. He walked out in front of a lorry. I've just seen three or four blokes on the top of the cab of a lorry. Our family tree looks like it was hit by a lorry. This is an annoying time to be stuck behind a lorry. Something's bound to have fallen off the back of a lorry. She walked straight into a lorry. I wish to support all the amendments on the ramp and the pigs, because I have watched the horror of trying to get pigs into a lorry.	I've just seen three or four blokes on the top of the cab of a lorry. He walked out in front of a lorry. This is an annoying time to be stuck behind a lorry. There you are, parked on a lorry. Something's bound to have fallen off the back of a lorry. I wish to support all the amendments on the ramp and the pigs, because I have watched the horror of trying to get pigs into a lorry. She walked straight into a lorry. Our family tree looks like it was hit by a lorry.

I am in a lorry.

12 results on the web

I am on a lorry.

18 results on the web

More popular!

There you are, parked on a lorry.
He walked out in front of a lorry.
I've just seen three or four blokes on the top of the cab of a lorry.
Our family tree looks like it was hit by a lorry.
This is an annoying time to be stuck behind a lorry.
Something's bound to have fallen off the back of a lorry.
She walked straight into a lorry.
I wish to support all the amendments on the ramp and the pigs, because I have watched the horror of trying to get pigs into a lorry.

I've just seen three or four blokes on the top of the cab of a lorry.
He walked out in front of a lorry.
This is an annoying time to be stuck behind a lorry.
There you are, parked on a lorry.
Something's bound to have fallen off the back of a lorry.
I wish to support all the amendments on the ramp and the pigs, because I have watched the horror of trying to get pigs into a lorry.
She walked straight into a lorry.
Our family tree looks like it was hit by a lorry.