80 Channel UHF- 40 Channel UHF

Hi all, does anybody here know how 80 ch corresponds with 40 ch, for instance. Most truckies seem to use ch 40 and gray nomads ch 18 on a 40 channel transceiver. The Simpson desert and CSR seem to have recognized channels for safety purposes, which channels do you use on an 80 channel set to talk with people with 40 channel sets? Probably a stupid question, but it won't be the last time I'll sit in the corner. Cheers,
Kanga...........