Duration

[craigwp]

(hope this post follows forum rules and such)
Heyy,
my problem is that, when i have ripped certain DVD's using dvdfab platinum, grouped all the .vobs together using vobmerge, and then input that one .vob into mediacoder the duration says it is only approximately 1h 15mins when the certain films i have had this happen to are about 2h 30 mins, it seems to completely half the movie (approximately) so im wondering if anyone knows i way to fix this by posibly doing something else.
craig