Which sentence is correct?
"I had more customers than usual." "I had more customers than usually."
"More than usual" sounds pretty common, but "more than usually" seems more correct when I think about it. There seem to be more adverb adjective conflicts like this, for example "drive safe" vs. "drive safely", which was explained here: Which is correct: "drive safe" or "drive safely"?. Is the rule the same in case of "more than usual"?
When shortened forms become idiomatic, it may be better not to try to identify parts of speech within them.
I had more cups of tea than Harry [had] [cups of tea]. No problem with classifying Harry as a noun (phrase) (though 'they did' reduces to 'them' nowadays).
...
I had more cups of tea than [I] [had] [cups of] coffee. Again, no problems.
...
I had more money than [proved to be / was] necessary. No problem; adjective.
...
I had more cups of tea [this morning] than I had yesterday. Temporal adverb.
BUT
I['ve] had more cups of tea than yesterday. Is this an ex temporal adverb? A temporal adverb modifying a missing verb?*
This becomes far more ambiguous when one tries to recover the undeleted form of say
It's warmer than yesterday. ... It's warmer than it was yesterday or It's [a] warmer [day] than yesterday was?
With 'more ... than usual', it can be very hard to determine the undeleted original (which may not have been used in particular cases anyway). This means that attempts at logical analysis are going to be unconvincing.
As CalifJim says on EnglishForums:
'[more X than] usual':
This is a fixed phrase. There is no [apparent] logic involved.
..........................
With simpler forms such as think big, drive safe, work smarter, take it nice and easy, adjectives are being used (perhaps unconventionally – often for emphasis) as [if they were] adverbs. The acceptability of this practice, and other considerations, have been addressed here before. Look up flat adverbs, eg at 'Using short adjectives as adverbs, such as “easy” & “short” '.
There is no doubt in my mind that most British people will say "I had more customers than usual", and not use the adverb. The theory of it I shall leave to my betters.
The question is whether you mean to say that the number of customers is more than the usual number (adjective) or that you had more customers than you usually have (adverb).
So either is correct. Practically speaking, they amount to the same thing. We (in AmE, and per David Pugh, in BrE as well) tend to use the shorter form "usual", and not dwell on this subtle distinction.
