Google’s mobile revenue? Depends how you do the math.

            Google wowed Wall Street with the revelation that its mobile business is generating revenue at a run rate of over $2.5 billion.

Not bad for a business that’s still in its infancy, and which was operating at a $1 billion run rate at this time last year.

Of course, a run rate is not the same as revenue that’s been booked – it’s simply a way of extrapolating what a full year’s worth of revenue will be, assuming the current rate of revenue holds steady.

So what is Google’s actual mobile revenue right now?

Many Wall Street analysts estimated on Friday that Google generated $625 million in mobile revenue in the recently-ended quarter -– a not unreasonable assumption, given that four quarters’ worth of $625 million totals $2.5 billion. (And since Google said the run rate was more than $2.5 billion, perhaps $626 million for the quarter would be an even more reasonable estimate).

Not so fast, says BGC Partners analyst Colin Gillis.

There’s no guarantee that Google based its run rate on a full quarter’s worth of revenue.

They could have taken mobile revenue from the last month and multiplied it by 12, said Gillis. They could even have used their best single day of mobile revenue and multiplied by 365, he noted.

As a result, Gillis estimates that Google’s mobile revenue in Q3 was probably closer to $500 million or $550 million.

“We have no idea what that number really is,” he said.