Yes...I find it a bit hard to believe...Gingrich as Speaker abstained on most votes (as do other Speakers)....although I’d be shocked if his ACU rating were as low as 42, or Michele Bachmann’s as low as 80.
(Yes, there are many other indexes besides ADA/ACU).
If you follow the link, what you find are simple rankings, from most liberal to most conservative -- based on their votes -- for every Congress from the first to the 111th.
If you look, for example, at the 111th Congress, you'll see they rank Ron Paul as the second most conservative Congressman.
I simply converted that to percent and said he votes more conservative than 99% of other Republican Congressmen.
And as of the 111th Congress, all Republicans voted more conservatively than any Democrat.
If you look on the Senate side, you'll see that, for example, John McCain ranks about half way down the list of Republicans -- in other words, half of Republicans voted more conservatively, half less.
For Santorum and Gingrich I went back to previous Congresses, and looked where they stood relative to their peers.
In his last Congress, Gingrich voted more conservatively than 42% of other Republicans. In other words, more than half of Gingrich's Republican peers voted more conservatively than Gingrich.
Santorum in his last congress voted more conservatively than 33% of other Republicans. In other words, two thirds of Santorum's Republican peers voted more conservatively than Santorum.
My point is, except for "severe" conservatives like Paul and Bachmann, these guys all come from the middle of the Republican pack.