In my post about Derek Jeter's defensive improvements this year, here is what I said about Teixeira's positive impact on him:
Teixeria's glove probably helps, but Jeter has never made many throwing errors. He is on pace for 5 this year and has averaged 6.5 per season since 2001.It was a pretty blunt measurement, and it turns out I might have actually oversold Teix's influence on fielding throws to first.
John Dewan, author of The Fielding Bible II, contends that there isn't much evidence to suggest that Teixeira is better at rounding up errant throws than the Big G: (h/t BBTF)
In fact, in 2008, Giambi's 29 scoops for the Yankees were good for 0.26 scoops per game started, while Teixeira's 2009 scoops for the Yankees are only 0.23 per game.
Dewan's Plus/Minus System keeps track of positive and negative fielding plays made by each player in every game and recently added "scoops" by a first baseman which would have prevented an error by an infielder as a positive play. These stats are tallied by actual people watching the games, so there is a human element involved, but on the whole would figure to be pretty accurate.
The only thing I can think of which would skew the results is that Teixeira can cover more territory with his foot on the bag than Giambi, and this extended range allows his to field balls Giambi would have had to reach for without scooping them. But I don't think that could occur often enough to make much of an impact.
So where does Teixeira surpass Giambi?
The true difference between Gold Glover Mark Teixeira and Jason Giambi is in handling grounders. In the last two years Teixeira has saved his teams 18 runs fielding grounders, while Giambi has cost his team 18, a 36-run difference in Defensive Runs Saved.
Keep in mind those numbers are over the course of two years, but that is a massive gap in terms of fielding ability. To put it another way, Teixeira's career UZR/150 (Ultimate Zone Rating averaged over 150 games) is 2.2 while Giambi's is -7.2. Giambi might think he's a great defender, but that's obviously not what the numbers say.
Defensive performance is never going to be as easy to put a numerical value on as offensive production, but this should at least help us appreciate what Teix brings to the table on both sides of the ball.