Over 50,000 NRG Systems #40 anemometers manufactured between May 2006 and December 2008 are potentially affected by a self-excited vibratory phenomenon termed dry friction whip (DFW). Affected anemometers can report wind speeds that are lower than true speeds by up to several percent. An analysis of post-deployment wind tunnel calibration tests on 99 anemometers manufactured in this period found that about 85% were affected by the problem. The mean wind speed bias between 4 and 16 m s −1 for most of the anemometers compared with pre-deployment tests ranged from −1.5% to −3.0%, and for some, the bias was as large as −6.5%. This finding was confirmed by field tests carried out for a mostly different sample of 53 potentially affected anemometers deployed in pairs with WindSensor P2546A or Vector A100LK anemometers. The field tests further indicated that the problem tends to worsen over time and varies with wind speed. However, the pattern of response varies greatly among anemometers. Based on this research, a method of adjusting data from NRG #40 anemometers manufactured in this period is proposed. The adjustments eliminate the mean bias in the field test data set, although significant scatter remains. Copyright © 2011 John Wiley & Sons, Ltd.