Photoshop :: CS6 Extended JPG Size

I am using Photoshop CS6 Extended to make a set of small images for insertion in a MS Word document. I want the images to be 36 mm wide in the printed document. I use File > Image Size to make a file 340 px wide at a resolution of 240 px/inch, save as a tif, and it works correctly in Word. But if I save as jpg, with the same size settings, the file displays in the Word document at 90 mm wide.
 
If I use Photoshop CS4 to make the jpg, this problem does not occur. The problem happens with Photoshop 13.0.1 x64 on Windows 7 (but not with Photoshop 11.0.2 on Windows XP).