Plug in diesel tuning box

Smartalex
Smartalex Forum Participant Posts: 11
edited March 2013 in Towcars & Towing #1

Has anyone used one of these plug in diesel tuning boxes which is supposed to increase MPG by 15 - 20%.

According to the hype it will also increase power and torque.

Other questions: is it legal, does it increase your insurance?

If anyone has used these and can give any valid answers to the above queries or has any advice I would be very interested to hear.