Train horns have been done:
http://www.smartcarofamerica.com/for...n-horns-41682/ - scroll down a bit for the video of his actual car and all the horns he has installed on it.
For mileage, I am tracking my mileage on Fuelly. I average just under 40 MPG - with a high of 44 and a low of 35 when I take it in for service. I think the service guys drive faster than I do
Click on the banner for current and past fuel consumption information. This is a great site - and it is free!