Did you check the EPA site? They have that info but I do not know how far back they go.
 
Later!
Mr. Ed

Quote from MrEd on 01/31/12 at 13:54:52:

 
 
Yeah, it only goes back as far as 1984.  However, because of this I can pretty much say that it is above 15 MPG city and 20 MPG Highway, due to the eagle wagon with the same drive train being getting those numbers.
 
Quote from amxdreamer on 01/31/12 at 20:47:04:

 
Dang, and here I was hoping for either 4, or 31+  
 
But yeah, jokes aside the Hornet is about 400lbs lighter, and has less drive-train resistance, so I'd imagine it'd have to get near 18ish in the city, and a little bit more than 20 during highway driving.
 
That's what I'm hoping, at least.

A 1973 Popular Science test reported 15.4 mpg @ 45 mph in a 73 Hornet 4 door with 304/2V. To put that in perspective, a 318 Valiant was next closest at 14.8, 302 Maverick 14.1, and a 350 Nova only got 11.7 mpg. All four doors with autos.
 
Motor Trend tested a Hornet four door with 258 w/auto and averaged 21.4 mpg. They had a Hornet Sportabout w/360 auto in the same test and averaged 18.7 mpg with it. Thye tested a Valiant four door w/225 auto and a Duster 360 in the same article. The Valiant only got 18.1 mpg, the Duster 14.3. They were far more impressed with the 258 Hornet than the 225 Valiant, but the Valiant did weigh 245 pounds more and had 33 fewer inches of engine. The 225 was only advertised with 5 less hp (105@3600, Hornet 110@3600) and 15 ft/lbs less torque (180@1600, Hornet 195@2000). From 0-60 the Hornet was 1.9 seconds faster than the Valiant. Standing quarter mile was 19.442@69.17 mph for the Hornet, 17.02@81.74 for the 360 Sportabout. Valiant quarter was 20.216@67.46 mph, 360 Duster 15.84@87.80.  
 
We always hear about how great AMCs and Ramblers were on gas mileage. Well, they were better than most, but not a lot better and no where close to what we'd consider "great mileage" today. Even VW Beetles didn't get more than 20-25 mpg (depending on terrain and how you drove). Low to mid 20s was considered "great" when most Detroit iron was averaging in the mid to high teens!