I had a 304 in my '75 and it never got less than 20mpg. It was mostly stock only had a mild cam and headers with a 2100 carb running 32" Firestone ATX. Of course it was when gasoline was gasoline. My '76 with the 258 gets way worse fuel mileage, around 15mpg.
My stock 1976 CJ-7 with a 304 only got around 10-12 MPG before the TBI conversion, that really helped, now around 18 mpg. But I am running a TH400(very heavy internally) and a full time QT transfer case. If it was not for the drive train most likely would get over 20 mpg. My 304 is getting tired, compression down to around 110-130 lbs on all 8 cylinders, but still running very strong with the junkyard TBI conversion. I have a 360 I have been working on for a long time, hopefully when I get that complete we will see how the junkyard TBI conversion works on a mechanically sound engine. I am looking forward to that day. It will be optimized with a special ground camshaft for TBI operation. That should help significantly.
My axle ratios are stock 3.54, and running 31 inch Dueler HT tires.
didnt think about tire size. so whats the ratio for 33's?
Well, that depends - larger tires can have a lot of variation between brands, style, and wear.
The best way is to use a GPS. Most have a setable trip meter. On your next road trip, write down your odometer reading, and zero the trip meter. At the end of your trip divide GPS miles by odometer miles. This will give you the adjustment factor. Be sure the GPS doesn't loose lock - no woods trip for this...
Or, you can use MPH - drive at 60 MPH on the speedo, and see what the GPS says - same formula - GPS MPH divided by speedo MPH = adjustment factor.
For to figure out true miles driven, take your odometer miles and multiply by the adjustment factor.
For my Jeep with 31's the adjustment factor is 1.08.