I have a pretty much bone stock 1968 mustang with a 289 and c-4 auto trans that I am about to drive across country. It does not have ac but it does have power steering and power brakes. It is my first long distance trip in this car. What kind of fuel mileage can i expect? Oh yeah it has 2.72 gears in the rear end.
Done a few short open road trips. As long as things flow smoothly and very little up the hill - down the hill action, then I can manage 24 to 26. Remember! Drive like you have an egg between the bottom of your foot and the throttle pedal (Plus gas prices will help keep that in check). In addition, if you have the time to do this trip, then keep it out of the "red zone" on the speedometer (70 or below).
My setup is a 68 coupe with 289 +30 with a stock intake, lean running primary side of a 4100 carb, stock config automatic (C-4), 1800 stall torque converter, lowered with 24" (hgt.) tires on 14s and 3.00:1 final ratio.
My '67 with p/s, p/b and a ,040" over 302 with a few mods, C4 trans, and 3:1 gearing manages around 18mpg on the highway at a steady 60mpg. I never go faster so I can't comment on higher speeds, but without overdrive but keeping your speed down will be essencial. With a light foot though I still even average over 17mpg on my commute to work/school so I think you'll be in good shape. Your hardtop should be lighter than my droptop too.
My '66 coupe is a 289 C-code, C-4 with A/C and a 2.80:1 rear end; not too different than yours. About the only non-original item on it is a Sears cruise control. In 1982 the 3 of us drove from NM to the World's Fair in Knoxville with the car filled with our normal travel 'stuff'. Being summertime we had the A/C on every minute of the 3821 miles which included a side trip to Indiana on the way home. Most of the interstate miles were also with the cruise control. Trip average 21.08 MPG - I was surprised. Normal MPG around town: 14.
I am the car's second owner but I have the original owner's records of maintenance and gas from day 1. He didn't usually calculate gas mileage but all the numbers were there so I (and Excel) did it for him 20 years later. The original owner usually got 12-13 around town, but like me, on a long trip through Texas in Nov 1978 he was consistently getting 20 MPG with a couple of tanks at 22-23.
Your mileage today may depend upon how much ethanol is in the gas you buy.
Just took a trip last week of 633 miles in my '67, averaged 18,32mpg with a bit of urban driving in an unfamiliar city as well as construction traffic pulling it down. Without those two I would have made a 19,1mpg average. I had a high tank of 20,82mpg over 127 miles. I drive a steady 60mph with tires up at 38psi cold, engine running on the cusp of danger at 230*.
Mine is running a ,040" over 302 (308ci total) with a 4bbl Carter (Edelbrock) on top of a Performer 289 intake, stock heads and cam, Pertronix ignition, Hedman Hedders, 2" duals with h-pipe and glasspacks, and 3.00:1 gearing. It's also heavier than yours since convertibles add some flab on their own and I further have subframe connectors. Other than that, it's the same as yours, with p/s and p/b, and a C4 trans.
My guesstimate would be low 20's with calm driving, 21-23mpg or so.
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