20th Jan 2015, 14:48

I have a 78 Mark V. I drive it back and forth from Naples to Miami on I75. Consistently average 13.5 MPG at 70-75mph.

If your mileage is poor, replace the brake calipers. Mine were not fully disengaging. I ruined two sets of rotors.

Replaced the calipers and gas mileage increased by 1-1.5 MPG.

11th Feb 2015, 15:42

I agree. I just came across a 1978 Mark V for sale on a local car lot in VERY good shape -- faded paint, but just typical dings and such -- and mechanically just as good, and for 1600 dollars I am probably going to buy it.

My question is where in the devil can I find parts for it?? Anybody have words of wisdom for me? I'm an excellent backyard mechanic, so I am not concerned on that score. Thanks!!!

12th Feb 2015, 00:03

Mechanical parts are at your nearest auto parts store.

Trim parts and other things specific to that model can be found online.

19th Feb 2015, 07:13

If you're willing to do work, you could swap out the C6 and replace it with a rebuilt THM200-4R to give it the overdrive. There are shops that make rebuilt ones for all vehicles. Next, rip out the rear axle and go with something taller than the 2.75 in your car. It'll likely hurt performance a little, but mileage should greatly improve.

A big Lincoln with even a 460 with a 2.47 from a 1979 Continental would only be turning at 1,319 RPM at 70 mph in OD. If your engine's in good shape, that will really get your mileage up, probably to the high teens/low twenties on the highway.

20th Feb 2015, 10:54

Isn't the THM200-R4 a General Motors transmission? Surprised to hear it recommended in a Ford motor car...

20th Feb 2015, 11:09

Since these are so cheap, just pay the extra gas. I wouldn't get into engine or trans swaps. Find a smaller, nice, daily fuel efficient driver, and keep this washed, waxed and garaged for weekend fun. Or a rare long trip. Keep in mind the fuel hogs are usually why you find cars like this inexpensive today. Often these are estate sale finds as well. I have taken a couple extra people in my cars on weekend trips and split the gas. A good alternative. Don't let them just sit too long though.

21st Feb 2015, 06:33

Well also don't forget that, while inefficient, 3-speeds of all types suck hard for drivability. Driving faster than 55 mph is a battle and your engine will be howling the whole time. Overdrive was a great innovation when it came out, partly because of that.

The THM200-4R is a GM transmission, but many professional shops now make special bolt patterns for use in all vehicles. Stock form, it's crap. But it can be rebuilt to take in huge amounts of input torque and be very durable. A good shop can take care of that.

If the car is just a weekend cruiser, then a swap is probably not worth it. But if it's your daily driver, well it may be worth checking out, especially if it has a 460 with the 2.75 gears.