i shoulda thought of that. now i'd be willing to bet your speedometer is off

I'm not sure if ford used a transmission driven speedo or the electronic sensor in the rear diff like the later models. If its the electronic style then get a superlift truspeed module. It allowes you to set the speedo for different tires and gears. If its the mechanical style, then you will need to pull the driven gear and count teeth and look at the colors. different colors had different ratios for trans and diff options. You might still be able to get the speedo gears from ford or possibly a good tranny shop. Hope this helps.

FWIW - I'd want to get the rpms down a bit - I turn ~2000 rpm at 70 mph getting ~ 20 mpg on my 2500, although slowing down to ~1700 rpm nets me 24 mpg. Hopefully correcting your speedometer will show that you are going much faster than you think you are.

With 35" tires, 3.55 gears, and running at 65mph in 5th gear (Diesel ZF5 has a .75 ratio), you should be turning 1661 rpms. Far cry from 2900 rpms.

You actually need to measure the height of your tires. Very rarely are the tires true to the size printed on the sidewall. Most 35's are only about 33 1/2" to 34" tall. Then do the math. If I remember the formula correctly it should be mph x trans ratio x axle ratio x 336 divided by tire size. In 4th (direct) with 3.55 and 34's at 65 mph, you would be turnin about 2300 rpm. If you want to know what the rpms are for 5th(overdrive), just put that gear ratio in. The numbers wont lie but tachs and speedos often do! Good luck.