ok lets see if we can't figure out what I screwed up today.

1988 f150 2wd 5 speed

i pulled cluster today to lube noisy speedometer cable and figured while i was there i would fix nonworking odometer or at least figure out what was broken. turned out the odometer quit working because the little brass plug the holds the gear in "fell" out. put it back in and spun the speedometer using only the part the cable drives and all was wonderful ... until i put it back in truck and test drove.

before this 55 mph in 5th gear was 1800-1900 rpms
now 55mph in 5th gear is 1400-1500 rpms

any chance i may have moved the needle? it seems to rest in same place as before when not moving.surely just lubing the cable would not make this much difference. although at 55mph before i always got passed by everyone lol

 

So your speedometer is actually registering too fast now. The older speedometers like this had a little deal to set the tension on that spring thing. Maybe you accidently moved that. I got an old speedometer out of my '88, I'll have to look at it if I can find it.

 

You have 2 options, figure out what RPM the engine should be turning based on the rear axle ratio and tire size, or use a GPS to compare to your speedo to see if it really is off.

 

well according to this it was off to begin with and should be more accurate now

Engine RPM Calculator
if you go to the end and use 3.08 gears 29 inch tires 55mph and .80 for fifth gear on m5r2 i get 1570 rpms