Weird RPM to Speed changes.

[1979C20]

After I firat put my new tires and wheels on my truck, according to my GPS unit my truck was running 1450RPMs at 45mph for a few days. And the past couple days its been running 1900RPMs at 45. What would cause a change all the sudden? Its the truck in my sig.

[scorpion]

I would say your GPS was wrong the first couple of days.  GPS units are computers and it's possible that the data it was gathering and prcessing wasn't accurate.  It seems unlikely that no change to your vehicle could result in a lost of MPH at RPM.  If it were an Auto I suppose the converter could be slipping but since you have a manual, I'd can't see a way.

The only way RPM could increase when your drivetrain configuration stays constant would be if there was some new drag on the drivetrain that caused the motor to have to work harder to maintain the same speed.  A gain of 450 RPM is significant if you ask me.  Even being in 4 wheel drive by mistake wouldn't likely result in such an increase.

[1979C20]

Well, today my RPM to MPH changed again. It went to 1700RPM at 45MPH. BUT, last night I cleared some mud off of my headers, control arms, trans xmember, and it rained yesterday and cleaned off my wheels, tires, and stuff. So I guess it was the mud. Maybe I had mud or sand in my drums or caked on my rotors. Ill take a power washer to everything and see if it helps. And I need to check my rear dif fluid level, its starting to moan while im slowing down.

[1979C20]

Also, my stock speedometer is off by 10MPH, 50 on the speedo is 40MPH. And that matched the GPS unit, 55MPH when the GPS said 45MPH. So, my RPMs did infact change.

[scorpion]

Would have been good to know about the mud.  Mud can add a significant amount of weight to a truck which can make yor motor work harder...

Just wash your truck and drive it dude.   

[beastie_3]

i dont understand how RPMs would change without the MPH changing in a vehicle with a manual transmission. Wouldnt that mean the clutch is slipping?

[78 Chevyrado]

i dont understand how RPMs would change without the MPH changing in a vehicle with a manual transmission. Wouldnt that mean the clutch is slipping?

x2

sounds like the GPS unit isn't accurate enough for this kind of testing.  whats the margin of error for the GPS unit?

[thirsty]

And I need to check my rear dif fluid level, its starting to moan while im slowing down.

Might want to look into this soon.

[muddpuppy01]

yeah mud or not you could pull a trailer loaded with 20k lbs with a manual rpm to mph is going to stay the same as long as the clutch is not slipping and if it was slipping by 500 rpm you would smell it and be on the side of the road before to long

only thing that would change would be the thottle position with the change in load

if you are running a 33 tire 45mph = 1879 rpm / 35 tire 45 mph = 1771 rpm

which one are you running

[1979C20]

Im running 31's, and its at 1450RPM.... And thats accurate witb my GPS speedo on my phone, my GPS on my dash, and my stock speedo. And its also accurate with speed signs around town.

[1979C20]

Alrighty, for the past few days, my truck has been running 1600RPMS AT 45MPH and 1450RPMS at 40Mph consistantly, and accurate with my GPS unit, phone GPS, and speed sensor/signs around town.

[Irish_Alley]

with running 4.10s and 31s you should be close to 1800 rpms so something is off. with what info your giving your running a 3.27 or around there

[1979C20]

No, its 4.11s. Unless my tach reads wrong.

[Irish_Alley]

with 4.10s and 31s you should be at 2k at 45mph

[beastie_3]

i thought ford had 4.11?