I have a 2001 Supercrew 4x4 off road, 5.4, 3.55 regualr axle, and my truck only runs about 1800 rpm at 70mph?? What gives? I`ve got a 2000 Lariat SUPERCAB that runs 2050 rpm at 70 and its a 4.6, 3.55 regualr axle.

So why does the 5.4 in my Screw run a lot slower?

 

tire size? is the supercab a 2wd? if so, then theres your difference.

 

The lariat supercab is a 2wd. It has 275/60/17 (stock BFG) on it
The Supercre is a 4wd. It has 265/70/17 (stock GY) on it.

Why does the engine in a 4x4 run slower in 2wd than a 2wd's engine does?

I guess i'm very uneducated.

 

your 2wd tires are shorter (less diameter) which means that is you were to put a white mark on a tire off each vehicle, and roll it one revolution, the taller tire (4wd) would cover more ground. All other variables the same, a taller tire requires less revolutions per minute (RPM) to go the same speed as a shorter tire. Your 2 vehicles tire are not turning the same rpms in order to acheive the same road speed.

 

Thanks for the help Jefflaws,

I guess basically when my 2wd says i`m going 70, I AM, but when my 4x4 says i`m going 70.... am I really going like 73 or 4?

so does ford not have a different computer for the 275/60/17 and the 265/70/17? Thats absurd if they dont, there is a HUGE difference in the tire size.

wes

 

no no no , the speedo is calibrated to be correct. Thats why the tach vs. speedo relationship is different between the 2. Your engine in the 2wd really does have to turn more RPMs even with the same rear end gear to go the same speed as the 4wd. Because the tires on the 2wd are smaller, they have to spin faster to cover the same ground, therefore more RPM at a given speed than a taller tire.

 

The computer does know the tire size, and if it wasnt compensated, then you would see the tach and speedo readings be the same between the 2 trucks, but since it knows that the tires on the 2wd truck arent going to get it as far down the road per revolution, it makes the speedo register slower.

 

Im determined to make you understand this. Try finding a small tire, wheel, ball , etc. and a larger of the same laying around the house. Now with one on each side of you, roll them across the ground keeping them up with each other. Guess what, the smaller one has to turn on its center axis faster than the larger one to keep up. Now, them rolling the same speed represents your 2 trucks both going 70 mph. Guess what, the rpms of the smaller one (representing the 2wd with smaller tires) is higher! Just like the 2050 rmps vs the 1800 of the truck going the same speed with bigger tires.

If you still dont really understand, ill try again

 

I think I got it now! Thanks a million Jefflaws!

No then, I put the set of 265/70/17 tires and wheels on my lariat and it still says 2050 at 70.... guess that is because I REALLY WAS going faster because of the larger tire size and the computer had not compensated for it.

WES
SA TX
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very correct, the speedo didnt know you changed tire size. Remember its not actually measuring speed, its comparing driveshaft rpm to a table that is based on rear end ratio AND tire size of what its programmed for. (if you use a GPS to measure MPH then the 2 trucks will tach the same at 70 with the same tires, and the one that has smaller tires will tach higher, the one with larger tires will tach lower.

 

Thanks for all the help Jefflaws, you are the greatest.


So it doesnt actually measure speed??? I did not know that, you learn something new everyday...

its wierd how they manage to tie the driveshaft and everything else into the PATS system...how does that work Jeff?

wes

 

PATS? , no (passive antitheft system)

PCM, yes (powertrain control module)

The transmission has a OSS (output shaft speed) sensor in the rear of the tranny right there where the driveshaft comes out. That signal is used by the PCM to tell it how many rpms* the shaft is turning. The PCM uses a number (determined by tire size) to put the speedo needle where it needs to be.


*Actually all the PCM sees is a AC voltage signal generated by the variable reluctance sensor. The signal voltage and frequency increases with speed. The PCM uses a type of rectifier circuit that functions as a AD converter (analog to digital) and gets a digital square wave signal that varies in frequency and compared to a oscillator in order to get a variable that is multiplied by the constant stored in the emissions calibration (for gear ratio) and by the tire size variable (stored in the non-volitile flash memory) to get a digital signal that is transmitted via the SCP network to the HEC (cluster) and then converted back to a voltage that reacts to the magnetic windings that moves the speedo needle. Whew!

 

Wes ,
Jefflaws gave good answers . I went on trip and used my GPS to check the speedometer calibration . The drive was straight and flat for miles with no traffic to deal with . The GPS has a speed accuracy 0.1 mph . I have the 265-70-17 tires . At 70 mph indicated was dead on according to the GPS . At 60 mph indicated the GPS showed 58.7 mph . At 80 mph indicated the GPS showed 81.2 mph . At 90 mph I could not get a good reading because the wife was smackin' me and yellin' something unintelligable . Women , I just don't understand them sometimes . The bottom line is that indicated speed is faster than actual under 70 and slower than actual over 70 .

 

Thats typical. All speedometers are off a certain bit, giving them a "sweet spot" that they are dead on correct. In fact, speedometer accuracy even depends on the amount of tire wear you have.