As I stated before, even though the signal going to the EEC-V on the stock F trucks is a "vehicle speed" signal (VSS), the computer is really interested in transmission "output shaft speed" (OSS) for shift scheduling. As such, on these applications the shift tables are really based on OSS, backwards calulated from VSS, the ring gear ratio & tire size. It's kinda complicated the way they do, because they were trying to save the cost of TWO (or more) different sensors. Many other manufacturers get the VSS from the wheel speed sensors to drive speedo and ABS and have a separate OSS in the tranny for just shift scheduling. Some newer Fords use this approach now. My Tundra had an OSS ("Speed Sensor #1") on the tranny for shift scheduling, a VSS ("Speed Sensor #2") on the t-case for the speedo & transfer case shift-on-the-fly computer, and TWO rear axle sensors (directly behind the wheel bearings in the rear axle tubes on both sides) for the ABS. Definately a better way to get those separate signals, but at least 4 times as costly as Ford's approach.

Think of it this way: if you have a hypothetical truck with 3.73's that shifts at 10 mph from 1st to 2nd part throttle, which is maybe 2,750 rpm. If you change the rear end ratio to 4.11, wouldn't you want it to STILL shift at 2,750 rpm under the same conditions? The ratio of engine RPM to output shaft speed didn't change, since that's a function of torque converter slippage and transmission gear ratios. What changed was the ratio of output shaft speed to vehicle speed. If you feed the computer an output shaft speed signal, then the computer will continue to shift at the same output shaft speed, which in this example is 2,750 rpm, even though the Vehicle Speed with the lower gears is more like 12-13 mph. However, the actual vehicle speed is lower now. If you used a VSS signal to the computer and recal'ed the PSOM to correct the speedo but DIDN'T reflash the computer to reflect the change, the shifts would occur at 15mph which might be 3,000rpm with the new lower gears. What you really want is the shifts to be based on engine RPM and load, but for some silly reason only WOT shifts are based on engine RPM.

It all has to do with the indirect way the OSS & VSS is measured in stock F -trucks. Since they use a ring gear (which equals wheel speed) sensor, then changing gear ratios would NOT require a speedo recal, but WOULD require a computer flash because the final drive ratio is different (ratio of wheel speed to driveshaft/trans output shaft speed). On the other hand if you change tires but not gears AND you DIDN'T recal the PSOM then the computer shift schedule will be right (you didn't change wheel speed to driveshaft speed ratio) but the speedo will be off. If you recal the PSOM, it will correct the speedo but also change the ratio of the VSS signal that it generates to the EEC. If you don't reflash the the computer to account for this then the speedo will be corrected, but now the shift scheduling will be off. If the vehicle used a OSS in the first place, only the speedo would generally be affected by gearing/tire changes.

Here's some math for you to think about:

Most mechanical speedo's indicate 60mph at 1,000rpm cable speed.
One minute at 60mph = 1 mile traveled.
Therefore 1,000 speedo cable revs = 1 mile.
Many of the GM cruise sensors give 8 pulses per rev.
1,000 revs X 8 pulses = 8,000 pulses per mile.
(That's where this "8kPPM" stuff came from in the first place...)
I just turns out that in 1:1 tranmission ratio "most" vehicles will be around 2,600 driveshaft (transmission output shaft) RPM at 60mph.
(That's why "most" vehicles speedo drive gear is around 2.6:1)
2,600 rpm X 3 pulses per rev (ie 3 tooth reluctor) = 7,800 pulses [per mile]
7,800PPM is close enough to 8kPPM that the computer is happy (in other words the shift points will be within a couple hundred engine RPM of the programmed points). So long as it's within 10% the computer will be happy. Worst case is the shift points may be a little early or late, which can be corrected by flashing the computer with a different "axle ratio" that what it was originally programmed. Once that's done, you can change axles & gears all day long and the shift schedule (shift engine RPM) will be unaffected.


In my case, 5.13 gears and worn 39.5" Swampers is 2,665 driveshaft rpm. Doing the math we see 2,665x3 pulses = 7,995 PPM, so most of the time the speed reported by the EEC-V (on my scan tool) is the same as the speed reported by my GPS. That's purely by coincidence.

The Pro Topline heads are an aftermarket replacement F3 head. They have the same basic design but improved water jacket design to reduce the spot cooling problems (and cracking) of the OEM heads.