I may have to try this stuff in my rig. I usually get about 14-15 average and if I could get up to 17 that would be pretty dang nice. . . .

 

I haven't seen 14/15 mpg since before just about every gas station in the area started infecting the fuel with ethanol. I averaged 13.5-14 when I first got the truck in 04 and got 15 one time on all highway driving but that might have had something to do with tires that were a little bigger than the pcm thought I had on it at the time. My average now is 12 +-0.5.

 

A lot of people forget to factor in the change from stock size tires without changing gears to match will throw MPG guage out as well as the odometer and speedometer. (that would give you a slightly lower mpg reading than your really getting I believe.

 

If I'm not mistaken, '96 and up rely solely on the tone ring sensor which is south of the differential, and the PCM determines speed.. increasing tire size will alter the PCM's interpretation of speed, but gears won't.. just to clarify.. '95 and down need a new gear in the tail of the trans to account for larger tires and gears.. But, I may be wrong.. if you run larger tires, you gotta get a flash.. I think SuperChips has this ability, but SCT does not..

There is a 6% decrease in perceived speed/distance with my rig between 33" and 35" tires.. as was observed with a GPS..

 

I thinks that's 98 and up. My 96 speedsensor for the speedo is in the side of the trans, and is gear driven. Makes re-calibrations rather easy.

 

DOH!!! you're right as can be... I dunno why I typed '96.. I swear, I knew that!!!

 

So if the speedo is off so is the mpg right?... Unless you change either the transmission (depending on year) or diff gears or pcm to compensate. Or am I missing something?
the next question is what does the pcm use to calculate it? real time engine load or things like weight and speed which hold both be altered from stock state...
I dont even have a mileage gauge so...I use the one in my torque app now with the OBD bluetooth adapter...it appears to use vehicle weight , engine load,rpm's and speed

 

i don't use the little gauge in the truck to figure mileage- i do it the old fashioned way, which also has it's problems- but is the most accurate way to do it..

i don't know how the PCM figures, but after installing a vacuum gauge, the two are eerily related... MAP sensor maybe?

yup- fuel mileage and speed/distance are all related.. the PCM in 98+ rigs listens to the tone sensor in the rear axle, which is downwind of the differential, and uses whatever calculations it uses to determine speed.. it can be altered at the PCM by simply changing one of the constants (tire diameter), and is a function of the PCM.. older rigs use a mechanical gear to drive the odometer/speedometer..

 

Now I am totally cornfused! So waht does the speed sensor in the tailshaft of a 99 46re tranny do if it is not to measure speed?

 

PCM uses that one to verify that when it commands TCC, or O/D, that something actually happens. In all reality, it's one right from the department of redundancy department......