Up hill down hill, head wind, air density..

Or you need to warm up the engine longer so it enters closed loop and uses the oxygen sensor.

 

 

Sorry I missed a few details there.

The RPM shift is due to either not switching to overdrive, or the torque converter not locking. The first is temperature dependant, the second is load based (so the wind and hills still apply).

 

Almost no where is dead flat, check GPS or a typographical map. Even a light breeze on the front end makes a difference, get on a bicycle and try going from 10-15-17 mph. Another bit is the air density 50-70°, it's substantial thicker in the morning due to the cold and any moisture..

Anyways hook up your volt meter to the transmission diagnostic. It will output a voltage that can be compared to the shift position.

Another quick check is the vafm air temp, it will have an effect on the fuel mixture. Which was probably a red herring on my part not noting your rpm to speed correlation.

 

 

I (we all) assume you have an automatic transmission. At 70, the transmission should be in overdrive and lockup, unless you pulling a pretty steep hill (6% or so), or going into one hellacious headwind. You can both hear the lockup and see it on the tach. Driving that fast out of lock up, or in 3d, will really affect fuel economy. Once you're in lockup, the relationship between rpm and mph is fixed by the gear ratio; wind and hills don't make a difference.

But the best method, as suggested by Co_94_PU, is to just ask your transmission. You do that with a voltmeter. Either use your multimeter (you'll need long enough leads to get the meter inside the cab), or drop $5 on my patented Gear Shift Indicator. https://www.yotatech.com/forums/f116...icator-303218/

 

 

When you're driving to work and the RPMs are higher, if you click the OD button off, does the engine RPM increase? If not, then it's not going into OD.

 

 

Well, that is kind of impossible, unless you have a electrical gremlin that is showing the wrong tach speed at times. That can be traced to GND.

I also have a V6 Auto that doesn't go into OD - check engine light will come on then too. Still stumped. So it's in the garage just sitting around. Good luck!

 

Even once in OD, Lockup can be on or off. With Lockup off, it's driving through the torque converter, so rpm can change with load.

 

 

The transmission won't lock up when it is "cold." Perhaps you're not warming up fast enough (thermostat?) Or perhaps the transmission temp sensor http://web.archive.org/web/201408160...34electron.pdf is dying.