RPM - am i just dumb???

[Stalex]

Hi,

I was conducting a little experiment this morning re mpg and set the cruise at 70 mph. At this point rpm read 2100. Car's auto & was in "D".

For the next 8 miles the speed obviously stayed at 70 mph (what a boring drive that was!!) but the rpm also stayed the same at 2100 despite some pretty steep inclines and declines. I would have expected the engine to have to rev higher on climbs to manitain the same speed...and rev lower on downhills...or is this all compensated for through gear changes etc....or am i just showing my complete lack of understanding of anything mechanical??

24.4 ave mpg btw...bit of town, bit of dual carraigeway over 11 mile journey.

[Aldouk]

Hum ...
RPM stays the same while the fuel used goes up or down as the load on the engine goes up or down.

[R33]

Yes I would just guess as the engine has so much torque it could maintin the rpm's with it's available torque. If you had gone up a steeper hill and it's didn't have enough torque to maintain the rpm then it would have changed down a gear. As Aldouk says the fuel consumption would have gone down as you were going up the hill as the engine load increased.