You are forgetting that going down a mountain in an automatic makes the rpm go up under cruise control, which uses more fuel. Also the mountain has to be really downhill for the car to keep the same speed while not using the overdrive. Even if I let go off the gas while driving down a mountain the car still slows down. It's complicated but it's a lose-lose situation going down a mountain in an automatic.
I will find a way to surpass 40mpg on long trips. My tires are at 40psi and I plan to do the underbody really soon.
I'm not sure I believe that. When I go down teh mountain in my manual in gear the RPMs are REALLY high. That's what engine braking does. The down side of an automatic is that the engine braking is always engaged when going downhill.
Odds are the engineers at Nissan anticipated this and made the injectors fire less frequently while going downhill.
I'm willing to bet that if you put a scangauge on your car and drove downhill your mileage would improve more than you realize.
The rpms do go down but the car slows down too fast. I have to speed up a lot.
If the throttle is closed and the engine speed is above a set limit, such as 1200rpm in the case of Hondas, the injectors are disabled. That's how engine braking works. Auto or manual trans will generaly slow the car an equal amount, it really has nothing to do with it.
Well you have to keep the engine turning and in gear to regulate speed unless you want to pop it into neutral so the next thing is to turn off the fuel pump so the injectors will not be operating. The ScanGauge will probably register the injectors opening even with no fuel or you could end up with an engine check light for no fuel pressure. From what I am seening my xB shuts off the fuel to the injectors until about 1000rpm when slowing in 5th gear - it really slows you down then starts bucking below 20mph even with your foot off the gas. ScanGauge shows like 80-120-150+ mpg depending upon speed when slowing down but it senses a reducing speed so if you can put the clutch in or get it in neutral and keep from slowing down then the indicated mpg goes up more. If you have to burn a little gas it still gives you some pretty high MPG. Air density really affects air drag so colder air slows you down more - made a trip in near zero to below zero weather one time in the Geo and couldn't get over 65mph except on down hills from a combination of too cold air into the engine and more density causing air drag. A lot of this shows up on my electric scooter with the amp meter showing drag in hot and cold weather and really affects range. Funny driving the xB feels like the scooter trying to stretch the range by slowing for stops way ahead of time and really light on the throttle.