[Randy]

Is it possible for a trip meter digital display to ever display a number higher than 150.0 mpg?

Saturday morning I was driving what would normally be my route to work. Since traffic was light and I wasn't in a hurry, I was trying for maximum fuel economy. There is a long (maybe 5 miles), down hill (5% grade) stretch, and I noticed that the segment fuel economy, after about a mile of this, said 150.0. I travelled the rest of the way down the long hill (at 70 mph) without touching the gas, and the segment display still said 150.0 mpg. I'm sure that the trip economy was over 180 mpg, but I don't think that the display is set-up to show those higher numbers. (It only occured to me later that I should have looked at the metric numbers to see if they were still going down.)

Has anyone ever seen trip mileage display higher than 150.0 mpg on the digital display?

Randy

 

[Tim Maddux]

No, it is not possible for the display to exceed 150 mpg. You were right to think you were probably doing much better than that.

I have confirmed this by resetting a trip meter at the top of a hill and then going down a grade with the engine in fuel-cut mode. At the bottom of the hill my trip mileage reported 150 mpg. However, I hadn't burned any gas. My metric meter reported 0 L/100km which is infinite mpg.

After I drove a distance equal to the length of the hill at an average of 70 mpg my trip mileage had decreased to 140 mpg ... consistent with the mpg down the hill being infinite. If it were truly 150 mpg down the hill I would have seen a combined mpg of 2/(1/70 + 1/150) = 95 mpg.

 

[amplifiedmouse]

i wish we had hills where i live...

 

[redpriest]

I had fun where I came to a stop at the top of a hill, so my engine shut off. I then left off the brake enough so that my car started rolling again, then I was rolling down this rather steep grade (about 15 deg) and was doing about 30-40 mph with the engine off. It improved my gas mileage tremendously for that trip =)

 

[Will M]

In Virginia, it is apparently illegal to coast down a hill. Apparently, that is considered to be failing to control your vehicle. I suspect it is one of those laws that they only pull out after an accident to weigh into the question of "fault". Otherwise, who can tell if you are coasting, and who cares?

Still, if you are having a conversation with a cop, it would probably be a bad thing to mention any of the times you were coasting down a hill...

 

[james]

I think fuel cutoff mode must not cut all the way to zero on a long downhill, or at least not zero all the time. If it did, I would expect the temperature gauge to fall, due to the effect of cooling air and no heat from the engine. That doesn't seem to happen...

 

[Tim Maddux]

I have seen my temperature gauge fall in the manner you describe while going down Malibu Canyon Rd. in California, which is about 5 miles long, all gentle-sloped downhill. Early, cool summer morning, around mid-50's F (typical gloomy marine layer even though LA was already hot). 5th gear, in fuel-cut mode, sometimes light regen-brake but not often. I lost a couple ticks of temperature and (as I said before for a different road) burned no gas as evidenced by the L/100km gauge.

http://www.insightcentral.net/encyclope ... elcut.html

 

[Will M]

In a normal car, when you turn off the engine, the indicated engine temperature actually goes UP because the water pump stops before the water absorbs the most recent heat from combustion. The water slowly takes on that heat, but faster than it loses heat everywhere else.

In fuel cut mode, however, I'd expect that the water pump is still churning.

 

[james]

True, so I wouldn't expect to see any effect on a short downhill, since residual heat in the block is being transferred to the coolant. And of course once the block reaches the thermostat's minimum temperature, the flow through the radiator should go to zero.

Still, the downgrade in question is about 12 miles, so I would expect to see cooling. Granted I haven't done it in temps below about 70. A definitive test will have to wait for winter, and trips down Mt. Rose. Past vehicles haven't even warmed to normal operating temp doing that...