Did you just add the two mpg figures and divide by two? That’s not going to give you the right result.
For example, say I drive downhill 10 miles then uphill 10 miles using 1 gallon of gas. That’s an average of 20 mpg.
Now divide the trip in two. Going downhill I use 1/4 gallon, so that’s 40 mpg. Uphill I use 3/4 gallon, so that’s 13.3 mpg. So 53.3/2 = 26.7 mpg, but this is not the average fuel economy over the 20 miles.
Tyres by themselves have a maximum pressure, and 45psi sounds about right for that. The recommended pressure is much lower, and relates to the load being put on the tyres, i.e. what car they are attached to. So it’s in your car’s handbook, and/or possibly on a sticker somewhere in the car.
Yep 28 mpg in to work and 15 mpg home. 25 miles each way. 21.5mpg averaging the two. Unless I don’t reset the trip computer, then it shows about 18 average. I think it resets itself about every 10 miles or so (contrary to what the manual says).
I live 2200 feet above work. The trip computer to calculate mpg may be inaccurate, but I think it would be consistent. That’s what’s odd.
It doesn’t really matter that much. Gotta do what ya gotta do.
I’m not a car expert, but it sounds like you are bouncing down the road at that pressure. There should be a sticker on the inside back of your driver’s door with the recommended tire pressures (30-35psi)…
Gahh… Your right. The great (some would disagree) MPG I get down hill and the work that needs to be done going uphill /2 isn’t an average.
In my experience, there will be a label on the door jamb with the recommended tire pressure. This information is courtesy of the auto manufacturer, not the tire manufacturer.
I have heard that you get worse gas mileage with your windows open than with them closed, because of decreased aerodynamic efficiency. This means that if you can’t drive with your windows closed, you might be better off using the AC.
But, I have no facts on which to back this. Anyone know?
I’ll go check 'em. I just threw the 45psi number out there, though. I don’t remember the exact number. If that sounds ridiculously high, I’m probably wrong.
edit: I need to read ahead to the end before posting
Actually is probably isn’t wrong. yes there are pumping losses, but offsetting that is the fact that the engine management system adds additional fuel at wide open or near wide open throttle to aid in acceleration. On the systems I am familiar with this ranges from 25%-33% additional fuel at or near WOT. This far outstrips any pumping losses that would occur at small throttle openings.
::: Sigh::: No coasting down hill with the throttle does not use any additional gasoline. On a modern fuel injected engine, when the throttle is closed and the engine is decelerating the fuel to the engine is shut off. Down a long hill there is zero fuel usage. Unless you are creating gasoline in your car with the throttle open, you aren’t going to beat zero fuel usage.
I don’t claim to know much about cars, but doesn’t an IC engine need fuel just to idle?
I was going to ask Rick about this, as he’s mentioned it before. Obviously the engine in my Prius turns off when it’s not needed. But what about my '99 Jeep Cherokee?
ISTR reading that the fuel did shut off on the downhill. The econometer will read 99.9 mpg during those times. But the tach shows the engine is still turning, so I assumed the engine didn’t actually turn off. But couldn’t it be off, and it’s being turned by the transmission? If so, that doesn’t seem efficient because it would slow the vehicle even going downhill. Maybe the slushbox disengages? If it did, whence the revs? (If I put the transmission in neutral going downhill, I get the same idle-speed revs.)
::: sigh:::
Yes of course it does. But the return to idle is not an instantaneous process. It takes time. During this time, the injectors are shut off.
Assuming your car has a tach, tomorrow on your way to work take your foot off of the gas at say 70 mph. Count off how long it takes for the engine to return to below about 1500 RPM*. During this time no fuel is being injected.
If you are going down a hill, this lack of injection can last for up to several minutes.
I have said it before, and I will say it again here. There are zero advantages to injecting fuel on deceleration, and several reasons not to.
Modern cars do not inject fuel on decell.
*actual cut in point varies with engine management system, engine temp and a few other factors. Lowest I have seen is about 1300 RPM and the highest is about 1800.
Johnny yup your jeep also. Your econometer verifies this. While in drive your transmission does not give engine braking, but due to the fact that the transmission output shaft is turning faster than the engine would turn at idle, there is a “windmilling” effect that keeps the revs above idle for a period of time. This windmilling effect is NOT engine braking but rather just the fluid coupling in the torque converter working backward.
To see the difference between this windmilling and engine braking do this simple test. Put your automatic in L or 1st gear. Accelerate to 4,000 RPM. Take your foot off of the gas. See how quickly the car slows? Now go back and do it again in D. This time accelerate hard (to keep the trans from shifting) when you get to 4,000 RPM again take your foot off of the gas. Feel the difference?
The only cars I have ever seen that completely freewheeled on decell were old SAABs.
We went over this a month or so ago. You could shift your car to neutral on a long downhill, and perhaps the decreased rolling resistance would offset the increased fuel consumption. I am not a fan of this, as shifting back into drive puts a hell of a shock on the drive line. Doing this on a regular basis is bound to wear out something sooner than normal. Given the choice of replacing a transmission early, or gaining a couple of a hundredths of a MPG, for me there is no contest.
What do you keep sighing for? I asked an honest, valid question. If it’s such an exhausting effort to cast your infinite car knowledge down from atop Olympus then don’t do it. I’m sure plenty of other people here know more about cars than you do and could pass the sacred fire to us grovelling mortals with a lot less attitude.
Try answering the exact same question about once a month for 6 months or so, and then have people argue with the correct answer. After you do that get back to me 'K?
Rick, it’s been a lot longer than 6 months. I’ve gotten your back on the fuel-cutoff-when-decelerating issue a few times over the last year or more.
It really is taking longer than we thought. 
Cisco I’m sorry if I came off as snarky in my first couple of posts. I’m working at a friend’s auto shop this week, and it was about 108F in the shop this afternoon.
I plead a severe case of I’m whooped.
I thought that ::sigh:: was directed at me, but from the tone of this I’m guessing it wasn’t. I’ve never doubted that the fuel was shut off on decel.
But (and you know I’m not much of a mechanic) I’ve been curious about the revs. Bear with me… Going down a long grade gravity maintains my speed and the injectors are shut off. The RPM drops to idle (it’s been a couple of months since I’ve driven the Jeep, and longer than that since I’ve been on a long grade, so I’m guessing at the number), about 1,000 or 1,200 rpm. Does the fuel management system turn the fuel back on to allow the engine to run at idle, just as if I’m sitting at a stop light? (It’s so much easier in the Prius, because you can actually hear and feel then engine actually shut down.)
Thanks. Sorry if I was too rude in response. I’ve never read these discussions before, so it’s new to me. My question was just that - a question. I genuinely don’t know enough about cars to take your word at anything other than face value. Thanks for the info provided.