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Absolutely agree with mugen...The most accurate mileage gauge I have seen is in my Mom's prius, most other cars can be off from anywhere from 2-10mpg. For the real value you have to calculate it yourself after a fuel stop. Pretty easy and a guarantee of accuracy. Also a nice way to track your progress...
I also agree. My 08 V6 Accord has ALWAYS avg. around 25MPG (according to the trip computer) no matter what I do to it, from day one. After 30K miles, it's still averages around 25 after deliberate spirited driving. This is why I take all the averages from any car with a grain of salt!
Good conversation or fun comparison sure, I wouldn't put $ on its accuracy.
Curious for to see how your trip to Dallas goes in terms of mileage Melbitoast. Keep us posted. Lobuxracer, not sure what the reason would be on why you could not duplicate the results of the other posters. In looking at the previous posts have you ever taken a pure highway trip and then calculated your mileage?
Understanding the efficiency as well as you seem to what do you believe to be the optimal RPM for the IS-F to run at 70mph? In my experience with my now gone 540i the lower the RPM the better...So I am curious why you believe that lowering the RPM during cruising is not the ultimate goal?
Pure highway trip with indicated and calculated numbers from my IS350:
I haven't been quite as **** with the F, but I have driven it from ATL to CLT a number of times, and I've made a road trip from ATL to Dallas and back, ATL to Cedar Rapids, IA and back, and I've duplicated the methodology I used to create the chart above. Never seen 29 mpg for a whole tank ever, no matter where I set my cruise. I've seen up to 34 mpg between the gas station and home (about 6.2 miles) if I fill up with the engine fully warm and drive very easy all the way home (more downhill than uphill), but never, ever have I seen a tank - even on the Dallas run where we filled up and drove until the reserve light came on without any stops - make 29 mpg according to the device in the car. The times I've calculated mpg from gallons purchased and the trip odometer (I reset it every tank) the results have been close enough to call the on board device accurate.
So until I see a picture with 30 mpg and 420 miles on the trip meter from an IS F, I'm calling shenanigans. Oh, I also have done this with my Scion tC:
The discussion points on engine efficiency and the maxim lower rpm means better mileage would take an entire thread of its own. Suffice it to say, efficiency on any gasoline engine is far from simple to understand and applying a simplistic "lower rpm means better gas mileage" ignores an entire realm of physics.
These are impressive spreadsheets Lobux and thank you for sharing them, but having an IS350 I can tell you that on fuel pump calculations I consistently average 30-32mpg on my 125 (250 round trip) mile drive from Urbana to my home in Chicago. The dash readings vary anywhere from 27-35 mpg, but I don't have spreadsheets or pictures as you do, but I can tell you I use 93 octane fuel from RoadRanger and average 73 mph for greater than 100 miles of the trip.
Out of curiosity how are you creating these sheets/ Do you have an OBDII computer or are you using a combination of the navigation and your standard fuel calculations?
I agree and understand with your statement about seeing the trip computer showing 400+ or 30 mpg and I would like to see that as well for verification, but regardless of your physics argument if the car is pushing through the oncoming air efficiently and the engine is not 'bogged' down in low rpms (ie something ridiculously low like 600-800 rpms) the MPG will improve given the same speed, conditions, vehicle, etc. Maybe your earlier claim that GA fuel has higher alcohol levels may be accurate?
Ex: Car A travels at 70 mph turning 2400 rpms and in the same conditions Car A travels at 70 mph turning 1700 rpms then the latter will have less fuel use and yield higher miles per gallon of fuel used. Assuming again that the lower rpm level is still within the engines acceptable range.
I don't want to deviate thread too far either, but I am happy to continue this discussion of efficiency elsewhere or am happy to review any sites you feel would further support your viewpoint.
The times I've calculated mpg from gallons purchased and the trip odometer (I reset it every tank) the results have been close enough to call the on board device accurate.
Lobux, is there particular reason it would require the full use of the tank? Would say, a half tank of all highway driving suffice?
I've managed my numbers with a trip between 80-90 miles. What other parameters would you add, to ensure validity?
The full tank runs are "best possible" conditions with only road construction to interfere with ideal numbers. Traffic on Interstates off of rush hour is a non-player, so ideally, you fill up with the engine as close to normal operating temperature as possible, then drive until the reserve light comes on or until your nerve gives up. The best I've ever seen in my F was 382 miles and I had less than a gallon left when I compared what I pumped with what the tank holds according to Lexus. The last 10 miles or so I was pretty nervous, and trusting we'd make it to the next gas station before hitting fumes.
There are lots of ways to measure this, but the full tank method should (in theory) give the best possible results because there isn't any reheating being done. The engine gets up to operating temp and stays there the whole time - very little fuel is wasted on keeping the engine hot (this is one of the counterintuitive things about efficiency).
AFA the lower rpm argument - not necessarily. An engine is at peak thermal efficiency at torque peak. Anything less than WOT at peak torque is going to be less efficient. Luckily for us, we don't need that much power to keep rolling down the road at a steady speed, so the trick is to find the spot where the engine's pumping losses are low (usually low rpm) and efficiency is good. It can't be great, but it can be good. The OEMs all know this and set up their operating parameters for part throttle to take advantage of it as much as is practical - they have emissions and efficiency targets to hit to meet EPA and CAFE standards. Is it possible to reduce rpm by 500 and improve efficiency? Maybe and only maybe. If you have to open the throttleplate more to maintain power (almost a given because HP required is based on a certain amount of air passing through the engine no matter how you slice it), then you'll gain and lose efficiencies in other areas. Longer time between combustion events means more opportunity for the cylinder to cool between those events which also means you'll need to burn more fuel to bring the cylinder head back to temperature with each cycle - so there are LOTS of trade offs going on and making a blanket statement "better fuel mileage at lower rpm" is ignoring a whole lot of the system's interactions which result in the actual fuel economy.
It's a lot like saying "Car A has more torque than Car B, it's better" which completely ignores the entire drivetrain's contribution to moving the car. More torque might FEEL better, but there's no guarantee it IS better unless the two cars are identical in all respects except one has more torque. It's why I laugh when people post their torque numbers - they're academic unless the drivetrains are identical, and usually they're comparing two completely different brands and coming to the conclusion one is better than the other because it has more torque. Not at all universally true. MIGHT be true, but no guarantees. Same with efficiency - MIGHT be more efficient at lower rpm, but by the same token, it might not. Certainly if you're running a lot of overlap on your cams for high rpm torque, running this configuration at lower rpm is only going to make things worse.
Very insightful answer Lobux. Thank you for the information. From your calculations have you found the optimal RPM for the IS-F engine in terms of efficiency in standard freeway driving?
Very insightful answer Lobux. Thank you for the information. From your calculations have you found the optimal RPM for the IS-F engine in terms of efficiency in standard freeway driving?
I wish I had a good answer, but I don't. I've not even seen over 30 mpg for any drive more than 6 or 7 miles, but I don't have the discipline to drive 60 mph on the freeway for any length of time. I imagine if I had the ability to drive 60 mph for a whole trip, I would have bought an IS250 and been happy.
Lobuxracer, my guess is that you are right about the ethanol in Georgia affecting your mileage. My best mileage so far was just over 28 MPG on a 186 mile roadtrip from Virginia back to Maryland. I feathered the throttle as best I could, but the trip was through rolling hills and on a highway where I got stuck at about 15 traffic lights along the way...plus it was about 95 degrees out. I guarantee that I could get 30 MPG at 80 mph on a flat road on CC using 93 octane with no ethanol.
Small update, I found the pics I took of the longest I ever went on one tank of fuel:
This was an ideal run, car was fully up to temperature when I stopped for gas, and I just filled it up and hit the road. I drove without stopping until the reserve light came on, then drove about 17 miles past the 0 miles indication. I filled with 15.1 gallons.
That's a good distance. I never take long enough trips to do anything like that. If somebody planned it out right and kept the CC on over flat roads on a long road trip, a 400+ mile take would be attainable.
Did some searching and found a few threads in the forum on MPG for the IS-F but none related to the aerodynamics. I am curious what some of you believe would be the result of the different MPG's experienced by the two cars? Obviously the engines are vastly different, but with the 8-speed trans and the low final gear I can't see why this car would not get into the low 30's on extended highway drives. Especially if you are turning 1500-2000 rpms at cruise?
In looking at both cars I wonder how much the front fascia has to do with the IS-F having lower returns? Anyone think that the larger bumper with much larger openings is a big reason for the difference? Anyone know the drag coefficient of both vehicles?
The reason for these questions is that the IS-F seems to have all of the parts and performance bits I wish I had on my IS350, so it is just easier to buy the IS-F. So a swap will hopefully be in my future. I can justify the higher octane fuels, but only if I can get north of 30mpg on my 200+ mile weekend drives. Tired of hearing my Dad and brother brag about there lame vettes getting 30+ on the highway and still beating me at the track.
That's a good distance. I never take long enough trips to do anything like that. If somebody planned it out right and kept the CC on over flat roads on a long road trip, a 400+ mile take would be attainable.
I agree and using non ethanol gas at 55-65. For sure 30
12-11-10, 07:53 PM
