How accurate is Avg MPG?
03-01-07, 02:14 PM
#16
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So, I would think that the skid control computer would then also control the odometer. Right? Or, is the odometer just a mechanical link? Do you think Lexus makes the speedometer show a couple miles over "actual" speed, to protect us from ourselves, or something like that?
03-01-07, 07:34 PM
#17
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The odometer show less than actual in most cases, and the speedometer show more than actual in most cases.
I was driving late last night pretty much alone on the freeway doing an economy run and I decided to check out a few things while in motion. I opened up the diagnostic screen (display UL - LL - UL - LL) then tapped all four corners in a backward z starting UR. There is a diagnostic screen that shows speed in km/hr. With the cruise set at 70 mph (actually the legal limit), the speed on the diagnostic screen was only 107 km/hr. Not even close to 70 mph, but within tolerance. So I suspect my speedo is reading faster than I am going. However, my tires are 65% worn, so it is possible this indication would be closer if the tires were full tread depth (still not 4 mph off though.)
Anyway, the FSM doesn't say anything about adjusting the speed or the odometer (I suspect it would be illegal to allow anything to adjust the odometer). Also, I am not sure how the speed I saw on the diagnostic screen is generated. If it is using the dead reckoning system, it is undoubtedly adjustable, and it may be that the nav system is not adjusted at the factory for 18" wheels so it indicates slow.
I'll dig deeper into this another time. Right now I have to pack for the wife's b-day mini-vacation this weekend and I am forbidden to bring any computer with me. Fair enough anyway, it's her birthday celebration, not CL research time.
I was driving late last night pretty much alone on the freeway doing an economy run and I decided to check out a few things while in motion. I opened up the diagnostic screen (display UL - LL - UL - LL) then tapped all four corners in a backward z starting UR. There is a diagnostic screen that shows speed in km/hr. With the cruise set at 70 mph (actually the legal limit), the speed on the diagnostic screen was only 107 km/hr. Not even close to 70 mph, but within tolerance. So I suspect my speedo is reading faster than I am going. However, my tires are 65% worn, so it is possible this indication would be closer if the tires were full tread depth (still not 4 mph off though.)
Anyway, the FSM doesn't say anything about adjusting the speed or the odometer (I suspect it would be illegal to allow anything to adjust the odometer). Also, I am not sure how the speed I saw on the diagnostic screen is generated. If it is using the dead reckoning system, it is undoubtedly adjustable, and it may be that the nav system is not adjusted at the factory for 18" wheels so it indicates slow.
I'll dig deeper into this another time. Right now I have to pack for the wife's b-day mini-vacation this weekend and I am forbidden to bring any computer with me. Fair enough anyway, it's her birthday celebration, not CL research time.
03-02-07, 12:21 AM
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The change from 17" to 18" tires might not even require an adjustment, since the 18 is a lower profile, it may be about the same outside diameter.
Anyone here have 17" wheels/tires? Try the speed test - set cruise control on 70 mph, then reset "Avg MPH" on the display to 0, and see if it next reads 68, or 72, or who knows what! Report your findings back to the committee here. Should you choose not to accept this mission, your IS will self-destruct in 5 seconds.
Anyone here have 17" wheels/tires? Try the speed test - set cruise control on 70 mph, then reset "Avg MPH" on the display to 0, and see if it next reads 68, or 72, or who knows what! Report your findings back to the committee here. Should you choose not to accept this mission, your IS will self-destruct in 5 seconds.
Last edited by CWS; 03-02-07 at 12:24 AM.
03-02-07, 06:27 AM
#19
The change from 17" to 18" tires might not even require an adjustment, since the 18 is a lower profile, it may be about the same outside diameter.
Anyone here have 17" wheels/tires? Try the speed test - set cruise control on 70 mph, then reset "Avg MPH" on the display to 0, and see if it next reads 68, or 72, or who knows what! Report your findings back to the committee here. Should you choose not to accept this mission, your IS will self-destruct in 5 seconds.
Anyone here have 17" wheels/tires? Try the speed test - set cruise control on 70 mph, then reset "Avg MPH" on the display to 0, and see if it next reads 68, or 72, or who knows what! Report your findings back to the committee here. Should you choose not to accept this mission, your IS will self-destruct in 5 seconds.
03-02-07, 07:23 AM
#20
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I've gone through this whole crappy MPG thing since I do mainly city driving...I usually dont push harder than 2500RPMs when accelerating. I was averagine like 280 miles/tank now I'm getting 306 with like 22.6MPG right now...
just drive with an egg under your foot for a while and you'll be alright
just drive with an egg under your foot for a while and you'll be alright
03-02-07, 12:07 PM
#21
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Someone once said "if you give a man his assumptions, you cannot refute his argument". I won't argue with your statement; I just am curious whether or not the 17" wheel models will show 68 MPH on the "avg speed" display, while the speedometer shows 70, like my 18" does.
03-02-07, 06:44 PM
#25
I'm an engineer, and I KNOW that you can screw up the calc by assuming that your gas tank is filled up the exact same level every time, and that your gas pump is reporting the exact amount of fuel. I would trust the ECU's calculation before I would trust the odometer's miles traveled divided by the gallon's filled method.
03-02-07, 10:42 PM
#26
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I'm an engineer, and I KNOW that you can screw up the calc by assuming that your gas tank is filled up the exact same level every time, and that your gas pump is reporting the exact amount of fuel. I would trust the ECU's calculation before I would trust the odometer's miles traveled divided by the gallon's filled method.
In 15 months of car ownership, my hand MPG calculation is consistently about .5 MPG higher than the "TANK MPG". The only way that your explanation would cover this would be if EVERY single time I filled the tank, I put in less gas than I did the previous fillup. Needless to say, that one doesn't pass the sanity test.
03-03-07, 06:22 AM
#27
If he averages his results over enough fill-ups, it won't matter that the tank isn't filled to the same level each time. Sometimes his hand calculation will be high, sometimes low. The average should be on-the-money.
In 15 months of car ownership, my hand MPG calculation is consistently about .5 MPG higher than the "TANK MPG". The only way that your explanation would cover this would be if EVERY single time I filled the tank, I put in less gas than I did the previous fillup. Needless to say, that one doesn't pass the sanity test.
In 15 months of car ownership, my hand MPG calculation is consistently about .5 MPG higher than the "TANK MPG". The only way that your explanation would cover this would be if EVERY single time I filled the tank, I put in less gas than I did the previous fillup. Needless to say, that one doesn't pass the sanity test.
03-03-07, 07:18 AM
#28
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Also, I buy gas from a variety of pumps at many different gas stations. While I could accept that a particular pump has slipped out of calibration, I find it much harder to accept that they are all miscalibrated by roughly the same amount in the station's favor.
03-03-07, 07:29 AM
#29
The gas pumps all have blue weights and measures stickers, attesting that some government agency has checked them for accuracy. The fuel injectors in my car have no such sticker.
Also, I buy gas from a variety of pumps at many different gas stations. While I could accept that a particular pump has slipped out of calibration, I find it much harder to accept that they are all miscalibrated by roughly the same amount in the station's favor.
Also, I buy gas from a variety of pumps at many different gas stations. While I could accept that a particular pump has slipped out of calibration, I find it much harder to accept that they are all miscalibrated by roughly the same amount in the station's favor.
03-05-07, 12:10 PM
#30
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Worse than that, I read an article in the local paper explaining this process in my county. It is a single point check at 5.0 gallons.
I don't know if you've ever done calibrations, but I know with great certainty if you only check one point, it's the only one you can be sure is correct. The indications between 0 and 5 gallons could be all screwy as long as when you hit the 5 gallon mark you have 5 gallons within the allowed accuracy.
For example - I went to a station with calibrated pumps and filled up my Scion tC. The gas tank holds 14.5 gallons. I put in (according to the pump) 14.7 gallons. I know I was within a gallon of empty, but not out of gas (unless I made a hard right turn). So how did I get more gas in the tank than the manufacturer says it can hold unless the pump's calibration isn't accurate?
Example two - I used to have adjustable torque wrenches in my shop while in the AF. They required calibration every 90 days. Most would not last more than 4 calibration cycles because they would not maintain correct tolerance over their entire range of operation. They worked fine at the two points I needed though. So, I requested a limited calibration for the two points I needed knowing they were not trustworthy outside that range. Our pumps in this county are in the same boat. Fine if all you want is 5 gallons. Anything else is not reliable.
I don't know if you've ever done calibrations, but I know with great certainty if you only check one point, it's the only one you can be sure is correct. The indications between 0 and 5 gallons could be all screwy as long as when you hit the 5 gallon mark you have 5 gallons within the allowed accuracy.
For example - I went to a station with calibrated pumps and filled up my Scion tC. The gas tank holds 14.5 gallons. I put in (according to the pump) 14.7 gallons. I know I was within a gallon of empty, but not out of gas (unless I made a hard right turn). So how did I get more gas in the tank than the manufacturer says it can hold unless the pump's calibration isn't accurate?
Example two - I used to have adjustable torque wrenches in my shop while in the AF. They required calibration every 90 days. Most would not last more than 4 calibration cycles because they would not maintain correct tolerance over their entire range of operation. They worked fine at the two points I needed though. So, I requested a limited calibration for the two points I needed knowing they were not trustworthy outside that range. Our pumps in this county are in the same boat. Fine if all you want is 5 gallons. Anything else is not reliable.
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