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I had my suspicion that the computer mpg on the CT is always reading too high based on my manual calculation. All I'm concern about is 2 numbers, the number of miles travel and the amount of fuel needed to replace what was burned traveling those miles.

So this is according to the September 2013 issue of Car & Driver magazine who wrote an article researching this discrepancy. They tested several vehicles over a number of fill-ups and thousands of miles of data gathering and concluded that the cars computer were all inaccurate that ranged from 4.5 to 6.4% too high. To summarize here are the reason's why:

- The density of the fuel varies
- Vapor recovery system, this canister is purged by the engine and this unmetered fuel isn't counted by the trip computer
- Plastic fuel tank can expand and contract by as much as a liter
- Fuel delivery to each cylinder per combustion cycle in microliter must be measured accurately, how accurate are the measurements?

They go on with a short article on how to maually calibrate your car accurately to get the true mpg.
 

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- Plastic fuel tank can expand and contract by as much as a liter
Not sure about this one. The 2nd Gen Prius had a bag for a gas tank, and it was still the same 2 mpg over-estimate as all the other Toyota hybrids, rigid gas tank or not. Seems to just be something with the way that Toyota measures it.
 

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I wish there was a method for manual calibration when the tank is near empty. When the range reads 0 mi on my dash, I still have 2.5 gallons left.
 

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- The density of the fuel varies

If this were a real factor, why wouldn't it work both ways? I have no doubt that this is true, but sometimes it is more dense, therefore sometimes it is less dense. That can't be a reason that the MPG is always higher than actual.
 

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There is some truth to it as obviously temperature is a factor....but there was definitely a moron working on our fuel gauge and miles remaining calculator. Shows up as zero miles when there is clearly 2 gallons left in the tank and the reading constantly off by -2mpg (once I measured -3.2mpg)!
 

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it is obvious to me that Toyota is protecting us from ourselves. Apparently dire consequences result from running the fuel tank dry.

I really don't understand what the big deal is. As long as the error is fairly consistent, what difference does it make? I am not trying to set a world record for gas mileage for the Guiness Book, all I care about is if I am driving as efficiently as I am able, which is obviously not as efficient as some who post on here and not even close to some of the figures I used to read on the Prius site. Some of those people need to start writing fantasy books.
 

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The CT fuel gauge annoys me to no end. I came from a 2001 BMW 330xi and that car's fuel gauge was incredible. It would give me range readings right down to zero before conking out. No, I didn't do that on purpose, when I first picked it up in Munich, I misunderstood the owner's manual, given that it was the euro manual. I thought I had 2 gallons left, when it showed zero, kind of like how you have 3 gallons left when the CT shows zero. Nope, when the BMW shows zero, you've got zero.
 

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I was told that Toyota does this for different markets. Their cars in Europe and Asia are setup so that if the fuel gauge says 0, it means 0, but in the US, Toyota claims that Americans want/need buffer or they panic, they made it so that 0 doesn't really mean zero, it means that it is now on reserve.
 

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Regarding Fuel Tank Volume Inacuracy:
Is there a fix? My gauge sets on "0" and has nearly 3.25 gallons left in the tank.
I have a SC430 and a LS430 which are both incredibly acurate in MPG estimations and fuel tank volumes. Very frustrating overall.
 

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I have a scan guage with a speed read out. The read out varies with the displayed speed on the dash. The dashboard speedo says 35 mph whilst the scan gauge says 32 mph..... also my garmin sat nav says 32mph. I have always thought that the manufacturers deliberately adjust their speedos to show a higher speed than the car actually travels at because they wish to avoid being taken to court by car owners who blame the manufacturers speedo settings for the causes of speeding infringements or worse.
 

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I have a scan guage with a speed read out. The read out varies with the displayed speed on the dash. The dashboard speedo says 35 mph whilst the scan gauge says 32 mph..... also my garmin sat nav says 32mph. I have always thought that the manufacturers deliberately adjust their speedos to show a higher speed than the car actually travels at because they wish to avoid being taken to court by car owners who blame the manufacturers speedo settings for the causes of speeding infringements or worse.
Oh sorry... what I forgot to point out is that in order to compute fuel consumption, the car needs to take the speedo reading in order to calculated the distance travelled verses the amount of fuel used. Therefore, if the speedo isn't correct, the fuel consumption is never going to be correct.
 

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Nice to know about the reviews about it. But whenever i owned car there is no fault in the accuracy of its MPG. Always it shows the original value.
 

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I have a scan guage with a speed read out. The read out varies with the displayed speed on the dash. The dashboard speedo says 35 mph whilst the scan gauge says 32 mph..... also my garmin sat nav says 32mph. I have always thought that the manufacturers deliberately adjust their speedos to show a higher speed than the car actually travels at because they wish to avoid being taken to court by car owners who blame the manufacturers speedo settings for the causes of speeding infringements or worse.
I've noticed the same ~2 mph difference in true speed vs speedo indicated speed. I figure it has to do with tire diameter. They need you to have an accurate speed with the biggest possible tire diameter (new) and I suppose it gets a bit off as the tires wear down. But 2mph difference?? Still seems a bit off.
 
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