mpg calculated vs computer

In various test with road side "here is your speed" radars and my Garmin GPS. I have found the dash speed to be correct up to 25 mph. It then varies by 1 mph. The GPS/Radar shows 26 mph when the car shows 27 mph. This seems to make the same jump in the lower 50~55 mph range of another 1 mph. At 57 mph on the dash, the GPS/Radars show doing 55mph.

 

Yes, that sounds right. Since it seems to be off by about 5% for everyone, you are seeing the display rounded or truncated to the nearest MPH. At 20 MPH, the error would be about 1, at 40 it would be about 2, etc.

 

It just occurred to me that, since the speedometer display seems to be purposely miscalibrated by Toyota, there may be some way to recalibrate it through an OBDII device. Does anyone know if this is possible?

 

You could use tires that are 5% larger circumference (or 5% fewer revolutions per mile), yielding 5% greater distance per revolution, which would match dashboard indicated to externally measured speed.

 

lol! I would have to care about the error a lot more to go to that expense!

Seriously though, I did look at the tire size as a factor. My V5 has slightly larger wheels than the other versions (P215/50 R17 vs P205/60 R16). Using a tire size calculator at tire size calculator, the overall diameter is slightly under 1% larger on mine. Assuming Toyota did not compensate for this, the speedometer difference would actually be 1% less rather than 5% more.

I think Toyota purposely calibrated the speedometer higher by 5% so they could not be sued if we got a speeding ticket and they calibrated the fuel economy 5% higher so we would tell our friends the misleading number and they would sell more cars. I was just thinking that since they had a way of miscalibrating it, there might be some way of recalibrating it using an OBD2 device.

 

Recalibrating the instruments usually involves getting into software and data files and changing equations and/or look-up tables. Not something you can do with conventional OBD-II stuff.

 

Yeah, that sounds right. Given that OBD-II showed the more correct speed before Toyota miscalibrated it, I was hoping it was possible. Thanks.

 

I wouldn't assume that all the gas pumps are perfectly correct either. If they are off by 5%, 20 gallons actually pumped is rung up as 21 gallons. That is an extra $3.50 that most people aren't going to notice. If 100 people fill up that day, that is an extra $350. That isn't an insignificant chunk of change. I'm just saying that the error might not entirely be on the car side.

 

In this case, I'm talking about the speedometer error itself. The speedometer differs from my iPhone GPS, radar speed signs, and even from the internal speed number returned by OBDII. There is no question in my mind that the speedometer is purposely miscalibrated since so many people see the same thing. I suspect it is related to the miscalibration of the fuel economy number as well, which is a consistently similar error reported by many people also.

 

Yeah, I think the speedo is intentionally off. That is a normal thing among all makers. Odometer has to be within a certain range by warranty requirements.

 

If the odometer has to be in a certain range, why would they intentionally introduce a bias error in speed (and fuel consumption too)? I would think that the goal would be to put it in the middle of the range to allow for errors plus or minus. I haven't checked the odometer reading yet so I don't know if it has the same error.

 

I did a little web research and discovered that such errors in speed and fuel consumption are the industry norm. It seems that reading about 5% high, as the Prius seems to do, is average and some brands are much worse. I couldn't find a reason for this and manufacturers are vague at best when queried. Personally, I think they don't want to be sued for speeding tickets and they want to mislead people into thinking the fuel economy is better. Odometer readings tend to be very accurate though. Sorry for not doing this research sooner.

 

If you think about it though, by showing a slightly higher speed, say 65 when actually doing 63, it may result in only small amounts of MPG gained. However over time, even in one full tank, this would most likely bring MPG up. This might also account for some of the fudge numbers that the on board MPG computer displays. Is it also off by 4-5% too?

 

Yes, that's what I was thinking. The MPG is indeed high by about 5%.

 

The attached PDF contains my entire gas fill up logs. Some of them have the computer generated AMPG.

Here are my last two:
2013/07/07, TripA: 383.3, 9.739 gallons, AMPG: 39.36, Computer AMPG 41.2
2013/07/14, TripA: 337.4, 8.168 gallons, AMPG: 41.31, Computer AMPG 42.8

I typically fill to the first click. I also fill at the same gas station and at the same pump location at that gas station.

 

Does anyone know why the Toyota calculated mpg is so far off? Is it off by an even greater amount for those of us who try to maximize mpg by tweaking the pedal for glide and EV mode versus just driving normally and ignoring the power gauge and modes?

I don't top the tank off but just fill it until it clicks and everytime I've filled up I've checked me versus the Toyota calculation and they've been higher every single time. I calculate between 2.0-3.5 less than toyota displays. (I will say I tweak the pedal a lot to glide or get into EV mode, just below 45 so I wonder if all that EV and coasting is causing the gap.

If Toyota can't calculate it correctly, they might as well just put the digit 1 in front the number and tell me I'm getting 152 mpg instead of 52 when in really only getting 50mpg.

 

Mine are way off too now.

I've been filling up past the first four clicks; getting about 1 gallon more into the tank. My AMPG and calculated MPG are off by over two MPGs (calculated).

 

After 33,000 miles Computer 42.8, Fuelly 40.6

 

You will notice the speed of the car is also slightly off too.

Find one of the "your speed is" signs with radar.
At 25 mph it is correct

However at 27 mph it is really at 26 mph.
At 55 mph it is really at 53 mph.
I have not found one on the interstate at 75 mph, but I would suspect it to be really 72 mph when the dash shows 75 mph.

I guess I could take my garmin gps and check it for sure at 75+

This slight variation seems to match the slight conservative numbers found on the MPG estimates.
Most likely this is intentional by Toyota to get people to slow down, even slightly to increase their MPG numbers.

 

One of the first times that I filled up, the actual mpg matched the computer mgp. Since then, it has always been around 2 mpg off, with the actual being lower than computer. I don't use the same gas station, so I wonder if that is part of the issue. I always fill until it clicks off. If I go to one station, and it clicks off at 10 gallons, whereas at another station it might have gone until 10.5 gallons, that can make a big difference in mpg. At 420 miles on the tank: 420/10 = 40mpg. 420/10.5=40mpg. I'm going to try and use the same station to see if I can get some consistency.