Ok, disregard my previous reply. I see where we disagree now. I am assuming that the error doesn't change from driver to driver, type of driving to another, fuel type to fuel type, or location to location, and you think this is a bad assumption. However, I think that this is a fair assumption. No one here has falsified this assumption; so far everyone has reported that their dash reads too high for mpg. I'm not saying there doesn't exist a person who's dash reads perfect or actually underestimates, but I don't know of one yet. It would help if I could find some information about how the BRZ calculates its mpg. Fuel type and driving conditions might not make a difference, maybe the computer just polls how much fuel is currently being consumed and how fast the vehicle is moving.

It will take a bigger experiment to figure out why the dash is reading higher than expected. Maybe the source of error is that the mpg measurements are made discretely and not often enough, resulting in an overestimated approximation. Sort of like approximating the area under a curve numerically and not using a small enough dx (change in x). The experiment I am performing now is to conclude with evidence that the dash reads higher than expected.

that is pretty similar as to my results. Out of 42 fillups, my brz is reading on average 1.73 mpg higher than actual. the highest was 3.1 higher, lowest was 0.68 higher.

5% optimistic
 

I have been recording my fuel usage since I got my 2013 FRS and have not reset my avg mpg indicator yet, afaik. For about 70 fill ups I have averaged 5% lower real mileage than the average mpg readout on the car, with about 2/3 of my driving on highways.

Thanks for suggesting this comparison, OP.

Brian in Atlanta

I've uploaded a lot more mpg data here https://docs.google.com/spreadsheets...P78/edit#gid=0

I started taking notes about what kind of driving I was doing, where I refilled with fuel, what kind of fuel I filled with.

The avg OEM mpg is the average mpg I got on petrol, stock tires, stock everything except air filter. The avg e85 stg2 mpg is the average mpg I got on e85, stock tires, stock everything except air filter, OFH and OFT stg2 tune. The avg stg2 mpg is the same thing but on a 91oct tune.

Too many variables.
Transmission.
Type of driving.
Air pressure in tires.
Driver.
Location.
Fuel matters.
Here is my fuelly http://www.fuelly.com/driver/swarbrz
I gave up after a few tanks of e85 as the website uses current fill up to calculate costs, and previous fuel to calculate mpg, which throws some metrics off.
I was right around 28mpg 90% highway at 80mph on 91octane manual transmission.

BRZ MPG Data Analysis
 

Am guessing the car computer excludes stationary idling fuel consumption; therefore the average will read better overall.


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Hi all, UK owner here and from my experience, I can confirm that the dash always shows a higher figure than the actual mpg. This isn't just the BRZ that does this, but pretty much all cars. If you have a sat nav that shows your actual speed, you will see that you are also driving slower than the car reports.

As an example, a display from the car of 70mph is actually only 66mph.

As for fueling, a UK gallon is slightly bigger than a US gallon (something like 1 UK gallon = 1.2 US gallons) so my mpg figure is going to show slightly higher than what you can achieve in your cars.

The BRZ manufacturer's figures show that an extra urban figure of 44.1mpg is achievable and in most cases, you can never achieve what the manufacturer's figures show because the test is carried out in perfect conditions (e.g. exactly one gallon of fuel, no extra passengers or weight). In a recent 440 miles journey which I completed on one tank of fuel, the car reported a 47.3mpg return, but the actual was only 45.4 meaning that the car's figures are 4.2% out.

This is slightly disappointing, but I was very pleased I was able to beat the manufacturer's figures. :thumbup:

I've been tracking mine in my Owner's Journal Post 1 since I purchased the car (6AT) in 05/2012.

Results so far:

77,057 miles. 198 Fuel Stops.
Average Mileage Overall (Calculated): 32.72MPG
Average Mileage Overall (Computer): 33.80MPG
Average Fuel Cost $0.10 per mile.
Best Average: 36.6MPG (All Highway)
Worst Average: 27.6MPG (About 50% stop and go city)

Most of my travels is commuting which sees an average speed of around 45MPH.

With the original Primacy tires I averaged 32.65MPG (around 62,000 miles). Surprisingly, once I switched to Bridgestone Ecopia all-season tires (@15,000 miles) I've average 33.40MPG.

I've also "saved" $5,184 and 1,655 gallons of fuel over my previous daily driver that averaged around 18MPG even when adjusting for the difference in regular vs premium fuel.

I only live 11km from work and used to let the car warm right up when I first started driving it a year back. My fuel economy back then was pretty terrible vs now when I simply wait for the rpms to settle down to around the 700rpm mark then drive off.

Pretty sure the lengthy idling during warm up was / is being calculated by the onboard average fuel consumption metrics.

Same here.
Last winter was cold and snowy so mine sat long enough for the defroster to melt some ice. I averaged (by computer) 7.3 liters per 100 kilometers (32 mpg). This year it has bee almost ice and snow free so I jump in let it drop from high idle and drive off and have averaged 6.8 L/100Kms (34 mpg) . My driving is very regular and my average MPG does not waver one little bit so the only difference between the two is how long the car sat idling last winter.
The 6.8 figure is also my summer average when the car never idled past high and has not moved from that number since last April.

Idling would definitely impact your mileage, but that would be the case regardless of whether the computer is doing it or you do it manually.

Imagine the mileage if you filled up, drove one mile and then let it run until empty! Wouldn't matter if the computer calculated it or you did, it would be atrocious.

Assuming you filled the tank to exact capacity (13 GAL), then drive exactly one mile, it would be 0.076923...MPG

Practically though, you would most likely be marginally worse than that because fuel capacity does not normally take into account the fuel filler so you would burn slightly more that 13GAL. Also, I'm not sure if capacity is listed in USABLE fuel or overall capacity when talking cars.

I was too lazy to do the math so at this point you are 100% more ambitious than I.