Poor First MPG Computer is 12MPG Off

Aggregate at least 3 tanks for meaningful results

 

In the Gen 2 Prius, it took as much as 3 Gallons for the gauge and computer to notice you had filled up. I understand that the Gen 3 requires 2 gallons. I have no minimum for the Prius c, but I bet there is one, and it is over 1 gallon. You may have confused the computer on that tank.

 

Noticing a fillup only impacts the automatic display of the price adjustment screen as far as I know.

On that score though, I put 1.63 gallons in today and this is the first time that I recall the price adjustment screen not coming up. This was my record smallest top-up... amusingly it comes right after my record largest (which was 6.8).

 

My C2 shows me doing between 58 and 60, but my fuelly.com record (partial fillups using odometer reading) shows 53 mpg each time (4 fill ups so far).

I'm still testing...

 

I read somewhere on another forum that overfilling the tank might cause the computer to miss read the amount of gas on the tank resulting in bad calculations. Your tire pressure is way to high, at most you should do 40 psi, ideal 38. Car comes at 32-34 from dealership.

 

On my Gen 2, different brand/models of tires 'liked' different tire pressures. If it works for you, go for it.

 

I tried out the 40/38 tire pressure, but so far didn't notice anything. Going to go back to standard pressure and see what happens. Also, I never overfill my tank, so that's not an issue for me.

I started to note my trip computer reading in my fuelly logs. I am showing a consistent 4 mpg loss from the car's calculations to the real thing. I've heard in this forum that it has to do with the inherent inaccuracy of the speedometer.

 

Welcome!

First rule of Prius ownership: Never top-off the tank. It won't be accurate if you do.... which you have now observed firsthand.

To stand a shot of it measuring right, you pretty much have to wait until 3 gallons or more are needed.

 

Thank you.

I said in my post that I never top-off the tank. I don't do it in any car, ever. I fill it till the gas pump clicks them I'm done. Also, I use up 60% of my tank every day, so my fuel ups are 6 gallons or so every day (as you can see in my fuelly log). I think right now I am going to try and see how close I can get to the car's own mpg calculations as my next test, through A/C use and tire pressure maintenance (which I do weekly).

 

There's actually more than one meaning of "top off". Sorry to confuse.

Going beyond the click is one. The other is filling up long before it is actually needed... which is more often the common thought, since Prius offers much greater range for a medium-sized tank than other vehicles.

So, same advice, but instead remember: "at least 3 gallons"

 

try slowing down if you can, speed kills mpg's.

 

I got a C Four two weeks ago, and computer mileage has been consistently off by about 5-7 MPG. This is worse than the 2011 Highlander 4WD I traded in (off by 2-4 MPG). My other vehicle is a 2012 Sienna AWD and is off by 2-4 MPG as well. The incorrect calculation seems to increase with increased actual MPGs.

BTW: I am getting 50 MPG with just under 1,000 miles on the odometer - 45 to 55 miles per hour each trip.

 

With 250mi. on my new C/3 I drove 540mi. and averaged 41mpg during this first long range trip.I`m noticing now with 2100mi. on the car my average both city and hwy are increasing nicely.Considering the alternatives(Diesel,Plug-in)we still have the best economical mpg vehicle available today.Enjoy.

 

I found that there are too many variables in calculating mpg. For example, I drove my route once using no economy-minded city driving, and 50 mph on cruise control on all highways. A/C was on most of the day. I got 63.5 mpg (the car said 67.9 mpg).

I ran the same route again the next day, this time I drove economically in the city (braking with the regen as much as I can, and never hitting the PWR band when accelerating), and again sticking to 50 mph maxiumu on the highway using cruise control. I also barely used the A/C. I ended up getting 62.4 mpg (car said 66.8).

Due to how one run seems more efficient, yet netted 1 less mpg, it means that if I want accurate gauges of my driving style and conditions, I'd have to test each style/method over a longer period, like a week, instead of just one day.

Regarding speed, I stick to speed limits in the city, and on the highway, I've tried 50, 55, 60, and 65. The biggest observation is that 50 mph gets me 62/63 mpg (as stated above), which going to 60 mpg drops it to 57ish. To me, going below 60 mph is not worth the few mpgs vs the headaches from people on the road who behave irrationally and badly because they think you're going really slow.

 

The weather plays a part, too. Our weather has been between 0-50 F, with some very windy periods. I can lose 5 MPG easily on cold, windy, and rainy days. Wind seems to affect things the most, however.