- Filled the tank and reset trip
- Drove 321.3 miles
- Filled the tank; took 11.087 gallons
- Computer shows 30.9 MPG
- But 321.3/11.087= 28.97 MPG

Why the discrepancy between manual calculation and what the computer shows?

Has anyone else also experienced this?

 

Every car with OBC that I've had, across different manufacturers, has been optimistic in calculations. I don't know whether it's a big conspiracy, but it's typical.

 

I think computer shows Average MPG. Your calculation showed Actual.

 

Seems to me that the OBC was within about 6% of actual. I wouldn't expect much different.

Also, some gas pumps shut off earlier than others depending on the speed that they fill the tank. Slow pumping ones seem to fill the tank more than a fast one.

Try filling up at the exact same pump twice in a row to further standardize your calculations and remove a variable.

 

You have a defective car! Reject!!!

Seriously, the computer does a good job in estimating the mpg's but is not 100% accurate.

 

Wow 4 responses in 11 minutes guys.

I drive a lot so I'm careful about such calculations. Hence, not only did I use the same gas station, I used the same pump

Last 3 cars I've owned never gave me more than a 2% discrepancy, so 6.6% is a pretty large variation.

Either way, what should be considered the correct scientific MPG in my case, 28.97 or 30.9?

 

YOur manual calculation is the more accurate.

 

The mileage you calculate from the pump would be the accurate MPG--that is the actual measured mileage.

 

In that case 28.97 MPG average is pretty crappy . It included 65% highway & 35% city driving and I was "behaving" as I'm still in the break-in period.

Should I expect the mileage to get better with time?

 

Yes, you should.

BTW, how come your dealer didn't send you off with a full tank???

 

Roughly how much should the MPG be expected to increase after break-in?

I did get a full tank of gas when I bought it. Just didn't bother to calculate the MPG for the first tank as I was too excited

 

I would expect 2-3 mpg as things get "loostened up" inside. Depends on your driving and trip profile, of course.

 

The gas mileage will improve once all the mechanicals seat in. Every trip computer have had in any car could not be trusted for accurate numbers.

All you can do is try to make sure the things the trip computers uses are all up to snuff.

Like, the air pressure in the tires. Making sure the tires are the exact size that came from the factory. Using the same pump to fill with is good, but make sure to use the slowest fill rate. The pump pressure will change, depending on how many others are at the station filling when you do.

A slow fill insures the auto-cutoff will be at about the same place every time.

Spend as little time idling as possible, This throws computer calculations off a bit. This one is probably why most are off. Have you been idling a lot? The more idle time, the less accurate the computer is going to be. It also reduces the calculated fuel mileage as well.

 

It would be extremely difficult to get the most perfect MPG from any reading unless you go to a gas station that was inspected like the day before! :P

Gas pumps become uncalibrated themselves so sometimes they are off as well.. but the OBC tries its best sometimes its much more optimistic.

I filled up the other day and was 400 miles till empty.. Got on the highway and drove 30 miles at 65 mph and my fuel went to 455 miles till empty.. I gained 55 miles in my tank! :P

 

my obc on my r53 MCS is usually within 1 mpg of my calculated mpg. of course don't ever get above 25 mpg unless its all highway driving.

 

Was your computer attempting to give you a total MPG since your last fill-up, or an average over the last N miles?

 

My 1st tank of fuel showed the OBC to be slightly optimistic too: 32.0 MPG actual vs. 32.3 MPG via OBC.

 

The OBC calculated the average from some point in the past, to make it something of a running average. Even if the OBC is 100% accurate, it would be off compared to a tank-by-tank measurement.

 

Man the people getting 32mpg must drive conservatively, or just lots of highways..

I havent gotten past 30.6mpg

 

I have noticed a pleasent computer increase per tank
1st tank of gas (from the dealer) obc said 28.9 mpg, all city driving

2nd tank obc says 30.7mpg, again all city, it was warmer out, if that makes a difference?

my average speed is somthing like 29 mph, never got it over 30!

 

Well, you have to admit that's pretty good for all city driving...take her out on the open roads and you'll go up from there.

 

It was surprising to learn that now w/ 2000 miles on my 07' MCS the mileage averages are actually very consistent w/ between the OBD & my own math. Still can't do better than 34mpg on the highway.

 

We assume you zero'd both gages at the same time?

 

be sure to reset the average when you fill up... or the computer will just keep a running average....perhaps for the entire life of the car if it's never been reset!!!

 

Of course I reset the MPG after fill-up guys...