michail71

I had a 500 mile drive today, mostly all highway, with some interesting results on the trip computer and the range estimate.

After 500 miles of driving the estimated range was 120 miles. So in theory my range would have been 620 mile.

The first half of the trip I had a lower MPG, around 28-30 or so. Half way through (the return leg) I reset the trip computer. At the end of my drive it read 33.2 MPG.

So my "from start" trip computer was 500 miles at 30 MPG. The second at 33.2 was for the last 250 miles.

Now if I take the 500 miles driven plus a conservative estimate of 100 miles remaining it would equate to 35.3 MPG. Which actually seem to be where the real time usage meter seemed to hover around.

So if my roughly 30 MPG average for the whole trip was accurate my gauge should be on empty.

Sorry for all the math. But if the odometer and fuel gauge are to be trusted it seems the trip computer would have to be showing MPG on the low side?

Alfadude

The distance to empty is based on how you are driving the car at that very instant and can vary wildly depending on how you are driving (spot and go at low speeds vs. steady state at 65mph). Had you continued your journey until empty I bet the numbers would have pretty much matched up. Seeing as you stopped I will bet that once you get in the car again and start driving that distance to empty will go down really fast unless you quickly get up to temp and hop on the freeway again.

michail71

Yes, I do expect that to happen. But in my probably confusing math "word problem" I had a 2-3 MPG discrepancy between my actual consumption and reported consumption.

Even on the start my top range showed about 530 miles (going by memory). I completed the drive in exactly 500 miles with 1/4 tank left. I should probably top off first thing today to get a good fuel reading as I don't think the gauge is linear.

Perhaps I could be off in that it appears the range does not count approximately the last gallon of fuel?

It was a long drive, I had to have something to do, lol.

michail71

Ok, did the fill up and here are the results.

13.76 gallons of fuel were added back into the tank.
Trip odometer was at 502 miles.
This comes out to 36.5 MPG

Total trip MPG on the computer was roughly 31 MPG. Second 250 mile leg was 33 MPG.

Range was reading 120 miles at fill up. This actually corresponds with my real MPG given about 3.5 gallons remaining in the tank.

So the trip computer seemed to be showing about 5 MPG less for the the entire trip. This also fits with observations I've been making that I've been going further between fill ups than what was indicated by the trip computer.

I know the Brits have a different sized gallon compared to the US. Perhaps there is a mix-up in the system? I just don't remember which direction that goes.

michail71

OK, one British gallon equals 1.2009 US gallons.

So 30.5 * 1.2 comes out to 36.6 which is damn close to my old fashioned measurements.

Alfadude

The most accurate way to do it is fill up your tank at the same station (and preferably at the same pump)over several tankfuls. Fill it until the pump clicks off and don't fill it anymore. Then see how far you went since your last tank and see what the actual MPG is. That will give you a pretty good idea how it compares to the trip computer. The trip computer always seems to be about 35-5% different than actual mileage in my experience. I consider teh trip computer a reference and nothing more. Gas gauges always seem to be far from linear. They always take longer to go from full to halfway and faster from half to empty.

michail71

Very true, I'm going through my next run right now. But I've watched the range for a while and it tends to be accurate. Also, the numbers have seemed consistently on the low side for me. Which is why I did this test on the long drive.

I've also logged fuel levels through the ODB-II port and I would say the car seems to have a very good idea of the fuel.

Based off of my initial measurements I would say there is a good possibility this is a software bug and it's reporting the British gallon on the trip computer's summary and that the other values are correctly in US units.

This could be a setting somewhere that the dealer may be able to change. I have an appointment on the 11th so I will bring this up. Oddly, one reason for my appointment is the navigation system randomly switches between metric and imperial measurement.

michail71

Being a software engineer this stuff gets to me. On my last car I found out it reported the speed as 4 MPH faster than it was but the odometer was dead on accurate. Over a 100 mile stretch my GPS and odometer were in sync but off by 4 MPH. This means they purposefully added error into the speedometer.

So if the speed limit was 70 you had to be set on 74. It was a VW so ...