Richard320
Spends too much time on here
- Joined
- Aug 3, 2019
- Messages
- 2,182
- Reaction score
- 2,324
- Points
- 113
Like the title says, I’m still trying to figure out hoe the dashboard calculates the mpg and range. You’d think that once you reset things, that the mileage calculation would start there. But anyone who has been watching his dash knows this isn’t so. Fill the tank, reset mpg, and within yards it will show some small mpg and then soon after take an enormous leap. How are they calculating mileage and range? This really bothered me. So I started paying attention.
Turns out I had never reset the trip odometers (at least intentionally). I had something like 4000 miles on Trip B, which showed the same mpg displayed on the economy screen. But then things made no sense --- if I was showing (for example) 20 mpg on Trip B and then reset the mpg screen, it should show the same cumulative mpg on Trip B, but it didn’t.
So I started thinking…. Is it time? Maybe the last six hours? Or is it for only the last X miles? Either of those would account for the discrepancy between the displayed mpg and the hand-calculated value. I started to experiment. I reset Trip A but left Trip B alone. Trip A started low and began climbing. Trip B, having a 4000 mile headstart, changed slower. My hypothesis was that the difference between the two would shrink until the preset time or certain number of miles had elapsed. It seemed to be 180-something miles. I missed the exact moment when they matched; I was probably watching the road. 186 miles seemed odd, but 300 km wouldn’t be! I thought sure I had it. The two trip odometer mpg readings should match from that point forward.
But they didn’t.
I tried again, resetting everything to zero. I took a fairly long trip. I gassed up before the return trip. The two trip odometers showed 20.7 and 20.5 mpg. I gassed up again and zeroed Trip A. At 270.8 miles, both showed 21.1 mpg. Is 270 miles the magic number?
No. Because almost immediately the two values started differing again.
Another trip and at 490.2 and 953.9 the two aligned again at 23.0 mpg. This is not possible if the calculation is over the miles since reset. To bring it up to 23 over 950 miles would mean that the average economy over the 220 (The current 490 minus the 270 from before) would have to be closer to 30 to push the average up, but it was only showing 23!
Today I filled up again at close to half a tank. Before zeroing things, I showed 21.3 mpg over 974.0 miles and 20.7 over 510.3 miles. That’s a pretty sizeable drop from 23.0. Yet I was just making short trips in cold weather. Lots of idling at lights with a cold engine. This 20.1 miles driven used either 4.26 gallons or 3.35 gallons. Mathematically impossible.
This difference in displayed economy is a pretty sizeable discrepancy. However, when manually calculating the fuel economy over the 510 miles -- two fillups of 11.876 and 12.438 gallons – I get 21.0 mpg. Exactly midway between the two values.
What does it mean? I still don’t know. It sure would be nice to know over what distance the mileage is calculated and thus, how much once can trust that estimated range. Research will continue. If you have any input, please let me know.
Turns out I had never reset the trip odometers (at least intentionally). I had something like 4000 miles on Trip B, which showed the same mpg displayed on the economy screen. But then things made no sense --- if I was showing (for example) 20 mpg on Trip B and then reset the mpg screen, it should show the same cumulative mpg on Trip B, but it didn’t.
So I started thinking…. Is it time? Maybe the last six hours? Or is it for only the last X miles? Either of those would account for the discrepancy between the displayed mpg and the hand-calculated value. I started to experiment. I reset Trip A but left Trip B alone. Trip A started low and began climbing. Trip B, having a 4000 mile headstart, changed slower. My hypothesis was that the difference between the two would shrink until the preset time or certain number of miles had elapsed. It seemed to be 180-something miles. I missed the exact moment when they matched; I was probably watching the road. 186 miles seemed odd, but 300 km wouldn’t be! I thought sure I had it. The two trip odometer mpg readings should match from that point forward.
But they didn’t.
I tried again, resetting everything to zero. I took a fairly long trip. I gassed up before the return trip. The two trip odometers showed 20.7 and 20.5 mpg. I gassed up again and zeroed Trip A. At 270.8 miles, both showed 21.1 mpg. Is 270 miles the magic number?
No. Because almost immediately the two values started differing again.
Another trip and at 490.2 and 953.9 the two aligned again at 23.0 mpg. This is not possible if the calculation is over the miles since reset. To bring it up to 23 over 950 miles would mean that the average economy over the 220 (The current 490 minus the 270 from before) would have to be closer to 30 to push the average up, but it was only showing 23!
Today I filled up again at close to half a tank. Before zeroing things, I showed 21.3 mpg over 974.0 miles and 20.7 over 510.3 miles. That’s a pretty sizeable drop from 23.0. Yet I was just making short trips in cold weather. Lots of idling at lights with a cold engine. This 20.1 miles driven used either 4.26 gallons or 3.35 gallons. Mathematically impossible.
This difference in displayed economy is a pretty sizeable discrepancy. However, when manually calculating the fuel economy over the 510 miles -- two fillups of 11.876 and 12.438 gallons – I get 21.0 mpg. Exactly midway between the two values.
What does it mean? I still don’t know. It sure would be nice to know over what distance the mileage is calculated and thus, how much once can trust that estimated range. Research will continue. If you have any input, please let me know.
Last edited:
