DWPC

For those interested; just drove my 06 CTT (58K miles) from Sedona AZ to Boulder CO via Albuquerque: calculated the old fashioned way 16.8 mpg; trip computer mpg 18.4 (I knew it was way off!); indicated avg speed 68 mph (includes local miles miles during stops - actually held 75 to 85 on the interstate. Our first long trip in the CTT...what a super cruiser.

JohnnyBahamas

+1

I recently did a 520 mile round trip from Tahoe to the San Jose and back in 8 hours in my '06 (55900 miles) with the return trip in heavy rain. She dominated the road.

Ultimate highway machine.

AdMan21

14.9, 2004 CTT, all city.

Cole

Just curious how you are calculating this? Off the odometer? How can you be sure its correct? (Vs the trip data the trip meter is calculating with?)

gnat

miles/gallons = mpg

Unless he's done something crazy with his wheels/tires the ODO should be almost dead on. Your speedo, on the other hand, should be of by as much as 10%.

Both my speedo and PCM econo data exaggerate by about 8% which is within Porsche's specs so they won't do anything about it. This is actually the worst car I've owned in that regard. Normally they've been about 4% and the best was 2%.

Cole

I know how to do the math

But its stupid to assume that one source of data is dead on and the other I'd off when being supplied by the same source!

Some people use a GPS to calculate these numbers on trips. So wondering what his source was. I it was simply the odometer I would seriously question the data.

Not sure why people think their hand calculated data is more accurate when they are calculating it off the same data the trip computer is.

The real variable is what source is the cars computer using for the amount of fuel? I'm assuming that you guys are calculating it off of random gas station pumps........how accurate ore those and are you getting the exact same amount in every time? Just the difference in temperature and the expansion/contraction in the volume of fuel and when a pump shuts off could cause a variance.

Slow Guy

Same source being the vehicle? Then that would also apply to the trip computer too.

This has been an issue w/ BMW's for years and yes the speedo is off (reading high) but the odometer is dead on because this affects warranty issues. If the odo was off there would have been a class action suit long before now.

Speedo's/odo's haven't been driven by gears turning a cable off the back of the transmission in years, they're now run electronically which makes it very easy to "fudge" one number and not the other. As I said, if they were fudging the odometer numbers they would open themselves up to some serious warranty (lawsuit) issues.

Continue believing the figures the "trip computer" are telling you if you wish but I would suggest verifying w/ mileage/gal used calculation, I seriously doubt they would be open to lawsuits if that calculation was off.

DWPC

?? If the odo is off far enough to matter we've got a great fraud case against PNA.

Cole

You guys put an amazing about of faith in the accuracy of individually owned fuel pumps and your ability to fill the same density of fuel everytime, but are very quick to judge the trip meter.....its kinda hilarious.

Go fill up at the same station after the same miles driven, with the same pump, at the same temperature, and then compare them. I would bet the margin of error narrows.

gnat

Actually I recognize that the hand calculation is off too, but not by nearly as much. The problem with the calculation is that you don't really know how much was used. You are making the assumption that on the previous fill you got it to exactly 26 gallons and you are making the same assumption for the current fill up to get the gallons used. The reality, however, is that there are all kinds of reasons that your fill isn't as exact as is desired (including tampered pumps).

Contrary to most people's views, I drain my tank pretty damn dry (filled 26.05 gallons once ) so I'm reasonably sure that there is something close to the full amount in there each time. My fills are also consistent (so it's not like I had an air bubble that only let me put 23 gallons in one time and 25 the next). Based on the math I would consistently have to be burning multiple (as high as 5 in one case) gallons less than I am to get the MPG figure the PCM/MFD tells me I get.

Furthermore we can take a trip I did today as an example of the PCM's math ability. When I started off I had already put 300 miles on it and the overall average (I reset the total meter at each fill up) was reading 29mpg. I went 10 miles round trip and finished with a 26.4 average for the trip, yet the overall average dropped to 28.7. Either the current trip or overall values are obviously lying as 10 miles at 26.4mpg can not throw off a 300 mile 29mpg average by that much. If the overall value is to be believed then the 10 mile trip actually would have been 19-20mpg. Yes I have taken it to the dealer and complained about it. They've checked it and find it to be functioning normally as the variations are within allowable tolerances.

Additionally in regards to your "same source" argument. In the 958 (don't know about the older ones) you can view your trip meters both in the MFD and PCM. The two never agree for me. The MFD is always 0.2 higher than the same information on the PCM screen. My original dealer claimed that they were coming from a different source, but couldn't back up such a claim (that was the last time I talked to that dealer). My new dealer looked at me like I had a second head when I recounted that conversation... (which is about how I looked at my old dealer when he said it to me)

Slow Guy

Last post on the issue for me
1) Fuel pumps at the gas stations are regulated/tested. Are they 100% accurate, no, but I believe what multiple pumps/calculations are telling me over an (un-verified) on board computer any day.
2) there is no need to completely empty a tank to get fuel mileage, just fill it to the same level when you check.
3) How much do you think the density of fuel changes over 30-40 deg.? Enough to give you a 2 mpg difference in your calculations? I seriously doubt it.

gnat

1) There is typically one agent doing the testing for a few hundred square miles. In high density areas a station can go multiple years without being checked. While the cases of malicious tampering are pretty low, lax maintenance on the pumps can also cause discrepancies.

2) No there is not. I figure since I paid for it, I'm gonna use it

3) Actually there can be a surprising amount of difference, but no still not enough to answer for the gulf between the two measurements.

Cole is correct though. We shouldn't blindly trust the gauges (as Needsdecaf will attest regarding the gas gauge ). You should periodically check your odometer against a measured mile to verify it's still correct. There are regulations governing the accuracy of the odometer, but that doesn't mean they don't break.

mcbit

Measured against the GPS in my Kenwood unit over a 20,000 km service interval I did not get more than 100 kms variance between that and the ODO reading in my old 05 CTT. At 140 kph (GPS) the speedo is reading 147 kph and at 251 kph (GPS) the speedo reads 261 kph, so there is a speedo error but its not a percentage. Cayennes with factory GPS recalibrate the ODO and speedo after tyre changes. My new to me C4S has 91,000kms on the ECU and 96,000 on the ODO so go figure.

All the fuel pumps over here must be calibrated and verified on an annual basis; not the same in the states?

I would always take the hand calc over the computer but I would be looking at an average over time and would not particuarly believe the evidence of one trip for the reasons outlined by Gnat.

gnat

Ours are supposed to be too, but such regulatory bodies are often under-funded and (consequently) under-staffed to be able to enforce their stated goals. In this case they do a lot of relying on people to complain about bad pumps and then they'll go check. Most drivers, however, can't tell you how big their tank is or what they expect a fill up to be so they have no idea unless it's really egregious.

mcbit

But in reality the whole thing is pretty easy to regulate, in most places fuel vendors are licensed and the onus is put on them to to supply the certification. If they don't supply submit the certification they are visited or their licenses revoked.