Poor VW Bora TDI 115 mpg during cold ambient temperatures.

Well, there ya go. I live in Connecticut. Is there a winter mix for diesel as well?

Michael

There is, but I don't know how energy density is affected for winter diesel. It's definitely less for winter gasoline.

-- Mike Smith

Right. It couldn't be that your ability to *convey* sarcasm is not quite what you think it is, or that sarcasm generally doesn't carry well in a textual medium (thus the emergence of winkies and smilies et al). No, it has to be that Americans are stupid. Lovely. Just doing your bit for peace, love, and understanding, I guess.

-- Mike Smith

Calm down Mike. Please don't put words into my mouth, I certainly don't think that Americans are stupid. I was just hoping for an answer to my question not a discussion on the realtive temperatures of the UK and New York.

A question of clarification, are the mpg readings you mention based on a calculation of the fuel purchased at the petrol station and your odometer reading since last fillup? Or are the readings from the in-car trip computer? If the latter, I understand from an auto electronics technician that the trip-meter estimate the mpg based on car's speed, engine rpm, and vacuum pull. It could be the trip computer is getting some faulty data that is temperature related. Just a thought.

However, if in fact the mpg you mentioned are based on actual fuel purchased and miles driven, then that's an interesting relationship with temperature.

cheers.

The mpg values were calculated from the actual volume of fuel used and distance travelled. The values corresponded with the mpg displayed on the in-car mpg meter.

TDI has no vacuum. It's a diesel.

There may be other factors that haven't been measured. (aka "confounding factors") Just because they're not being measured, doesn't mean that they don't exist.

A diesel without a charger must have a vacuum. Otherwise air would not pass thru the air filter.

With the turbocharger, it may show a pressure, but you could say the same for a gasoline engine.

It has less vacuum than a gasoline engine at WOT. And it doesn't vary with fuel consumption.

The turbo complicates things. But only if you're relying on manifold pressure in the first place.

The TDI probably uses a fuel consumption signal from the ECU; which is probably based on the effective duty cycle of injection. It's then a matter of using distance pulses (as per speedometer and odometer) to calculate fuel consumption over distance.

i.e. the fuel consumption displayed is based on what the engine management system thinks it's putting into the engine, and how far the consumption compter thinks the car has moved.

The mpg values were calculated from the actual volume of fuel used and distance travelled. The values corresponded with the mpg displayed on the in-car mpg meter.

Did you fill up at the same station, using the same pump, and top off the same amount?

We bought fuel from various service stations on the outward and return journeys and I always filled the tank to determine exactly how much fuel was used. The improved fuel efficiency of the car (confirmed by the mpg meter) was limited to stretches of the journey which had higher ambient temperatures. Once we got back to the UK (a blistering 7 degrees), the economy has been excellent and I'm still using the same diesel that was in the car when it was returning lower mpg economy.

Weird. Do you usually talk that way?