What would cause this?

Mr Jack · 5 Dec 2006 at 22:24

This morning on my commute from Coventry to Derby my new car's trip computer reported that it averaged 63.5 mpg, on the way back this evening it reported that it averaged 52.0 mpg. I set the cruise control to the same speed both ways, and it returned similar results last time I did the journey.

Why would this be happening? :confused:

One More Solo · 5 Dec 2006 at 22:25

Gradient or wind perhaps?

Caerdydd · 5 Dec 2006 at 22:27

Could be air conditioning settings, or driving uphill vs downhill

Also i'm sure there were places that you didn't use cruise control. Accellerating quicker uses more fuel.

Alternatively your trip computer might just not be that accurate

Mr Jack · 5 Dec 2006 at 22:36

I didn't have air conditioning on either way. The only extra electricals I had on the way home was the lights.

The place I was going to in Derby is 50m below my starting point in Coventry, but could an average gradient of 1m to the mile really make that much difference?

Yeah, there were some differences in traffic; I get more in the morning. But the bulk of my journey is spent cruising along the M69, and then M1. I would have thought I'd get better fuel economy with less traffic?

Rhyzz · 6 Dec 2006 at 00:07

Weather is probably the culprit but trip computers aren't always spot on, they can be a few mpg out depending on how quickly you accelerated to the speed you're maintaining, etc.

Take the 2 numbers and average them out, that's your approximate mpg

Mickey_D · 6 Dec 2006 at 07:40

Did you have the windscreen defroster on? That will automatically run the A/C pump to act as a dehumidifier, hence lessening the fuel mileage.

Baron_Samedi · 6 Dec 2006 at 08:31

Conversely, for the same journey I find my MPG better on the home run as opposed to the outward journey. Given our weather at the moment (roughly the same at each end of the day) I put the difference down to the hills etc on the roads present at the beginning of the trip affecting the MPG as the car warms up.

For example, when I set off in the morning there are a number of inclines before the roads level off, but coming home the road is much flatter and as such the trip MPG climbs quicker on the return journey.

airchie · 6 Dec 2006 at 09:10

I heard somewhere (can't remember where) that accelerating hard to your cruising speed uses LESS fuel than accelerating there gradually.

Kinda makes sense, accel hard uses more fuel per sec but not much more (depending on car etc) but gets you to cruising speed much quicker.

Tesla · 6 Dec 2006 at 09:46

airchie said:
I heard somewhere (can't remember where) that accelerating hard to your cruising speed uses LESS fuel than accelerating there gradually.

I don't really agree with that, surely fuel consumption is related to engine speed, not road speed? (Assuming you are in the most efficient gear?)

paradigm · 6 Dec 2006 at 09:52

airchie said:
I heard somewhere (can't remember where) that accelerating hard to your cruising speed uses LESS fuel than accelerating there gradually.

Personaly experience would tend to agree with you, a quick blat at ~10MPG getting me up to cruising speed/throttle (~37MPG), actually means i have a higher overall MPG than slowly getting to 70MPH at ~20MPG.

Building Candy · 6 Dec 2006 at 09:58

Did you have a particularly heavy lunch

Increased fuel consumption due to hauling a larger belly?

lordrobs · 6 Dec 2006 at 12:10

Whatever the case I'd be happy if my MPG was anywhere near either of those figure

Mickey_D · 6 Dec 2006 at 17:19

lordrobs said:
Whatever the case I'd be happy if my MPG was anywhere near either of those figure

Oh, no kidding!!! I've been driving the truck the last couple days and I'm looking at 1/6th that figure!!!