Falling MPG any ideas?

Mr_Mink

I have noticed the mpg of my little Up has been steadily dropping over the past year/year and a half and was wondering if anyone had any ideas on what I should be checking.

The car is a VW Up, the 74HP engine. Mileage is around 33,500. The car has been serviced every 10,000 miles. The car was returning 62 mpg mid to late last summer. This year, the same route (with a little less traffic) and I'm getting about 51 mpg.

I keep and eye on the tyre pressures and they're fine (not that I think a low tyre could cause a 11 mpg drop anyway). There's never any clutter in the car. As far as I'm aware my driving style has remained the same. If anything there's a bit less traffic to deal with each day.

I'm a bit at a loss as to what could be causing this problem and how to/if I can fix it. If anyone has any ideas I'd be grateful.

forgotmyname

Is this the trip computer saying its 11mpg worse or your working it out at each
fillup?

Changes your tyres or brakes in that time? Less traffic = your going a bit faster than you used to?

Whats the service interval on the air filter etc?

Norman_Castle

Due to wear, the circumference of the tyres is now smaller which increases the number of revolutions per journey. This will increase the fuel consumption.

AdrianC

You're getting 51mpg in November, versus 62mpg in summer? Or is that 51mpg an annual average?

If it's a seasonal change then - yep - I'd expect that, especially if you're doing shorter runs. You're putting more load on, with electricity being required for lights, wipers, heater fan, rear window. You're starting with the engine colder (so "more choke") and it's taking longer to heat up, because the heater matrix is shedding more heat.

Relatively big numeric changes can happen from smallish changes in the amount of fuel used, when you get to relatively good economy.

62mpg is 72ml of fuel per mile.
51mpg is 88ml of fuel per mile.

Strider590

Wouldn't surprise me if it's got a "time for a new car" light.

But seriously, you might want to check the coolant thermostat.
Basically does the heater work quickly? and does the engine stay above 70 degrees when driving?

It's a mechanical device that diverts coolant into the radiator once the engine reaches operating temperature, if it jams open then the engine will never get warm and if engine doesn't get warm it'll use more fuel (10% or so more).

Mr_Mink

Well, it is the trip computer. I did once do the math when it was claiming 62 mpg and it actually tallied up so I've taken to trusting it's claims. I've also noticed that when I top up (always do this on a Friday on the way home from work) I used to have a 1/3 or more of the tank remaining, nowadays i'm around a 1/4 of the tank (I always just fill it full).

As for tyres and brakes, 4 new tyres just over a month ago and new disks and pads at the front about 3 months ago.

I couldn't say for certain as I don't have the printouts from the services to hand but my understanding is that the air filter was changed at this most recent servicing at 30,000 miles.

almillar

Trip computer isn't good enough. Most accurate way to measure it is to empty the tank (well not quite) and fill to the throat. Use the little odometer in the car and reset it when you fill up. Convert litres bought to gallons (/4.5 ish) then divide the miles done by the gallons you've just put it. That's the MPG for the tank you've just used. Repeat.

Mr_Mink

Ah, couple of replies whilst I typed that.

Adrian, the mpg's are on a month to month basis, comparing this November directly to last November is around 57-58 mpg last November to the 51 this November. I know it's a bit colder this year and the little thing doesn't like the colder, but it still seems a fairly big gap.

Stryder
The heater sadly has never been the quickest (I've always just attributed that to the poxy little three-pot engine in it). Is checking the coolant thermostat an easy enough job to do or am I better off sending the car to the garage for that to be checked?

bryanb

almillar wrote: »

Trip computer isn't good enough. Most accurate way to measure it is to empty the tank (well not quite) and fill to the throat. Use the little odometer in the car and reset it when you fill up. Convert litres bought to gallons (/4.5 ish) then divide the miles done by the gallons you've just put it. That's the MPG for the tank you've just used. Repeat.

Agreed, but why empty the tank?

AdrianC

New tyres, y'say? Same brand and type as before?

NeededToChooseAUsername

New tyres... have they been balanced correctly?

Edit: Check your tyre wear... although probably not easy to notice if they've just been done.