A maths question

[sparkythecat]

My car trip computer recorded that I’d averaged 41.3 mpg over a distance of 720miles
I then hitched up a heavy trailer and drove 109 miles.
At the end of that journey the computer recorded that over a total distance of 829 miles I’d averaged 38.1mpg

What was the fuel consumption of the car when it was towing the trailer?

[TheInternet]

How many gallons of fuel did you use to go 720 miles?
How many gallons of fuel did you use to go 829 miles?

[ianrb]

25.17 mpg

[mikeiow]

sparkythecat said:

My car trip computer recorded that I’d averaged 41.3 mpg over a distance of 720miles
I then hitched up a heavy trailer and drove 109 miles.
At the end of that journey the computer recorded that over a total distance of 829 miles I’d averaged 38.1mpg

What was the fuel consumption of the car when it was towing the trailer?

720 miles at 41.3 mpg is 17.43 gallons
829 miles at 38.1 mpg is 21.76 gallons

Your 109 miles took (21.76-17.43) 4.33 gallons....
...so 25.17 mpg

Heavy trainer indeed!

Unless I've missed something obvious....

[woodlands]

quite a simple calculation.

for the first 720 miles you consumed 17.43 gallons of fuel
for the total 829 miles you consumed approx 21.75 gallons
then for 109 miles you consumed 4.32 gallons giving and average of approx 25.2 MPG for the 109 miles while towing a trailer

[mikeiow]

ianrb said:

25.17 mpg

Glad we got the same result....I get a bonus pint for showing my workings

[sparkythecat]

Thanks very much for that chaps.
Landrover Freelander2 towing a 1400kg boat/trailer combo.

I’d expected better than that. Trailer brakes are working properly. I’ll check the tyre pressures.

[Halmyre]

I get about 30mpg (on a good day) in an A4 TDi towing ~1400kg of caravan so 25mpg from a Freelander 2 is not too bad.

[Bobupndown]

sparkythecat said:

Thanks very much for that chaps.
Landrover Freelander2 towing a 1400kg boat/trailer combo.

I’d expected better than that. Trailer brakes are working properly. I’ll check the tyre pressures.

That's about normal for a Freelander 2. Manual or auto? My Sd4 Auto will nudge just over 40 on a long solo run. Towing a 1450kg caravan, normally around 21-22mpg but got over 24 last week including a spell on the M25.

[sparkythecat]

Bobupndown said:

sparkythecat said:

Thanks very much for that chaps.
Landrover Freelander2 towing a 1400kg boat/trailer combo.

I’d expected better than that. Trailer brakes are working properly. I’ll check the tyre pressures.

That's about normal for a Freelander 2. Manual or auto? My Sd4 Auto will nudge just over 40 on a long solo run. Towing a 1450kg caravan, normally around 21-22mpg but got over 24 last week including a spell on the M25.

It’s a Td4 manual. I’m sure the auto makes for more relaxed towing. I might get it remapped to see if I can cut down on gear changes

[Bobupndown]

The 190hp Sd4 auto is superb at towing. Same power and torque as 2.7 tdv6 Discovery 3 but 400kg lighter.

[Export56]

I have noticed that these car computers seem to take the last few miles with a higher weighting. For example can drive 100 miles and get 40 mpg, then floor it for a couple of miles and it drops to 38mpg. This suggests I drove two miles at 11 mpg which seems unlikely.

[TwigtheWonderkid]

Export56 said:

I have noticed that these car computers seem to take the last few miles with a higher weighting. For example can drive 100 miles and get 40 mpg, then floor it for a couple of miles and it drops to 38mpg. This suggests I drove two miles at 11 mpg which seems unlikely.

You'd have to drive 2 miles at approximately 3 mpg to achieve that result.

100 miles at 40 mpg means you use 2.5 gallons.
102 miles at 38 mpg means you use 2.684 gallons

So you used 0.684 gallons to drive 2 miles, or 0.342 gallons per mile, so around 3mpg.

[Frankthered]

TwigtheWonderkid said:

Export56 said:

I have noticed that these car computers seem to take the last few miles with a higher weighting. For example can drive 100 miles and get 40 mpg, then floor it for a couple of miles and it drops to 38mpg. This suggests I drove two miles at 11 mpg which seems unlikely.

You'd have to drive 2 miles at approximately 3 mpg to achieve that result.

100 miles at 40 mpg means you use 2.5 gallons.
102 miles at 38 mpg means you use 2.684 gallons

So you used 0.684 gallons to drive 2 miles, or 0.342 gallons per mile, so around 3mpg.

Twig, it's early, I'll let you off - 2.684-2.5 is 0.184, not 0.684.

So that's 0.184 gallons for 2 miles, 0.092 gallons per mile - i.e. 10.87 mpg.

Is 11 mpg possible? It depends how hard you are trying. Back in the 90's I navigated for an old mate in a few road rallies. He had a Mk1 Astra, initially with a standard 1.2 engine. After one event (when the throttle was used as a switch) he calculated his fuel consumption using the old skool brim-to-brim method and reckoned he got 12 mpg - ISTR we had to fill up before we came home!

Bear in mind that on road rallies, your average speed is limited to 30 mph.

[TwigtheWonderkid]

Frankthered said:

TwigtheWonderkid said:

Export56 said:

I have noticed that these car computers seem to take the last few miles with a higher weighting. For example can drive 100 miles and get 40 mpg, then floor it for a couple of miles and it drops to 38mpg. This suggests I drove two miles at 11 mpg which seems unlikely.

You'd have to drive 2 miles at approximately 3 mpg to achieve that result.

100 miles at 40 mpg means you use 2.5 gallons.
102 miles at 38 mpg means you use 2.684 gallons

So you used 0.684 gallons to drive 2 miles, or 0.342 gallons per mile, so around 3mpg.

Twig, it's early, I'll let you off - 2.684-2.5 is 0.184, not 0.684.

So that's 0.184 gallons for 2 miles, 0.092 gallons per mile - i.e. 10.87 mpg.

Well spotted!

[Simpo Two]

sparkythecat said:

My car trip computer recorded that I’d averaged 41.3 mpg over a distance of 720miles
I then hitched up a heavy trailer and drove 109 miles.
At the end of that journey the computer recorded that over a total distance of 829 miles I’d averaged 38.1mpg

What was the fuel consumption of the car when it was towing the trailer?

Make the journey again, but reset the trip computer when you put the trailer on.

Saves maths

[skwdenyer]

Export56 said:

I have noticed that these car computers seem to take the last few miles with a higher weighting. For example can drive 100 miles and get 40 mpg, then floor it for a couple of miles and it drops to 38mpg. This suggests I drove two miles at 11 mpg which seems unlikely.

When I drove my 110 V8 Station Wagon + fully-loaded Brownchurch roof rack + 350kg trailer 250 miles from London to Yorkshire, I was very pleasantly surprised to average 14 mpg - I was expecting closer to 11