Weight and Fuel Economy

[JohnnyRims]

Quick question for the engineering types here - is there any way to work out/a good rule of thumb for how weight affects fuel economy? Assuming that I'm an impatient type who wont accept any drop in acceleration due to increased weight how much more fuel am I likely to be burning with say a 10% weight increase (3 passengers)?

[Papa Hotel]

Roughly a 10% increase in weight will increase fuel useage by 5%. Something like that.

[HellDiver]

Fold the back seat down, that'll help acceleration and fuel economy.

[kambites]

Depends massively on what type of driving you're doing. At a constant speed cruise, weight makes negligible difference to fuel economy; if you're constantly accelerating and decelerating it makes quite a lot.

[Papa Hotel]

HellDiver said:

Fold the back seat down, that'll help acceleration and fuel economy.

Improves handling too by lowering the centre of gravity. Possibly won't work if he's carrying three passengers though...

[JohnnyRims]

Papa Hotel said:

Improves handling too by lowering the centre of gravity. Possibly won't work if he's carrying three passengers though...

Rear seats don't fold but I try and maximise efficiency by driving with the windows rolled down AND the air conditioning on.

[Papa Hotel]

JohnnyRims said:

Rear seats don't fold but I try and maximise efficiency by driving with the windows rolled down AND the air conditioning on.

Good call, lowering the windows will lower the CoG too! I wouldn't bother with the aircon unless you have it on cold, you can then run a cold air feed from the vents to the intake, increasing power.

[redgriff500]

JohnnyRims said:

Quick question for the engineering types here - is there any way to work out/a good rule of thumb for how weight affects fuel economy?

Massively depends on whether you are stop start driving or on a run.

But it doesn't affect as much as you'd think.

Take a V8 from a 2ton car doing 20mpg and it won't do 30 in a 1 ton car (unfortunately)

[Howitzer]

I average around 28mpg in my 2.5 TDi 110 Defender, when I was towing a 3 ton trailer I averaged 22mpg, which was a nice surprise, the same speed for both, sat at a steady 60mph.

Dave!

[redgriff500]

Howitzer said:

I average around 28mpg in my 2.5 TDi 110 Defender, when I was towing a 3 ton trailer I averaged 22mpg, which was a nice surprise, the same speed for both, sat at a steady 60mph.

Dave!

Quite an increase in drag with a trailer too.