How much can DRL lights improve fuel consumption?

Lets say that DRL lights power consumption is about

P_DRL = 10[W].

Alternatively you can drive with “normal” lights on, that is: a front low beam + rear position lamps and registration plate lamps (in my case H7, R5W, W5W respectively)

P_NORM=2*55+2*5+2*5[W] = 130 [W],

we have omitted dashboard back-light but it shouldn't introduce too much error.

Volume of fuel V with energy density E_d needed to supply power P in time t is given by

where η_total is total efficiency.

Distance S is:

S = vt

where v is speed. We can also write:

lets insert t in the first fraction and we get:

Efficiency η_total is product of a engine efficiency η_engine, and a belt and alternator efficiency η_balt

If we want to calculate the volume fuel difference we should insert power difference into formula so:

Lets assume that belt and alternator efficiency is

η_balt = 50%.

In case of diesel engine

Ed = 37.3[MJ/ltr]

let also assume peek diesel engine efficiency

η_engine = 40%

The table shows fuel consumption at different speeds calculated according to presented assumptions calculated with formula (*)

In case of petroleum engine

Ed = 34.0[MJ/ltr]

and peek engine efficiency about

η_engine = 30%.

In real life we can expect much lower engine efficiency, depending on various conditions you can expect larger numbers, it is larger fuel savings.