The Natural Philosopher wrote:
Martin McGowan wrote:
Snakey wrote:
"Phil Kyle" wrote in message
snip.
snip

Spark plugs do
fail, and they also degrade. I used to have to record all fuel used
against miles driven for accounts purposes and as an experiment i
changed the spark plugs separately to the service(I serviced my own
vehicle) new spark plugs gave me about 2 MPG increase tank full to
tank full, replacing standard with high efficiency plugs gave me just
under 3 MPG increase. snip

Strangely enough, my brother in law who does about 60K miles a year in a
diesel, reports a similar loss in economy before a service..my guess is
clogged air filters.

I serviced the car without changing the spark plugs and did the tank
full to tank full readings. then i changed the spark plugs. I was
trying to see what i could economise on. the spark plugs accounted
for about half the service improvement in economy. I continued to
change spark plugs as it was financially worth it.
Martin McGowan