Tuesday, September 14, 2010

Save Fuel, Drive Smart

One of the biggest consumers of energy is our cars.

The best thing would be to not have one but that isn't practical in my line of work, having to visit clients and far flung offices at odd hours of the day and night.

Still, I try not to use my car if possible.  I spend quite a lot of time working at home. No fuel used for days at a time.  But I do often have to go to an office in Reading, some 65 miles away.

I'd normally go there by car but recently I had cause to go there by public transport (as some low life crashed into my car at the Reading M4 junction and ran off without stopping - GRRR!!!!). Two trains and a bus and over 2 hours each way weren't ideal (especially when I discovered the Reading night bus doesn't accept the return leg of your train "Plus Bus" ticket and you have to buy another ticket).
Any way, I digress...
I normally drive a Honda Insight, a cheaper cousin to the Toyota Prius, in that it is a hybrid petrol/electric car. This car has a driver feedback system that teaches you to drive more efficiently by showing you a colour change from green to blue in different shades as you waste more fuel by "lead-footing". But the lessons learned by driving the Insight are applicable to any car.

While my car was in the workshop, the dealer loaned me a little Honda Jazz. To my surprise, I found that it could get very good mileage by careful driving. It's a smaller car than the Insight and has a smaller 1.2L engine, which helped in traffic jams (less fuel wasted when idling). I could get it up to about 62mpg on the run to Reading. This is diesel territory normally and excellent for a petrol car. The Insight can do 72mpg on the same run.

The main lessons that need to be applied to get these sorts of returns are:

Don't overload the car - remove all junk from the boot, travel light. It costs fuel to accelerate a mass. The heavier the mass the more fuel. So ditch the weight to save the fuel. If your favourite petrol station is on your route, consider filling up only half a tank twice a week rather than a full tank once a week. Carrying a full tank of fuel around weighs a lot and wastes fuel.

Look ahead MUCH furter than you're used to and try to drive without using the accelerator or brake. An odd suggestion, but one that saves fuel and makes you a safer driver.

Don't accelerate agressively - use a light touch and change speed gradually. Don't accelerate to a fast speed if you can see that you'll have to stop or slow down seconds later.

Don't brake unnecessarily - stopping wastes your stored kinetic energy in the moving car and then you have to waste fuel to speed up again. If you can see far enough ahead, let the car slow naturally and with luck the lights will change before you get there. Accelerating from a rolling start uses less fuel than starting from a stop.

When driving a hybrid, the last point is a big factor as the electric motor acts as a generator when slowing down or braking. It recovers some of the kinetic energy that would have been lost in the brakes and stores it in the battery for use when speeding up again. In a non-hybrid car it still makes sense to preserve the kinetic energy you have, rather than waste it in heating the brakes.

Drive slowly - no, really... I mean it. Unless you are in a hurry, drive at the most efficient speed for the engine and gear box. If you are always in a hurry, leave earlier! Change up gear as soon as possible, keeping the RPM counter below 2500. The Jazz had a shift light for the manual gears to prompt you to change up and it's earlier than you'd think. The Jazz did well for such a small engined car - 53mph at 2300 RPM. The Insight has "long legs" with the CVT only making 1600 RPM at 53mph.

Air resistance is much lower at slower speeds too - An average car only needs 10hp to drive at 50mph but at 100mph it requires 80hp just to overcome the air resistance. I've noticed that Tesco lorries have reduced their regulated speed from the usual 56mph to 52mph. This must be to save the company fuel costs as the frontal area of a truck is huge and the aerodymanics are bad so the air resistance is very high.

Consider the trip average speed. It's meaningless to drive round the M25 in the morning at 70mph between traffic jams where you travel at 15mph. You'll just get to the next traffic jam faster. For instance, I know there's no point in driving at 70mph until I get past the M3 going round the M25 so I drive at 53mph max until then (and sit in the traffic jams 5 cars behind the guy who sped past me earlier). Once past the jams I'll drive at up to 70mph, as I know it will actually make a difference to my arrival time.

If you get through the jams more quickly than usual, drive more slowly afterwards. A sat nav will tell you your estimated time of arrival and "range to target". Use that information to adjust your speed accordingly!

Drive up steep hills more slowly than when on the flat or going down hill (especially on motorways)... Stressing the engine to maintain high speed on a hill wastes a lot of fuel with little saving in time.

Chose a motorway route if available. Motorways allow for constant speed driving, increasing efficiency. Use the cruise control if you have one. It maintains your speed with much greater accuracy than your right foot can and saves fuel. You can't use cruise control on a twisting A road and you have to stop for round abouts and so on. Plus motorways allow other faster traffic to go harmlessly around you. A roads suffer from "caravan syndrome". Even if a motorway route is slightly longer it may be actually more fuel efficient.

Don't put anything on the roof! Roof bars, boxes and so on increase air resistance significantly.

Inflate your tyres properly. Under inflated tyres waste energy as the rubber side walls flex and generate heat and can be dangerous at motorway speeds, increasing the risk of a blow-out.

Have your car serviced - make sure fuel and air filters are replaced to avoid wasting fuel from poor air-fuel mixture ratio. Use the thinnest grade of engine oil suitable to the climate you live in (the manual usually suggests a range of different oils for different climates). Thinner oil increases engine efficiency. Change the oil regularly - old oil gets thick with goo and doesn't protect the engine well.

Where's the solar power relevence of this post then?  The Insight is big enough to ferry my panels from their various original homes to their new one... using the least fuel possible.


  1. I was really glad to come across your website - we are on a similar trajectory in portland, oregon having already installed 2100 watts of grid tied solar here, had a heat pump water heater installed, had a mitsubishi 12000btu ductless mini split heat pump for house heat installed - and seeing quite a reduction of our power-demand.

  2. Nice one. Sounds good.

    I'm still adding "post dated" entries for this summer when I moved the solar array to the roof.

    Basically too lazy to post when things are actually happening :D