Fuel saving tips thread

AdrianHi

Mobeer wrote: »

When you coast to a stop the battery is charged from the rotation of the wheels. Then when the engine is restarted energy is taken from the battery. This makes more sense that braking harder when slowing down, then running the engine whilst stationary, then using power from the battery when restarting the car.

Almost all cars are continually charging the battery with the alternator under some load, more load when the battery needs charging. When moving in gear (not coasting) without the accelerator pressed you get no fuel used and battery charging because the motion on the car turns the engine, the engine turns the alternator.
The exceptions are cars that have some form of regenerative braking mechanism such as the Toyota Prius Hybrid and any of the new BMW or Mini's with "efficient dynamics" technology.

Pew_Pew_Pew_Lasers!

Mobeer wrote: »

When you coast to a stop the battery is charged from the rotation of the wheels. Then when the engine is restarted energy is taken from the battery. This makes more sense that braking harder when slowing down, then running the engine whilst stationary, then using power from the battery when restarting the car.

No it isn't. When you are coasting the engine is not physically linked to the wheels.

wba31

tomstickland wrote: »

Such is the nature of the system that I expect that you find the downhill part doesn't fully compensate for the extra work needed on the uphill part.
I believe that you will get slightly better mpg by allowing the car to slow on the uphill part and then picking up speed on the downhill.

when i had a car with cruise control, firstly i was rarely able to use it due to the middle lane hoggers and varying speeds of the fast lane and constantly having to come off the power to pull out as people in the fast lane refused to let me out...

but in fairness i never really drove over any vast mountains that meant the fuel consumption was all over the place. now the speed is all set by my sized 12 right foot i really struggle to maintain a constant speed without staring at my speedo, due to all the middle lane hoggers and varying speeds of the outside lane, but at least now i drive a bigger car more people are willing to let me out when i indicate...

the biggest thing that helped my fuel consumption is my car now has 6 gears, so from 50 up, you can cruise in that gear and get maybe 60 mpg+ with it (its a 1.9 TDi VW Bora). 70mph gets about 55mpg, 80mpg about 50, and 90 about 47mpg.

meester

ABN wrote: »

There are soooo many errors and misconceptions in this thread it's incredible.

But the one that stands out the most is the above.

Any item that uses electricity also uses fuel. The charge taken out of the battery has to be replaced. This places an increased load on the alternator which in turn places an extra load on the engine and hence lowers the fuel economy.

But a car idling at rest is burning a lot of energy for no output. Starting a car takes a high current for very little time. A small extra load on the alternator while the engine is already doing its normal job (propelling the car along the road) does not compare.

top.position

meester wrote: »

This is generally not true. Being in a higher gear means the car goes faster for the same amount of work. But in top gear at maximum torque your car will be generating a substantial amount of air resistance due to the high speed. This is part of the reason for the 56mph speed limit, even if it is not maximum torque.

The amount of work your engine has to do to push you along a given stretch of road at a given speed is completely independent of the gear you are driving in. The engine itself has an efficiency profile - the best gear for fuel economy is the one that keeps the engine revs nearest to its most efficient speed.

top.position

ABN wrote: »

There are soooo many errors and misconceptions in this thread it's incredible.

But the one that stands out the most is the above.

Any item that uses electricity also uses fuel. The charge taken out of the battery has to be replaced. This places an increased load on the alternator which in turn places an extra load on the engine and hence lowers the fuel economy.

Not all electrical usage costs fuel. The alternator is rated to be able to charge the battery and supply some current to the accessories, lights etc. Particularly in summer there are periods when the electric output from the alternator is not charging the battery as it is already fully charged. The excess output is lost as heat from the rectifier.

top.position

tomstickland wrote: »

You can safely inflate tyres by a few psi over the book figures. Some driving manuals tell you to do so to improve handling.

I agree with most of the above but personal experience tells me this last is wrong. I overinflated my tyres by 3psi and drove like that for a year (about 22k miles). At the MOT I was advised that the tyres were almost at the legal limit on the centre line. This year I have kept my new tyres correctly inflated. I estimate they have another 18k miles life in them. The difference in fuel consumption was about 5% (32 gallons). Two new tyres cost me £160. At today's prices that's about the same, a year ago the petrol was much cheaper. I lost money.

tomstickland

It's not wrong. You're complaining about increased wear on the centre line. That's a downside of over inflating tyres. The benefit is improved handling.
I pay 50pounds per tyre fitted and I don't normally get more than 10K out of a tyre, but I regard them as consumables.

Pew_Pew_Pew_Lasers!

You should change your driving style then. I pay £120 a corner and get about 40,000 miles from them.

banger9365

well the tyres on the lorrys at work have 5 pound more in them to save fuel and all of them are automatices and we are getting over 10mpg plus out of them and we change down gear when we are going down hill with cruisecontrol on to cut off the fuel to the engine and it brakes the engine to the anoyance of other drivers,put we are all tort how to do it at work.
it's nice to see car drivers thinking for a change about how they drive it's like a breath of fresh air keep it up,
i am getting 50 mpg out of my car by driving how we are tort and that round town no motorways
ho yes i would not us cheap tyres on any car has they are the only thing between you and the road and £50 is a bit cheap try some better one they will work out better value ,get more mile and mite even save you fuel