I wondered if when driving at constant speed (which I do a lot) the cars needs say 20bhp to hold 60mph, whether with/without remap or any gear ratio changes that the energy and therefore bhp required will actually change.
To be honest, I have forgot the in's and out's of the way you work it all out. Probably due to them being particulalry complicated calculations only of any use for that particular need, and me not having any need for them.
(Currently looking at nozzle discharge rates, and height and speed for setting an application rate to apply anti icing agent to 66,000sq m of ashpalt. need 25grms pr sq m, SG 1.28)
There was a sheet with the proportion of air to speed ratio kicking about on this site somewhere which would give some starting point on answering that. Probably the 56mph thread.
Then you need to work out the weight of your car and the power required just to move that. this should include the frictional resistance of the tyre. all of which increase with speed. Your engine power graph, if you have one, should give you an idea. (You may have one in the orignal manufacturers brochure but potentially that it it's design output and the car could be a bit less, or more, then age to take into considertation, some horses will have bolted) and you will now know your car's output at that given engine speed.
As far as the original question is concerned. if you need lets say 20bhp for 60mph, and your car is doing 38bhp at those revs, and 33 at 250rpm less. you should save fuel as it is producing more than it needs. In fact, its not running as effeciently as it could, so you could probably increase the speed to equal the 33bhp for the same fuel consumption and thus go further. ie increase the mpg. If you lower the power to meet the exact at 60mph, you will lose out at 61 plus.
Of course, if you fill the car with people, it might need 34bhp to do 60mph.
That said, I am sure you need more than 20bhp to shift 1.5tonnes at 60mph.
because your car will accelerate reasonable at 60mph, it is a fair chance to think it will be producing more power than it needs, and could probably drive almost as well by 250rpm reduction. But the effects will be increased proportional to the increase in speed.
They have slowed most efficient aircraft down to save fuel. The reason is that they have an efficiency operating window in which you can set the engine to power ratio for its most effective, but as they chase 1 or 2% fuel reductions, you can see this by reducing 70 or so knots of airspeed, adding about 25mins to the time for a trans atlantic flight and 1% of 170,000ltrs at 60p a ltr is a pretty big saving.
your reciprocating engine is not as efficient, and you are only taking 250rpm out of it, so again you are likley to see a very small reduction in fuel if everything was in your favour. You may even see the same increase in fuel efficienty by using better fuel or having your remap to suit 98ron. Entirely financially cost ineffective as it costs 4-8% more, but you could see a 2 or 3% better mpg. Now add that to a 250rpm reduction and it might start to break even, assuming you write off the costs of doing the work.
And that's petrol. Diesel engines being much more efficient to start with, so any 1% increase will be even less noticable.
To downside will be depening on the engine being used to do this. If it already a fuel efficient set up you may find you are on the edge of the performance line as it is, in which case, its all down hill. Or you may take out the builders in built comfort margin, which means you will not see anything, as you can't measure it to any set of specs.
So lots of rolling road sessions, increasing tyre pressure, fuel type changes, different oils, different tyres needed all of which to get a set of baseline results from which to work.
finger in the wind, if the car is easily capable of over 100mph as it is, then you will see a fuel saving by changing the ratio. Whether you can measure it will be a good question! (easliy capable means 10% or more)
If its a FIAT Panda 4x4, forget it.
how long are you planning to take over this project? Its a very green and admirable way to lower your CO2 emmissions.
I would just walk or cycle to work one day a week to do my bit.
( I assume the larger, heavier wheels would reduce efficiency, or have i got this wrong?)
