Theoretically it is correct. Quote:And hence the answer. The harder working engine may be tuned to be more efficient, thus (theoretically) uses less fuel.

In reality, it almost always makes more economy at lower RPM because our engines are larger than they need to be. Even the A12 is bigger than needed to sustain 100kph.

That's in a theoretical sense correct, if you cranked an engine 4000 times and then 3000 times the volume of air pumped by the engine 4000 times would be more, assuming that the manifold pressure were at ambient.

Changing the manifold pressure causes the cylinders to be filled with a proper A/F ratio at a pressure below ambient, and containing less fuel.

Opening the throttle more at 3000 rpm could result in pumping more air than a more closed throttle at 4000 rpm.

which is why i said it's hard to do without a vacuum gauge.

As you get the manifold pressure closer to ambient, you burn more fuel, and get worse mileage. ( unless you have a turbo {in some cases} )

A lower revving engine has less oilpressure, i'll take my chances at 4k with 50 lbs vs 35psi at 3k.

hi dodgeman.
I kind of don't understand, if you have a engine reving at 4,000rpm, compared to a engine reving at say for example 3,000rpm
in the same conditions at both the same speed on the road, the lower reving engine wouldn't drink as much juice as it's not working as hard to keep up the fuel demands of the higher reving engine.

Also the lower reving engine would not wear out as quick as the higher reving engine as there is not as much load on it.
Thanks

H165 disk brakes whole rear axle.

I'm not in a position to do it now unfortunately.

I'll have to wait till i break the current rear end before i can do that.

WEll I bought this one for $9.99 on ebay a while back

Which 200sx diff you looking for?
I know of 2 79-81 complete cars

yummy!
that would be fun with the higher ratio diff.

I'd just hate to spend money on a H150 diff, since sooner or later i need a 200sx rear axle swap.

Well what do you think about the 3.9 dif?

I have the 3.7's in the car now and a NOS set of 3.9's on the shelf.

Ideally you have a turbocharger in those ideal super efficient high MPG low rpm schemes.

It takes a vacuum gauge to get the throttle position just right to pull off the low rpm thing too.

want better mpg? do something about the frontal cross section of the car, a front spoiler helps at highway speeds.

the band of efficiency on the 60 series 5th gear with the 3.7 diff starts at 75mph, driving 60mph in 5th is not a fun thing at all. yes an A15 will get a car going well over 95 mph pretty easily.

wow, so far off topic ;)

Quote: This statement is not necessarily true.
It takes a certain measure of power to push a given object along at a given speed against normal rolling resistance & against the air.
For the purpose of simplicity, lets just say that in 4th gear you are doing 4,000rpm
At this speed you have the throttle kicked open about 1/3rd & the engine develops X amount of power.
Now we slip it into OD 5th & the revs drop to 3,000 rpm [for example]. If the cylinders are still operating at the same measure of volume fill for each intake stroke, then at that reduced engine speed, we would have less power & the car would slow down since we are now burning less fuel [per minute] than before.

Since the cylinders now have more time to receive a fresh charge of fuel, assuming the same throttle setting as before, then more fuel [& air] will flow in, giving each intake stroke a greater volume, but for each time unit, like a whole minute, we have fewer intake strokes. [lower rpM]
So now we have fewer intake strokes with more fuel/air per stroke. This in itself may not produce the required power at this reduced engine speed to maintain road speed & if this proves to be true, then we must open the throttle a little more to bring the power back up to the previous figure of X, but at the new lower engine speed in order to maintain our previous cruise speed.
So now we have even more fuel per intake stroke at our reduced rpm levels.

Simply put, we must burn fuel to make power. Each engine design has its own power-to-engine-speed curve for any given throttle opening & we must remember that as our vehicle speed increases, so does the air resistance, at the rate of the square of speed increase. [eg. multiply the speed by 2 & square that number [2x2=4] for increase of air resistance]

In the end, we may not be getting a fuel economy improvement at all, but instead, at certain speeds, we may be causing our engines to run below their designed point of greatest efficiency & doing both it, & ourselves a disservice.

Even the claim of reduced wear may not always be true since the load at the reduced engine speed is greater, so once again, there are fewer crank rotations, fewer piston reciprocations, but each one is done at a higher loading.

If you can engage 5th gear & still have the engine 'in the zone' of maximum efficiency, then it's all good, but pull 5th gear when too slow, or when battling a headwind [or both] & the economy thing can easily go straight out the window.


So what was the point of all this?
Do not assume that the presence of an overdrive gear will automatically provide a reduction in fuel consumption or wear rates in all instances at all times as even the engaging of the overdrive gear involves the use of indirect gearing which immediately drives up the drag through the gearbox itself which imposes a small additional power burden on the engine.
Knowledge is power. Get some.

The 60's use the same clutch and rear input as the 56's. The 63's are the same as L series.