I would tend to agree with Partie pn the diff gear issue!

Around town a lower diff ratio will cause an increase in fuel economy, assuming that it is a true stop start town cycle with limited freeway or highway sections.

Acceleration = Torque mulitplied by gearing. If the vehicle is required to stop and start constantly (ie peak hour or lots of traffic lights) and the accelerative efficiency is reduced due to the lower diff ratio, more energy is required during the initial or accelerative phase of driving. More energy required ='s more fuel useage. AGAIN, this is assuming the vehicle is used solely for city cycle driving with limited freeway or highway driving!

At one point or another, depending on gross vehicle weight, power, transmission, state of tune and driving style the ratio of city to highway driving will be more greatly benefited by the useage of a taller final drive gearing. Ie if you are predominantly using your vehicle to drive on National Highways with limited to no city cycle then yes a 2.7 final drive ratio will reduce your fuel useage. Accelerative efficiency is almost irrelevant when cruising at 100-110 k's.

Just remember it's horses for courses! The manufacturer of a car has to design them to be acceptable in all common useage conditions. Middle of the road 3.2-3.5 final drive ratios are the most versatile when combined with the Ford 4 speed box and the T5 manwell!

My 2 cents anyway!

a 2.7 will be more economical in all conditions - stop/start or highway if you accept the fact that acceleration will be slower. if you are trying to achieve the same or similar acceleration as with 3.08 diff then you will be needing to put your foot down a bit more and thus it chews a bit more fuel. the falcon 6 puts out 350+ nm of torque so its not going to struggle with the taller gearing.

A slower engine needs a larger throttle opening to get the same amount of air in. Yet you claim that the smaller the throttle opening, the better the economy. Could you please explain your contradictory theories?

so you claim that driving with a light foot will chew more fuel than driving more aggressively and using a larger % of throttle. i don't think so.

Stick with your current gearing unless you drive the Nullabor most days!

Despite Prydeys persistant "don't let the facts get in the way of a good story" information (no offence mate........you're not wrong but you are only half right), you will seriously waste your money on changing your diff ratio. The cost and heartache of setting these diffs up properly is will outweigh any mileage gains you may achieve.

Without knowing what economy you are getting now it is hard to say whether or not any measures will improve your economy.

How far are you getting out of how many litres?

Also, how does it idle? does it make good power everywhere? do different fuels prvide different mileage?

I never, and will never say that. You seem to be very hostile with very little, or no understanding of which you talk and when someone queries you on your theories, you respond with a flame, which can only mean you can't back up what you say.

Feel free to search the net to prove me wrong and I'll eat my words. Provide quotes, links, whatever you want. Put put your money where your mouth is instead of letting your ego do the talking.

lmao @ right foot

i am not flaming anybody although some of my comments may come accross as a bit sarcastic. i am merely trying to come to terms with a few of your theories. i do tend to think of fuel when we talk about throttle percentage so maybe this is where i'm confusing myself. i know the throttle controls the butterfly in the throttlebody which lets in less/more air depending on throttle position. however the butterfly also operates the throttle position sensor as well and wider opening = higher voltage which tells the ecu to richen mixture - more fuel.

a 2.7 dif will make the engine slower (rpm) for a given speed. after having a re read through the posts i will admit that i was wrong regarding throttle position being less. i assumed the slower the engine the lower the throttle percentage but now realise its the gearing making it slower. so maybe the suggestion of a taller ratio wasn't the smartest after all. hell we all make mistakes and the forums here to learn from.

That is very true to some extent. The throttle position sensor has two jobs in a MAP based, or the technical term is speed/density, injection system like the ford system.

1. It looks at the rate of change of the throttle to determine the right amount of accel/decel enrichment to give the engine. Faster movement = more enrichment.

2. It also tells the ECU when to go into open loop mode. I.e, it's not using the oxy sensor feedback to correct the injector timing. Open loop mode is generally in the 70%-80% throttle range. Anywhere below this throttle percentage and the ecu runs in closed loop mode targeting 14.7:1 AFR.

When you start using an 'alpha-n' algorithm for fuel calculations, the ecu looks at throttle position and RPM to determine engine load. Speed/density looks at MAP and RPM.

Also, don't forget that the EEC-IV and V only use narrowband oxy sensors, so any small deviation in mixture from 14.7:1 AFR results in a rich or lean reading and that's about it. Any voltage from the oxy sensor other than 0.5V is at best a stab in the dark at the actual AFR and for this reason, the ECU is incapable of targeting small changes to the AFR with a change in engine load.

As for a taller diff ratio affecting the acceleration, you guys are looking at it from teh wrong perspective. A 2.7:1 diff being driven by a driver accelerating @ 0.5G is going to use the same amount of fuel to accelerate the car as accelerating with 0.5G using a 3.45:1 diff.

There is one difference however! As stated previously, the harder you accelerate, the more fuel you use. F=ma, assuming you don't factor in slight changes in AFR. The difference is that the ENGINE accelerates faster. The pistons have weight and are constantly accelerating and decelerating through the cylinders all the time. If you reduce the time they have to change their speed, you're increasing the g forces on all the engine components and an increase in g forces results in more power required = more fuel used.

In general, people get used to a driving style and an acceleration rate that they're comfortable with. Make the diff ratio taller and all it means it that people instinctively open the throttle more to get the same acceleration.