Over-riding auto box to improve MPG

[Highfiddleedee]

I noticed when driving my W124 that when it changes up, the economy meter goes up also, particularly so when going up hill where the economy meter will redline as the car plods up in top gear. So, I now overide the auto on hills to save fuel. Now, most advice is to change early, government sponsered advice states that gear changes between 2000 and 3000 revs are for best fuel economy. I disagree! I think if the engine is allowed to gently attain a higher rev before changeing, say 4000 revs on hill climbes, fuel will be saved. The economy meter is I believe basically a vacuum gauge and the theory is that the greater the vacuum the more fuel will be drawn into the engine. In lower gears, the engine spins faster, the load on the engine is reduced, hence less fuel is required. So, that's how I see it, can anyone explain the low revs high gear bit, cos it doesn't make a lot of sense to me!

 

[grober]

I think for hill climbing you want to be close to the maximum torque range of the engine. For your engine its actually quite high at 4,400 rpm. and this perhaps explains your observations.
The 103.980 engine (181 in.³ or 2960 cm³) produces 177 hp or 132 kW at 5700 rpm (with catalyst), 185 hp or 138 kW at 5700 rpm (without catalyst), and has a torque of 166 ft·lbf or 225 N·m at 4400 rpm (with catalyst), 191 ft·lbf or 260 N·m at 4400 rpm (without catalyst). It uses the CIS-E (Continuous Injection System - Electronic) and has a bore of 88.5 mm (3.48 in) and a stroke of 80.2 mm (3.16 in). The compression ratio is 9.2 : 1 with a redline of 6550 rpm in 1987 and 6200 rpm from 1988 on. The firing order is 1-6-3-4-2-5. Oil capacity is 6.0 litres and coolant 8.5 litres. Number of valves 1 intake, 1 exhaust with V-shaped overhead configuration. Valve operation by single overhead camshaft driven by a simple roller-type chain. Recommended fuel octane is 91 RON/MON (96 RON)(86 MON). This engine can be found in the W124 E-Class, W126 S-Class and Mercedes-Benz R129.

http://en.wikipedia.org/wiki/Mercedes-Benz_M103_engine

 

[Stratman]

The economy gauge is vacuum operated, but your theory is wrong. The greater the vacuum the better the economy.

The vacuum is measured downwind of the throttle plate, so the greater the vacuum the less air is being drawn into the engine. Since the air/fuel ratio will always be around 14.7:1 less air means less fuel.

Zero vacuum means the tap is wide open and suitably large amounts of fuel are needed to maintain that ratio.

 

[Uberwagon]

The amount of variables in working this out is huge.

The smplest way is to listen to your engine. What is the lowest engine speed you can maintain where it isn't labouring for your chosen speed.

If your engine feels like it's struggling sightly, go down one gear.

Dave!

 

[Dieselman]

I don't think a car fitted with an auto gearbox make the engine labour, it will change down accordingly.

Back to the O-p.
As has been said the vacuum gauge will show higher vacuum when the engine is operating more efficently, but the actual overall efficiency will be compromised because you are selecting a lower gear ratio.

The vacuum will be lower in a higher gear but due to the engine turning slower it may well be using less fuel even though the throttle flap is wider open.

After checking the gearbox throttle cable setting is correct I would let the car do it's own thing.