quote Originally posted by Wasii: I have Insignia B sports tourer (Buick regal tourx) with diesel engine , 416 Nm (307 lb-ft) (I measured it on Dyno) and 245/45 18" wheels. When cruising long distances at 130 km/h (80mph), I have around 2300 rpm. This is 61% of my nominal rpm. Just for the cruising it is too much. The turbo of my diesel engine is efficient till 2250rpm, cause according to its spec. the engine has its top torque band between 1750 and 2250 RPM. It means that any speed with the higher rpm than 2250 would be the moment, when the engine would work less efficient. I just have an awkward feeling driving more than 130 km h knowing that my engine is performing at lower efficiency than it is capable of and I am just burning fuel hence money for nothing. Also higher rpm cause a tiny bit more wear on the components. I maintain my car and want that it functions for a long time. During my trips I measured the engine load with app over db2 interface. On the highway the engine has the load of only around 30%... I made a research about the brake-specific fuel consumption of the diesel turbo engines depending on load and rpm. The engines burn less fuel, if the load is at 2/3 and the rpm is relatively low. So, my aim was to lower the rpm and to increase the engine load on the highway to burn less fuel. Those are three reasons why I am changing the final drive from 3.545 to 3.091. I calculated, at 130 km/h I should have 1950 rpm. Still in the middle of the top torque band and still it would be at 52% of engine nominal rpm (where the peak power is). Well, honestly, even with 3.545 that car doesn't have power to accelerate quickly in the sixth gear. It accelerates slowly like a yacht. No one of the passengers feels it. So, I will not be surprised by low torque on the wheels after the rebuild. I really hope that it would reduce the fuel consumption of the car. Diesel engines of BMW, Mercedes , VW and so on have low top gears for cruising. It means they have proved the concept. I bought it stock with 70k km (43k miles), on highway it had 6.2 l/100km (38 mpg). Now after some mods it has 4.8 l/100km (49 mpg). I am trying to get the best efficiency on long highway trips from the car to be able to travel with clean confidence and some money left in the pocket

What 6th gear ratio do you have?

As long as a diesel is making boost, there won't be much change in efficiency from slowing it down. There will be some, but the change will not be as significant as it would be with a gasoline engine.

quote Can you tell me what are the mods you have done?

I have done the following:
Lowered by 2.5 cm,
Installed upper and lower active grille shutters and programmed them to Arduino to work on highways on light and middle engine load
Installed engine underbody cover
Installed underbody side covers up to the rear axle
Replaced 1 muffler with original 2 muffler version.
Increased diameter of exhaust pipe from 60 to 76 mm from catalytic converter up to the Y-connection of two mufflers.
Straightened muffler tips
Changed the fuel tank to AWD fuel tank, thanks to it I have a straight exhaust pipe without bending after the catalytic converter.
Made and installed underbody covers of the exhaust pipe tunnel from the engine up to the rear axle.
Made and installed smooth diffuser from the rear axle till the end of the fender.
Covered the outer openings of the 18" rims, so that only some holes in the middle of the wheel are left.
Put a heat shield blanket on the turbo.

quote Going from 3.545 to 3.091 will cause more that 12% reduce in acceleration... so if you think it is slow now it will be even worse with 3.091 and you will have to downshift for every overtaking which for me is annoying.

Well, on the highway I overtake slow vehicles at constant speed on the second or third lane.
But if I need to accelerate more than a bit, I need to downshiftnto the 5th.
After the rebuild I would just downshift directly to the 4th.
That is my theory yet. To be confirmed...