It's not the flywheel torque that's 50% less but the roadwheel torque.
If you consider how well the car accelerates in first gear compared to how poorly it accelerates in fifth gear, this is all the result of different gearing of the same engine.
If the engine produces, say, 150lb/ft of torque at 2000rpm it'll do that regardless what gear it's in. What the gearbox does with this however is crucial. Short gear ratios (low gears) convert the 2000rpm into something much, much slower - the engine can rev it's proverbials off but the road wheels turn fairly slowly, hence a low top speed on the rev limiter in first gear. Total energy out equals total energy in (ignoring powertrain losses which don't really vary with gear ratios) so if your RPM goes from 2000rpm into the gearbox to 200rpm out of the gearbox, then the torque of 150lb/ft in has the opposite applied to it, and would be multiplied up to 1,500lb/ft of torque.
Short gear ratios step down the RPM and multiply the torque. Tall gear ratios do the opposite.
Ignoring the fact that the engine can actually generate more torque at 3000rpm than 2000rpm and just assuming it's a completely flat torque curve like an electric motor has, then the simple act of re-gearing the gearbox to give 70mph's worth of road wheel RPM for a 50% lower flywheel RPM (i.e. less step-down of RPM from engine to wheels) means that you get 50% less multiplication of the engine's torque at 2000rpm compared to 3000rpm.
The engine produces the torque and RPM certainly, but it's what the gearbox spits out at the other end that you actually drive with.
You're spot on about fuel economy though. Practically all naturally aspirated petrol engines are at their highest efficiency at full-throttle. At full throttle pumping losses across the throttle body are minimised and these can be significant. The best way to use minimum fuel is to open the throttle wide and change the gearing to reduce engine RPM to bring the actual power output down to what's required. That would need a gear which gave you absolutely nothing to spare to fight headwinds, inclines etc with though. It is however how piston aeroplane engines are operated - full throttle and variable pitch propeller used to drag the engine RPM down to something like 60~75% power output.
If you consider how well the car accelerates in first gear compared to how poorly it accelerates in fifth gear, this is all the result of different gearing of the same engine.
If the engine produces, say, 150lb/ft of torque at 2000rpm it'll do that regardless what gear it's in. What the gearbox does with this however is crucial. Short gear ratios (low gears) convert the 2000rpm into something much, much slower - the engine can rev it's proverbials off but the road wheels turn fairly slowly, hence a low top speed on the rev limiter in first gear. Total energy out equals total energy in (ignoring powertrain losses which don't really vary with gear ratios) so if your RPM goes from 2000rpm into the gearbox to 200rpm out of the gearbox, then the torque of 150lb/ft in has the opposite applied to it, and would be multiplied up to 1,500lb/ft of torque.
Short gear ratios step down the RPM and multiply the torque. Tall gear ratios do the opposite.
Ignoring the fact that the engine can actually generate more torque at 3000rpm than 2000rpm and just assuming it's a completely flat torque curve like an electric motor has, then the simple act of re-gearing the gearbox to give 70mph's worth of road wheel RPM for a 50% lower flywheel RPM (i.e. less step-down of RPM from engine to wheels) means that you get 50% less multiplication of the engine's torque at 2000rpm compared to 3000rpm.
The engine produces the torque and RPM certainly, but it's what the gearbox spits out at the other end that you actually drive with.
You're spot on about fuel economy though. Practically all naturally aspirated petrol engines are at their highest efficiency at full-throttle. At full throttle pumping losses across the throttle body are minimised and these can be significant. The best way to use minimum fuel is to open the throttle wide and change the gearing to reduce engine RPM to bring the actual power output down to what's required. That would need a gear which gave you absolutely nothing to spare to fight headwinds, inclines etc with though. It is however how piston aeroplane engines are operated - full throttle and variable pitch propeller used to drag the engine RPM down to something like 60~75% power output.
