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I’ve recently been researching how engines and transmissions work (Im a novice) and I was curious about what determines a vehicles top speed.

Whilst trying to figure out gear ratios, I asked myself, why can’t a normal road vehicle reach redline in the highest gear.

I’m finding it hard to formulate the correct question as my knowledge is extremely limited but here it goes.

If a vehicle is in neutral, applying enough gas (and air) will increase the engine speed to red line. So why does a connected transmission limit the speed of the engine depending on the gear? I assume you could normally redline in first gear, but probably not in the highest gear. If it is to do with external forces, then if I was to raise a car off the ground, could I reach redline in the highest gear?

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Simply because the aerodynamic forces opposing the vehicle have matched the power capable of being delivered by the engine.

The rotating losses (tires, gears etc) are small in comparison at this point, the balance between aerodynamic and rotative usually « cross » about 40mph (based on all the studies I have seen).

If you raise the driven wheels off the ground then you can, of course, redline in every gear. But when you try the same on the racetrack it is down to the forces ie air resistance that oppose the vehicle.

James May (Top Gear) did a good explanation about how much air has to be moved when he drove the Bugatti Veyron at max... if I find it... but you will find it on youtube.

Found one : enter link description here

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The top gear is for fuel efficiency, not for top speed.

At high speeds, air resistance dominates. Air resistance force is proportional to speed squared, and air resistance power is proportional to speed cubed

An engine can be very roughly modeled as a machine producing constant torque up to its redline and then the torque falls to zero. (Yes, this is a very rough simplification.)

With a certain gear, the torque produced by the engine turns to a force on the wheels that matches the air resistance force at the top speed, which is the redline speed at this gear.

With a higher gear, the same torque equals less force due to the gear ratio. So, with a higher gear, you go slower: 1.2x the gear ratio, sqrt(1/1.2) = 0.91x times the speed due to air resistance force being proportional to speed squared.

With a lower gear, the same torque equals more force, so in theory you could go faster, except there is the RPM limit of the engine. So, without the RPM limit, you would go faster at lower gears. But, with the RPM limit, you can't quite reach those speeds due to the limited RPM.

There used to be a time where cars had 4 or less gears. Then, the top gear was often optimized for top speed.

Nowadays, with 6 gear manual boxes becoming abundant, the top gear is optimized for fuel efficiency, and you typically reach the top speed on second highest gear. At least that's the case for cars optimized for general purpose driving. Sports cars are an entirely different kind of animal.

  • Great answer. So without external forces acting upon the vehicle... it is possible to reach red line in the highest (fuel saving) gear? – Tyler Durden Jul 19 '19 at 19:39
  • @TylerDurden Yes, without external forces (tires spinning free), it is. – juhist Jul 20 '19 at 10:22
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As speed increases, drag increases.

Assuming level ground.

Drag is rolling resistance (pretty constant), and aerodynamic drag, which increases with speed. If you double the speed the drag increases by a factor of 4. At low speeds aerodynamic drag is quite limited, but rapidly increases with speed.

To overcome drag you need thrust (effectively torque at the wheels).

Thrust is a factor of torque (which varies with rpm) and gearing. As speed increases you need higher gearing for the same engine revs. But the higher gearing costs you torque at the wheels. If you can keep producing torque at higher rpm (which in effect means more power) then you can use lower gearing for the same road speed without running out of useful revs.

Top speed is the point where drag = thrust.

Ages ago I knocked up this web site (I used phpBB just to use its security) to help set up gearing for motorcycles doing top speed runs:-

http://www.bikegraph.co.uk/TorqueGraphs.php

You can select a vehicle and adjust the gearing (final drive gearing is quite easy to adjust on most bikes), and draw a graph of thrust against speed, overlayed with an estimate of the drag based on the CdA factor.

  • I'm not entirely sure this answer actually answers the question. – juhist Aug 2 '19 at 15:57
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it depends. Top gear ratios vary from car to car. If it's optimized for max speed you'll have max power rpm as max rpm attainable, if it's geared for "sprinting" you'll probably be able to redline it, if it's geared for fuel economy no, it'd be a long gear designed to have the engine working in the most efficient way when at highway speed and that's usually pretty low in the rpm band.

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