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A T-Rex (for instance) uses a P275/40R18 rear tire, which is 678 mm overall and so turns 485 revolutions per kilometre (780 revolutions per mile); that's only 808 rpm @ 100 km/h or 780 rpm at 60 mph. With a 2:1 drive ratio the motor would be turning less than 2,000 rpm at a reasonable highway speed. Yes, that's faster than the spec for 44 horsepower, but that doesn't seem fast enough to get peak power out of any suitable motor. Unfortunately, the torque of a motor like this will drop off radically far before 6000 rpm, because torque depends on current and you won't have the voltage to push 1280 amps through the motor at high speed.

Again comparing to a T-Rex, their electric version weighs 599 kg with 26 kWh of battery. I think you should expect your finished project to be substantially heavier than the 700 pound gasoline trike.
 

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I have a good bit of experience with RC brushless motors, can I assume a car DC motor also has a kv rating?
Not really. The voltage constant (KV) describes a linear relationship between applied voltage and resulting no-load speed, but it assumes that the variable field (the rotor in a brushed DC motor, stator in an AC motor) is reacting with a constant field (such as a permanent magnet stator in a brushed DC motor or permanent magnet rotor in an AC motor)... however, many DIY conversions still used series-field brushed DC motors such as that WarP family, in which the same current flows through the armature and field (rotor and stator windings), so none of the basic relationships are so simple.

Series brushed DC motors have a similar issue with the torque constant (KT): that constant assumes that torque linearly increases with current, but with both more field flux and motor armature flux, the torque actually rises something like with the square of current.

In AC motors used in EVs, KV and KT could be applied, but are typically not used. There are so many factors to consider (maximum current of both controller and motor, maximum power of controller, etc) that these motors are generally described by their maximum torque (at maximum current), speed at which they transition from current-limited to power-limited or voltage-limited, and power limit. Serious motor suppliers publish full motor performance charts, and vehicle designers match a motor to the vehicle requirements.
 

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What I don't know and had not considered was the best rpm range, and 60 mph seems a very good speed for max effciency. I may change that to 50 mph as I would tend to drive on surface streets rather than the highway. Plus highway speeds where I live tend to average 80mph. If max speed is important I have never gone above 150 mph, even in my twin turbo viper (just 800 rwhp). I doubt I would even attempt 150 in a roller skate with a motor like a T-Rex.
Peak motor and controller efficiency is typically found at moderate speed and relatively high load. In practice, production EVs tend to be designed so that the motor reaches maximum usable speed at the vehicle's maximum rated speed, which determines the gearing. To make power available at moderate speed, and to maximize torque to the wheels at low speed, that top speed is kept down as much as the market for the vehicle permits (e.g. much lower for a Nissan Leaf than for Tesla Model S), so the highest possible gear reduction ratio can be used. This assumes a single-ratio transmission, which is normal for a production EV.
 

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What is more important for EV performance, total voltage or cell discharge rating? I expect that delivering the necessary amperage for a given rpm is the limiting factor, and doing it with the lowest weight is the goal.
Neither, by itself. With the same number of the same cells you can build a high-current and low-voltage battery, or a low-current and high-voltage battery, or combinations in-between, all with the same energy capacity and same power capability. The system voltage is selected to work with the motor (and other practical limitations), and is nearly universally about 360 V (nominal) in current production EVs.

Given the same cell design and module construction (cooling, etc) the discharge rate (or "C rate") is about the same, so the energy capacity implies the power capability - a battery with more energy stored can put out more power.

Am I ontrack here, or should I be more concerned with kwH ? Its easy to convert HP to kwH, but with an electric motor I think you would need to integrate over the torque curve for a real comparison of engine vs electric motor. Are there any links to the math needed for a true compare?
Horsepower (HP) is power, and kilowatt-hours (kWh) is energy - one does not convert to the other. Did you mean kW (kilowatts), not kWh?

The nice thing about typical EV motors is that they are limited by the battery, or the controller's power-handling capacity, or motor cooling, so regardless of what the motor could possibly do it is limited to a constant torque up to some speed, then a constant power from there to near the maximum speed. In a high-performance vehicle the low-speed torque limit isn't a big deal because there isn't traction to use more, so for much of the speed range it's just a constant-power situation.

Edit: Just to be clear, my comments above about "typical EV motors" refer to the motors used in real production EVs, and similar motors available aftermarket (which are rarely used because they are so expensive), not brushed DC motors such as those salvaged from forklifts.
 

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Probably the only OEM batteries that would have a chance would be Volt batteries, as they have a relatively small size while still being at 400V. But they are much lower density so it's not a complete slam dunk. You'd just have to see if you could fit enough in to get to the voltage you need.
It's not just the Volt - any plug-in hybrid is in a similar situation of having modern EV voltage in a smaller package than a typical battery-only vehicle.

Of course the Volt is by far the most common plug-in hybrid, but there's also the Chrysler Pacifica Hybrid, the Mitsubishi Outlander PHEV, and various Volvo models. The Volvo battery is particularly small, made to fit in just the tunnel. I don't know of any comparison of these alternatives, but what appear to be the Pacifica modules are sold by various vendors under the cell and module manufacturer's name (LG Chem) if anyone wants to assemble the numbers.

The other challenge is power. You want your pack to be small but still pretty powerful. But most OEM packs use low power cells and get their power by making the pack very big....
This is the other area where the plug-in hybrids are a somewhat better source - they have a smaller battery (physically, and less energy because less range is required) but they still need enough power output to drive the vehicle.

Just don't try to use a non-plug-in hybrid battery - those are just too small, and typically are not even lithium (NiMH still works well for that application).
 
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