Choosing the Right Vehicle for Your EV Conversion: The Select Phase of SPARK

If converting an ICE vehicle into an EV sounds appealing to you, one has to wonder: why? It doesn’t really matter. It might not make sense when you first think about it, but we’ll try to help you see things from a different perspective. It is likely that you are the type of person who needs hard, objective and unquestionable data to answer some of life’s most interesting questions, so this article will try to explain the rationale behind selecting a vehicle to convert, and what to convert it into.

The SPARK Methodology acts as a step-by-step guide to help you navigate these decisions with clarity and purpose. In the Select Phase, the focus is on defining what kind of EV you want to build. Deciding WHAT you want to achieve with your conversion is the very first step and it will dictate the most important decision of them all: what vehicle should you convert to an EV? In this phase, you will produce your Engineering Decisions: a set of parameters that will become the basis for selecting your donor vehicle, motor, battery, and charging system sizing, among other things.

SPARK Methodology Review

In our previous article, we defined the SPARK methodology, which ensures your EV conversion follows a logical progression. Here’s a quick review:

Select: Define your vehicle requirements (Engineering Decisions).
Plan: Calculate energy requirements (Energy Blueprint).
Architect: Choose components (Component Strategy).
Route: Integrate your systems (Integration Map).
Kickoff: Test and validate the final build (Operational Readiness Profile).

In this article, we’ll focus on the Select Phase, where you will determine the performance metrics and operational requirements that your vehicle must meet.

Select Phase Overview

The Select Phase focuses on defining the operational use cases of your conversion and arriving at the Engineering Decisions that will shape your EV’s performance capabilities. In other words, you’re about to define what the vehicle will be able to do. This crucial step forces you to think about some common and not-so-common scenarios that your EV will encounter, so you can make sure that the powertrain meets your expectations every time. From effortless cruising to moving maximum loads on steep inclines.

What Moves a Vehicle?

We need to start with the basics: the physics of how a vehicle moves and what influences its performance. These considerations are rooted in First Principles, which simplify complex systems by breaking them down into their most fundamental elements.

In other words: we need to talk science, and we’ll try to make it as simple as possible, so bear with us. Keep in mind that the intention of these calculations is to be able to preliminarily determine if your EV conversion requires a $2,000 USD or a $20,000 USD motor, for example. Maybe you are absolutely not interested in an EV conversion that won’t go from 0-60 MPH in 2 seconds, but you need to make sure your budget is enough. Or maybe you want to know what the components that you already own are capable of doing if used in a certain donor vehicle. Or maybe you just like to learn new stuff. Either way, this phase explains the process.

At its core, every vehicle can be represented as a single mass with different forces acting upon it, according to the Lump Mass Theory.

Some of these forces include:

Traction Force: Generated by the motor to move the vehicle forward.
Resistance Forces: Including aerodynamic drag, rolling resistance, and gravitational pull (on inclines).

In short, in order to move the vehicle, it needs to be pushed by a force (Traction Force) that will be greater than the Resistance Forces acting upon it, like air drag, rolling resistance from tires, and the vehicle’s own mass, among other things.

By analyzing these forces, you can determine the power and energy requirements needed to achieve specific performance goals like speed, acceleration, and range. This will be done in the next phase of SPARK: Plan.

Engineering Decisions

Total Vehicle Weight

At the heart of your engineering decisions lies the total vehicle weight. This represents the total weight your vehicle must handle in operation, including its curb weight, passengers and cargo: this is referred to in engineering as Gross Vehicle Weight (GVW). The weight of a trailer needs to be accounted for in applications that require it, and the total weight of the vehicle (GVW) plus the trailer and load is known as Gross Combined Weight Rating (GCWR). Basically, you need to know how much mass the motor needs to push!

What kind of vehicle? Your intended application will dictate the type of donor vehicle. Are you looking for a sporty convertible, a practical sedan, a versatile hatchback, a rugged pickup truck, a spacious van, or something else entirely? Each vehicle type brings different baseline characteristics that influence its performance and powertrain needs. Consider what the vehicle was originally designed to do and you’ll get an idea of what you can easily achieve, but don’t let this be a constraint for your mind. Do you want a spacious van for Formula Drift that has only 60 miles of city range and fully charges in 20 minutes? Take note of that; you might be able to pull it off! Remember: think creatively; not everything needs to make total practical sense.
How many passengers do you want to move? The expected number of passengers directly affects the GVW, as each individual adds weight that impacts acceleration, braking, and overall energy requirements. More people means more power, which means a bigger motor, which means more energy.
How much payload, or stuff, other than passengers? Think about the items you typically transport. Are you regularly hauling heavy appliances like refrigerators, or is your day-to-day load as light as a gym bag and a laptop? What about that occasional track day—do you carry your spare set of wheels and tires, tools, and picnic setup? Defining your additional load expectations ensures the powertrain is appropriately sized for practicality without overengineering. For example, a small passenger car (sedan, hatchback) can normally carry 100 to 200kg (220 – 440 lbs) of cargo. An SUV or crossover 200 to 400kg (440 – 880 lbs) , a pickup truck 500 to 1000 kg (1100 – 2200 lbs) and vans/minivans 300 to 600kg (660 – 1300 lbs). Use these as baseline ideas if you can’t think of specific cargo requirements.
How much do you intend to tow, if at all? If towing is part of the vehicle’s intended purpose, you also need to consider the combined weight of the trailer, and the payload in the trailer.

Maximum Grade

What’s the steepest grade the vehicle is expected to climb? Determining the steepest incline your vehicle will need to handle is vital for ensuring it performs reliably in its intended environment. You don’t want a car that won’t go past 20 mph at full pedal on the slightest of uphills—unless what you want is a golf cart, which you might get for cheaper off of Facebook Marketplace. To determine the steepest slope that your vehicle will have to drive through, check your local and state infrastructure data to make sure you calculate the steepest around you. For highway driving in the continental USA, using an estimate of 7% will do the trick. If you live in Pittsburgh or San Francisco, these requirement might be vastly higher, so pay attention to your actual needs.

Maximum Speed

More decision-making. What do you want to prioritize: highway driving, city driving, off-roading at low speeds, or off-roading at high speeds? This decision doesn’t mean the vehicle can’t do more than one, but you have to focus on one first. If this is an autocross car that prioritizes acceleration but doesn’t ever need to go above 50 MPH, then that’s what you should design for. If the vehicle is to be a road trip machine that should be able to maintain speeds of up to 90 MPH (WHEREVER LEGAL AND SAFE!), your requirements will be different. Decide on a maximum speed that will satisfy your worst-case need.

Maximum Acceleration

The magic trick of EVs. Whether merging onto highways or navigating city intersections, everyone wants crazy acceleration these days. This seems to define the character of most battery-powered cars, but is it really that simple? Think here about your use cases: do you really want crazy acceleration, or will you settle for less? The powertrain needs to be sized accordingly, and cost tends to increase with performance, so keep that in mind. Be realistic. Though being realistic sometimes means chasing that sub-3-second 0-to-60 MPH time in your 2-door 1999 GMC Tahoe with the off-road tires you always dreamed of.

Desired Range

Yet another thing to be realistic about. When defining the range your EV needs, try to stay clear of the so-called “range anxiety.” Make sure you really understand what your range requirement is. If you only want it to cruise around town, go out and cruise around town today. Do it two or three times. How many miles did you really drive? Remember that more performance means more cost, so if you don’t need 300 miles of range for your project, don’t chase them.

Charging infrastructure available to you should also be considered when determining range. If you want to drive 150 miles to a neighboring town, you have to make sure that you have access to charging infrastructure or that you have the right amount of energy storage (batteries) to make it back home safely.

Weather

We have all heard it by now: range takes a big hit during very cold or very hot weather. One of the reasons is that the batteries, mainly Li-ion, need to be kept at a stable temperature, which requires cooling or heating the cells. While this might be irrelevant for those living in relatively climatically stable places like Mexico City or very close to the equator, more extreme climates, like in the northern USA or the Middle East, call for further analysis. More decisions have to be made since you might choose to use your EV conversion only during the summer months but leave it stored during the winter months, the other way around, or use it in any and all climates. Choose wisely.

Why Define These Use Cases?

The Select Phase allows you to make some creative decisions and forces you to think about the specific aspects that you want your EV conversion to achieve through the visualization of different operational use cases. Defining these use cases is about more than just meeting performance expectations. It’s about engineering a powertrain that is reliable, efficient, and capable under all conditions. No one wants to drive a car that will leave you stranded when its battery runs out due to poor Engineering Decisions or one that feels so slow that safety starts to become a serious concern. Part of the art that comes with an EV conversion is making it enjoyable, useful to some extent. By understanding what your EV needs to do—whether it’s merging onto a highway, climbing a steep hill, navigating city traffic, or all of the above—you ensure that every component, from the motor to the battery, is sized appropriately.

What’s Next?

The Engineering Decisions produced in this phase will be the basis of our next one, which will explain how things like Tractive Force, Rolling Resistance Force, Aerodynamic Drag Force, and Grade Resistance Force are calculated. Simple, fun, and effective, the process we will work on will give you the basis of what the motor, battery, and charging system should be able to deliver. This will in turn give you the ability to hunt for component suppliers to estimate the cost of our conversion.

Choosing the type of vehicle for your EV conversion is more than just a technical choice—it’s about imagining the potential of what your converted vehicle can represent, both functionally and emotionally. Does the project excite you? Then by all means, go ahead! If it does not excite you, we urge you to reconsider. We do not believe that an EV conversion makes much sense for practical reasons alone in 2025 wherever there are many new and used EV offerings in the market. This type of project has to have some passion or personal connection behind it.

Remember: in engineering, like in life, most decisions are a compromise. The SPARK methodology embraces this reality and tries to guide you, the reader, to think things through and balance performance, cost, a sprinkle of practicality, and a lot of passion to create an EV conversion that truly meets your needs and inspires you. If you want boring or you don’t care about the car, just buy a used EV!

Good engineering comes from graceful compromises and relentless decision-making. Anyone can design a car with the highest speed, craziest acceleration, incredible payload capacity, etcetera, but no one will be able to build it for a reasonable price. Even art has its limits sometimes!

Join the conversation: share your thoughts, ask questions, or tell us about your EV conversion journey or plans in the comments below. We’d love to answer any questions we can and/or learn from your experience.