This is dictated both by the size of your dog and your particular car.
A labrador or other large dog will need a wider ramp than a toy poodle, and perhaps something at less of an extreme incline too. There is no use trying to save space in your car with a slimline model if the dog cannot use it.
An SUV may need something which is longer to reach up to the boot compared to what would be needed for a little car. Short distances could still reach, but you would have a much steeper incline the shorter you go.
The majority are made from plastic or aluminium. This makes them lightweight and also possible to fold up and take with you, which are two of the most ideal properties.
Another material is wood. These are often not foldable and are raised in a similar motion to a deckchair with grooves to place it at different heights.
But there can be a fine balance between lightweight and durability. You don’t want to get something which is affordable and as light as a feather if it wobbles when your dog walks on it and puts them off using it.
Paying that little bit more may see stronger materials without losing any other positive attributes, and would also be more likely to last for your dogs lifetime.
Maximum Load Capacity
You will need a stronger ramp able to take more weight for a heavy dog compared to one suitable for a whippet or small dog. Always look at the load capacity and ensure this is generous too. If your dog is 20kg, go for something which can take 30kg+ so you’re not leaving it too risky.
This is for you as opposed to the dog. Ramps can either unfold with hinges to get to the full length or there are some telescopic models which extend.
The latter could be easier for you to manage but overall there isn’t much difference. Telescopic models may be slightly thinner when folded up but usually not by much. Their main benefit is that you can often get the length just right so they are adjustable.
When the ramp is folded up, some may come with a carry case or bag, but either way, you will need to ensure it is safely fastened in the car so it can’t fall over or injure any pets, or become damaged.