Many Powerchairs are offered in a variety of Speeds.
So called "Speed Packages. Chairs like the Quickie Groove, Pride Mobility's Quantum Rehab Powerchairs, or Bounder wheelchairs all offer slow 4 mph, 5 mph, 6, 8, and above "speed packages". 

So why wouldn't you simply choose the fastest one you can get - (money or "insurance" permitting)?

Well its basically down to physics, and battery capability. To go faster requires "more power". Power Wheelchairs have a finite amount of this...

With current battery technology at an affordable price that means that we are stuck with the standard "group 24" 70ah approx Gel or AGM lead based batteries on price alone.

And size too. Why?  Because to keep a sensibly low seat height, and compact overall dimensions on the powerchair to allow good indoor abilities, its the biggest batteries that can be fitted in! So the amount of "fuel" on board is "fixed". 

Now Physics tells us that to go twice as fast requires double the "gearing" and requires 4x the energy!  Horrible physics isn't it!

Most heavy (busy) full time grown adult powerchair users genuinely struggle to complete a full days use with a 6mph powerchair. I know I do. But I have a few solutions too! (later)  Some people who THINK they are heavy users manage all day powerchair use fine with the same 6mph powerchair. Which just proves they are wrong! 

Any slower than 6 mph would be plain dangerous crossing busy junctions etc, but would save battery power and increase range.  But who wants a 4mph or 5 mph chair?

If you chose the 8 mph version of your powerchair instead then you can expect to suffer around 20 to 30 percent less range than the 6 mph variant.

In other words I would run out of power on a busy day by about 5 pm! Yet in my 6 mph powerchair I can make the whole day and go to the pub in the evening as well.
 

Most 6mph powerchairs barely have enough torque (power) to properly climb thresholds, steep ramps etc with full control and authority. When you choose a faster powerchair, you also sacrifice about 20 percent of your available torque (power) generally speaking as well - making this worse still.

And you sacrifice some control capability and some acceleration.

You decrease the expensive battery service life too. Because the one thing that "wears out" your precious batteries fast is being discharged! The deeper they are discharged the greater the "damage".  Even deep cycle batteries only have a few hundred (300 approx) cycles in them at high discharge levels caused by higher speeds and the taller gear ratio required to do this.

What's the point of having 8 mph powerchairs then? (*Or even the 12 mph Bounder wheelchairs)

For some users they may be a great idea! For e.g. if you weigh say 7 stone rather than 17 stone the loss of range, and loss of torque would be a non issue. If you ACCEPT that it will have less torque, but never really need to do steep ramps, or never exceed the all day range of your 6 mph powerchair then it may also be OK for you.  Everything is a compromise.

*The bounder gets away with its even higher speed because it is huge and can accept bigger group 27 sized batteries. But the compromise here is one of size and indoor capability, and its still a little borderline on torque...

So which do you choose?

Heavy fully grown adult? Live in a hilly area? Want lots of range? Need better torque and control on ramps, or thresholds? Expect long battery service life? Then get the 6 mph version of your chosen powerchair with 4 pole motors and the biggest control system available for it if there's a choice.