285/65 or 285/70?

When I was looking to swap the standard tyres out on the Cruiser for some proper off road tyres, I faced a choice: should I stick to the standard size, or go a bit bigger? (Read about choosing tyres for outback touring and why you should change your factory tyres here.)

The most popular choice for the 200 Series is to increase from 285/65R17 to 285/70R17, giving just over an inch of extra diameter. What that means, apart from being able to brag "I've (almost) got 33s!" to my mates, is a ride height and ground clearance increase of about 14mm. Probably a bit less in the real world. This is perfectly legal in most Australian states.

There is also a wider range of tyres in 285/70, including a selection of mud tyres which are rare in 285/65.

There are three reasons I decided to stick with the standard size.

1: The Golden Rule

My theory for reliable outback touring is that almost every modification and change from factory specifications makes the car less reliable, unless you're very careful and you're solving a specific problem.

14mm of extra clearance is neither here nor there for me. While I have given the diff a bump or two over the years, 14mm of extra clearance doesn't make much difference in the desert. The Cruiser's rampover angle is its biggest limitation for outback touring, and it's virtually unchanged by such a small difference in ground clearance.

So changing the tyre size doesn't really offer me much in the way of improvement. Don't fix it if it ain't broke.

2: Downsides

Let's say the idea of 14mm more clearance did sound good - what are the possible downsides? Two, as far as I'm concerned: it increases the gearing, even if it's just a little bit, in a vehicle that is already geared too high. And guys on the forums report the occasional contact between the bigger tyre and the bodywork, sometimes even the upper control arms.

For the average suburban user or weekend trail warrior, this is probably fine. It won't happen often enough to damage either the vehicle or the tyre. For the outback adventurer crossing the Simpson or pounding out the PDR at maximum weight, this kind of contact happens all the time. Not only is it really annoying, it can quickly damage the tyre.

3: Availability in the bush

Another consideration is spares availability. 285/70R17 tyres are not hugely popular in the outback. They exist, but if you ask for one at Wiluna or Mount Dare, chances are you will have to wait for it to be shipped. 265/70R17 tyres are like red dirt and kangaroo shit - they're everywhere. And they have an identical 802mm diameter to the original 285/65R17.

So if it came to it, I could fit the smaller 265/70 to the same axle as a 285/65 and drive perfectly safely until I could get home. There isn't really a corresponding size to the 285/70 - you'll get close, but not perfect. You can run different sizes if you absolutely must, but it's definitely not something you should do for long.

Second thoughts

Before the Cruiser build, I had another couple of rules of thumb:

1. The maximum width for desert work is 265.

2. The minimum profile for desert work is 70.

Given that the 265/70 is interchangeable with the standard 285/65, and has a huge range of suitable tyres available, and is a bit cheaper, and is still suitable for the 8" wide standard wheels... it's quite tempting to downsize to 265/70. These tyres would be more suitable for desert driving, but are they legal?

In NSW the rules for fitting narrower, rather than wider, tyres are not clear.

VSI6 talks about changes to overall diameter and increases in rim width, neither of which is relevant.

VSI9 says:

• you must comply with the TRAA's Standards Manual

• you must comply with the ADRs

• vehicle track must not increase more than 50mm

• speed and load ratings must meet or exceed manufacturer's specs

• "it is your responsibility to ensure the vehicle remains safe to drive".

VSB14 contains a lot more technical information, but nothing that isn't captured in VSI6 or VSI9 when it comes to tyre size.

Looking at the requirements of VSI9 as the most relevant:

• The TRAA Standard Manual is not available to the public, but mostly it talks about the types, sizes, and pressures of tyres suited to particular wheels, rules for balancing and repair, and other technical data. 265/70R17 tyres are suitable for 8" rims and are routinely fitted to other 4WDs, so this won't be a problem.

• The ADRs mostly talk about the design and manufacture of the wheel and tyre itself. ADRs date back to a time when we designed and manufactured vehicles in this country, and aren't very relevant to minor modifications like tyre size.

• Fitting narrower tyres to the standard rims decreases, rather than increases, the wheel track so there is no issue with the +50mm limit. My track difference would be -20mm (half of the 20mm tyre with change, times two tyres).

• Speed and load ratings are a function of the tyre I choose, not its size.

This last point about ensuring the vehicle remains safe to drive is where it gets vague. If I make a change to my vehicle that gives it less grip, then I could fall foul of this sentence. I have two points in my favour.

Firstly, the 200 Series GX is sold in the Middle East market with 245/75R17 tyres. Clearly Toyota think this is an acceptable size.

Secondly, a good 265-width all terrain tyre offers at least as much grip, and probably more, than a cheap 285-width mud terrain tyre. Cruiser owners think nothing of using grossly inappropriate muddies for road driving all the time.

Are these two things together enough to hold my head up and say 'this is safe'? I think so. But I have been down this road on a previous vehicle, and it can get tiresome explaining this again and again to rego inspectors each year.

I think I'll stick with the 285/65, but perhaps I'll keep a set of 265/70s on aftermarket wheels for big desert trips?

What are your favourite tyres? Would you have gone for the bigger size? Comment below.