One of the most common modifications done to a vehicle going off roading is to fit bigger tyres to gain more clearance and perhaps go for an All-Terrain instead of a highway pattern to get the crossover ability.
In my experience the most likely cause of not been able to get through a track is contact with the chassis (not traction!) and this is even more the case when it comes to sand, once your chassis is on the ground the wheels no longer have the friction needed to drive the vehicle forward and your effectively stuck.
No matter how many wheels spin or what the traction control system can brake to divert power to another wheel no push down with gravity to create friction at the wheels means you aren’t going nowhere without help.
Putting higher springs in will pick your chassis up and since the differentials on an Adventra sit above the lowest point of the frame any chassis lift helps, remember though contrary to popular belief, lift does not give you raw clearance things linking to your wheel could still affect what you can and can’t clear and there is a limit for the front CV’s before you hit new problems.
The only way to gain raw clearance is to install bigger tyres. I won’t go into detail about this but a quick google will explain why.
Going for a larger tyre and a basic lift gives more clearance & improved traction results in less to contact with rocks/uneven terrain/ruts lowing your chance to hit an obstacle and with an All-Terrain you will get improved traction off road with non-flat/tarmac surfaces due to the rigid tyre patterns to assist in finding something to grip to.
Adventras and tyre diameters exceeding 27.8” with lift (just springs) but no other modifications will give protentional scrubbing on the front inside arch plastic when hard lock on a turn, mostly seen in reverse but does happen when say parking into car spot on full lock.
If you don’t want this stay under 27.8” diameter in your overall tyre choice. I’ve found an acceptable maximum with no other modifications other than lift is 28.4” and it fits in line with a few common All Terrain tyres in 16 & 17” rim setups and you only just contact. On my vehicle I’ve made a small cut so I don’t have any contact which has no downsides seen so far and you can’t really tell I’ve cut it anyway.
Adventras came standard with 17” alloys and a 225/55 tyre giving a rolling 26.7” diameter, whereby the Crewman/Cross comes with 16” alloys and a 215/65 tyre giving a rolling 27” diameter to give you an idea where you are starting from.
16″ VE steelie rims can used on the Adventra but they will need to be drilled out to 71.5mm from 66.9mm (taking note VZ commodore size is 69.5mm). They clear the brakes on the front which is important to know as some 16″ rims will not clear front brakes on an Adventra.
They clear the brakes on the front (you need to be careful as some 16″ steel rims wont).
This tyre calculator does well if your browsing All-Terrains or Mud-Terrain tyres to put on enter the specs in and it will tell you the rolling diameter so you can tell if it will fit or not plus sidewall & width difference. https://tiresize.com/tyre-size-calculator/
Take note that All-Terrains come at a disadvantage as do Mud-Terrains, do your research well, don’t compromise too much on wet weather traction on highways if your vehicle will continue to be daily driver as it is more important than say if you make it up climb or not when out and about with friends.
How much difference size increase really make?
In general, going from factory 26.7” dimeter to my current 28.1” I get an extra 18mm clearance really it isn’t a lot but enough! General rule of thumb is for every 2” extra rolling diameter you get approx. 25mm more height and you can’t get to 28.7” diameter easily without starting to cut into the plastics in the front if you enjoy using your full turning circle.
Is it worth it? Some say it does, and some say it doesn’t. I would say the right Tyre pressures and tread pattern would make more difference and keeping the Adventra factory looking at this point is a better option if it’s a daily driver as well.
Credit Photo Provided by Clive William Gray
From all my research the best is to go to 16” with a rubber that will calculate to around 28.4” if you want to maximise your off roading as if you decide to go hit the sand dunes once you deflate your tyres to 15psi remember your clearance drops again due to the lower psi but you still get good ride height and not a lot needs to be done to keep this setup for a daily driver.
If you frequent the camp sites and tracks 16” > 17” simply due to the sidewall having more rubber height. Off road this helps with getting additional traction for both when in low tyre pressure situations and really rough uneven terrain – and well looks better!
Take caution however that the Adventra has larger front brakes than its commodore-based variant, only so few fit alloys will fit around them. The Crewman/Cross 16” alloys factory is a good choice, there is also many aftermarket steel options as well.
If shopping aftermarket a size 16″ x 7.00″ PCD (Bolt Circle) of 5 x 120 and an Offset of 25 is normally the spec to go by without any verification though I’m told a centre bore of 70.2 but care should be taken to ensure whatever you get clears front brakes as they are bigger on the Adventra than the commodore-based variant.
Trade offs after going to a larger tyre?
Speedometer goes out nothing someone with a Tech2 can’t fix in a few minutes be aware after you’ve had the change to get this fixed or know the difference on the speedo otherwise you might get a surprise. Generally, per 2” extra diameter on the wheel is 10% slower (100km/h on speedometer = 110km/h true speed). My insraunce with Club 4×4 advises that I can have up to 10% variation in my speedometer vs true speed to be legal reading instrument.
You lose power & torque (apparently) I’ve not noticed anything to support this but if it is true its very minimal in the range we are talking, but this can also chalk up to worse fuel efficiency so just keep that in mind, also if your speedo hasn’t been recalibrated with updated PPK than your computer will report incorrect fuel usage per 100km as well.
Braking efficiency is reduced (slightly) to what you might be used to before. I have no evidence to support this in my changes but it’s a well-documented downside by people with tyre size upgrade.
Finally if you want to fit a spare Tyre in the space under the boot liner a 225/60 on 17″ alloy will fit flat you will need to be also carrying a compressor to get air into it if you need it. Your only option if you want a fully ready to use spare is going 225/55/17 or going slightly above normal size and going 235/55/17.
There are a few other things that I haven’t mentioned here but they aren’t heavily applicable to the Adventra such as higher centre of gravity, been able to get more modifications for off roading etc.
Can make em fit, but is it legal?
Information is based on my experience, because I’ve done it doesn’t mean its allowed, I’ve done my own research and you must do the same.
Credit Photo From Adventra & AWD Holden Facebook Group
You are responsible for operating a vehicle on the road that is safe, and legal. If your vehicle can be proven to be unsafe and not legal in some cases your insurance company may not fully cover, you in a claim. This can lead to all sorts of legal ramifications for me I’ve declared my tyre size used with my insurance company and I’ve only been able to get insured by Club 4×4 on my slightly oversized tyre configuration there may be other insurers out there just take due diligence when making these types of modifications and make sure you are covered in any event.