Oversized Tyres And Why They Often Aren't The Legal Or Smart Choice
Skip to content

GEAR THAT WORKS

Oversized Tyres And Why They Often Aren't The Legal Or Smart Choice

26 May 2022
Oversized Tyres And Why They Often Aren't The Legal Or Smart Choice

Buyer Beware – Oversized Tyres are neither Legal nor Smart

What is the best tyre for my 4WD?  

It's one of THE MOST OFTEN asked questions in the 4WD World and it is similar to inquiring about the length of a piece of string, as there are many variables to be considered.

If I had to go on record, and choose just one tyre type and size - and this is going to be a 'coming out of the closet' moment for this tyre whisperer - I'd have to say that the best 4WD tyre, in the history of the world, is the...LT235/85R16 120Q.

There we go, I feel much better now.

We’re still using this size on Steane’s mighty Defender Rolf

In that size there are a couple of my all-time favourites and twenty-plus years later, they are still readily available.

The Bridgestone Desert Dueler D661 was my first choice for SA Government clients operating in the APY Lands of South Australia when fitted to Troop Carriers back in the day.

Bridgestone's D661 Desert Dueler is a ripper skinny that's tough as for Outback Australia

The other was Toyo’s fab M55, a tyre that was suited to just about any mining operation, again applied to 70 series Land Cruisers in all their forms.

Toyo's fab M55 is a mining company favourite and it could be yours too if you crave a skinny

Different patterns for different jobs, but these two were clear winners for durability, availability and grip.

But Facebook is always right, right?

So many times when I’m perusing the Facebook forums there’ll be a “sexy” pic of a 4WD, all pumped-up on steroids and before you know it, the punters start lining up and firing off questions about “what tyre size is that mate” and “those wheels look awesome!”

Sex appeal vs legality on display here. Arctic Truck AT35 D-MAX

Right then you know there’s going to be some Bankcard action over the following days, as the shekels are exchanged for wheels and rubber totally unsuited to reality.

Is it, that going for a 33” or bigger, perhaps a 35” or 37” diameter, will remake your fourbie into something super-capable and tough, or ruin it completely?

I’m leaning toward ruin, particularly at the bigger end of the size scale.         

About the only 4WDs sold in this country that will take such a big increase in rolling diameter without either total compromise, or major re-engineering, are an Iveco Daily 4X4 or a Jeep Gladiator/Wrangler Rubicon. No, no, no, don’t tell me that your 79 series Land Cruiser is in the same league, because that is a bald-faced lie.

Jeep's Wrangler and Gladiator can cop 35s without too much trouble because that's the way they were engineered

I thought it prudent to talk directly to why a change in rubber can be good, but not to the point when all you’re doing is patting the pony.

Stock rubber is nasty

Stock rubber on just about everything sold in this country new has “passenger car” written all over its sidewalls and tread face. That uniform and tight pattern and low-depth (9mm) tread, is all about low-friction and rolling resistance (for fuel efficiency), being quiet (so you can hear your favourite tunes) and matching the ninety-percentile of buyers (who are just swanning around town on the blacktop).

Isuzu's MU-X now offers a twenty-inch wheel fitment which is painfully thin on just about any 4WD tyre benchmark

Those same tyres for me are pure evil. Because I rarely hang out in town, I crave something that has some gravel and goop-goodness, plus an ability to carry a load and resist the point of a rock.

When they are thin, they go pop real-easy on the gravels at just about any speed

That only comes in a Light Truck (LT) spec and even better, when it has a “120” or greater load index.

Let’s talk about size and how it does matter.

A new 4WD vehicle (like a D-MAX/MU-X, Hilux/Fortuner/Prado, Ranger/Everest, Triton/Pajero Sport) will likely have a rolling diameter around the 760mm mark when using these common OEM (Original Equipment Manufacturer) sizes:

P265/50R20 107V

P255/60R18 108S

P255/65R17 110S

At this size and using American tyre-speak, you might call them 30”.

The vehicle maker has selected that size for balance across all of the likely driving parameters (and thus plenty of compromise), firstly to get a degree of clearance under the vehicle for off-roading (but not too much, because that affects airflow and creates turbulence downstairs, that will cost you fuel economy). Secondly, to arrive at a gearing nirvana, where the engine can pull the jigger and its load down the road at a reasonable pace. Lastly, satisfy the safety-Nannas, both the legislators and the vehicle’s own electronic safety systems.

Toyota's Prado has plenty of fans but most of them will tell you the tyres are underdone

If you are only going to be spinning your wheels on the bitumen, you might, just might, be fine with OEM, but towing a bigger load or checking out a bush track will bitterly disappoint, because they will fail and always when it is most inconvenient.

MU-X can now legally tow 3,500kgs but the tyres might now be on a knife-edge

Looking at the sidewall-scratchings, the game is given up when you zero in on the load indices.

Revealing very modest carrying capacities of 107 (975kgs), 108 (1,000kgs) and 110 (1,060kgs) at maximum inflation, that flimsy reputation is further reinforced when you do the sums on the speed-ratings, V (240km/h), H (210km/h) and S (180km/h).

What you’ve got with this little lot, is a bunch of high-speed and paper-thin pretenders, that should be fitted up to anything other than a 4WD in this country.

How big is big?

Having made the decision to replace them (and that’s good), just how big should you go?

Typical modern legal alternatives to the previously mentioned OEM sizes will be either an LT265/65R18 122Q or an LT265/70R17 121S. Both roll out around the 800mm or 32” mark. Notice, no mention of anything in 20”, because they are a new category which I’m calling BC/T (that is Boulevard-Cruiser Terrain).

Toyo's new Open Country AT3 in LT26570R17 121S is about the perfect size for a modern vehicle

Legally, you are permitted a 50mm increase in diameter (that’s over the stated size seen on your tyre placard) and that should be enough. You’ll end up with another inch under the truck in clearance and a spare that will still fit.

Typical tyre placard detail will include pressure recommendations and OEM tyre and wheel size as fitted from new, from there go up 50mm

But that doesn’t stop some people pushing the legal boundaries and then some, and I’m banking that the fog of Covid has erased the memory of three short years ago, when the constabulary of QLD and NSW went bonkers on lifted 4WDs (that is tyres and suspension), making them instantly illegal. I reckon another blitz is likely due.

It's that AT35 D-MAX again, just teasing you with a glance down its flanks

Let’s say you’ve bought your brand-new Volcanic Amber Isuzu D-MAX X-Terrain and want to make it into an Arctic Truck AT-35. In the UK, somehow that is possible, straight off the showroom floor, but here you will be going your own way having a crack. This is a checklist of what you will be considering…

Will those wheels fit?

There is only so much room under a guard, so in short, no. Some finessing will be required via a big suspension lift and maybe a body lift too, and a fiddle with the position of the mudflaps, plus a bit of a wave around with an angle grinder on the body mounts up front.

How much poke?

With that much wheel there’s going to be a gargantuan change to the offset of the rim and that spells plenty of poke, so a big old flared arch is in order to disguise a sidewall that might otherwise be revealing itself.

Squeezing 35s under the guard of a D-MAX is no easy thing to do. Arctic Trucks do some serious finessing to make this happen

What mass does #1

Because the physical weight of that new wheel/tyre combination will be greater, expect longer stops. Yep, the added inertia created by that heavier-than-normal wheel, multiplies the torque-rolling effect and puts heaps more strain on puny brakes (especially rear drums), making crash-stops extremely problematic because they’ll take forever to slow down.

What mass does #2

It is a story about weight. A stockie 30” (tyre and wheel), as supplied on the vehicle new, tips the scales around the 27kgs mark. With a 35” (tyre and wheel) you’ll be knocking on the door of 50kgs. Add up the difference across five wheels and that’s the equivalent of putting an extra bod in the back seat. That will put a dent in your GVM, because I bet, you will still want that 3,500kgs towing ability, won’t you???

What mass does #3

That extra weight applied to an IFS hub/front end assembly that was never designed for it will create new loads on all componentry – bushes, ball joints, control arms and mounts, wheel bearings, steering arms and rack, driveshafts and constant-velocity joints, the whole lot. Expect greater servicing costs and plenty of breakages off-road.

The old IFS weak link, a busted CV joint

Get the slide-rule out

Additionally, the geometry will be changed. The arc of the suspension will be different and you risk more of the damaging, changed-angle potential on those same front-end components, coupled to out-of-alignment moments on that first rock contact, increasing premature tyre wear. It is a good way to promote evil handling, plus the breakages.

This diff centre is smashed, the sort of thing that can happen with torque loads dramatically increased above their design-spec

Under pressure

Of course you mob will still want to run your 40psi/275kPa when the vehicle is empty and wonder why the ride is so rough? Big tyres were designed for big vehicles. Little vehicles with big tyres will need less air. That’s simple tyre science.

How to bugger your gearing… and Royally

Going oversize will affect gearing. The traffic-light Grands Prix will be slower, so too overtaking on the highway and if you put a trailer or caravan on the back, you’re heading towards a cooked transmission in hilly areas with the new loads, as the trans starts hunting for the right gear and you start fiddling with overdrive manually.

You’ll be looking for a diff-gear swap, if it exists, to return to some semblance of normal.

Going down… very quickly!

Off-road that same increased (taller) gearing will affect the descending speed of the vehicle and adding extra speed to LOW range 1st gear descents when you don’t want it and also affecting available torque at the wheels, when say on sand, you want to crank it. It’ll be slower to respond and unable to sustain consistent momentum.

On test in the Flinders Ranges in the new D-MAX revealed it was a comfortable descender, BUT make it into an AT35 without changing the gearing and it will need a new level of brave

More bump and thump and fuel consumed

Wider also increases resistance in sand and mud. You’ll create more bounce and pull reaction on all surfaces, as the wheel assembly reacts to imperfections. The increased height will add to fuel consumption too. More turbulence under the car, coupled to more wind resistance on top, will make you feel like you’re driving at $2.75/litre every day of the week.

Safety electronics mischief

Going too tall also creates mischief with electronic safety systems.

Advanced radar and camera safety systems seen in a modern windscreen are calibrated to read the road ahead from a certain height. If the vehicle has been jacked up too high, it will impact on the computations the vehicle has to compute.

Isuzu's safety tech called IDAS computes collisions and things from a nominal height up in the windscreen

Some vehicles have to be reset at the dealership, but that will only be possible within the boundaries of the manufacturer’s design parameters, whilst other vehicles have a degree of AI (artificial intelligence) where they’ll “learn” the changes and adapt. There is a likelihood they will become reliably inoperable and make the car a defect-magnet.

Whacky speedo

Stock rubber will usually provide a speedo reading that tells a story about ten percent shy of reality. In other words, you’ll be travelling 90km/h instead of the sign-posted 100km/h.

Going to a 32” fitment it will usually become dead accurate, so fitting an LT265/70R17 or an LT265/65R18 will put you right on the posted speeds. Go bigger again and the speedo will be up the duff.

Here's another one that'll make your speedo dead-accurate on a D-MAX or an MU-X

Over you go

Increased wheel/suspension height also increases the risk of a rollover in high-speed changes in direction, like when avoiding animal strike and doubly so if the vehicle is setup with roof-rack or tub rack with a roof-top tent and the like. Expect instability on a grand scale.

Brand new Ford Bronco having a bit of a sleep on the side of a steep hill, where raising the centre of gravity often messes with vehicle balance. Pic Bronco6G

What warranty?

No-one is going to warrant this type of modification(s), so when that first driveshaft snaps and leaves its shrapnel in the diff, you’ll be needing a day’s profit from a Columbian cartel’s funny-white-powder trading to make it all right again… until the next time.

Breaking the fragile bits on your IFS fourbie might have you looking for some extra revenue like selling bags of powdered scorpions

It’s a wrap

My idea of tall, is usually accompanied with skinny, hence my past love affair with a 235/85. Sadly, manufacturers have robbed me of that option, as brake rotor dimensions have increased to the point where a 16” wheel won’t fit around a modern brake assembly, with room to spare.

An 85-aspect ratio (the sidewall height of the tyre) was awesome with a capital A, offering lots of protection for the bottom of the rim. That additional acreage with more sidewall to flex, meant better dampening over corrugations and most importantly, a huge volume-to-footprint length that no modern tyre can equal when off-road and looking for grip. Best not forget better fuel efficiency too, as you disturb less air when you are skinny.

So, when you are busting your chops to get onto a set of 35x12.5R17 to make an Arctic out of your D-MAX, just remember that you better have an engineering certificate sitting in the glovebox, and I suggest you get it laminated, because the coppers will be likely, and daily, be pulling you over and wanting to put their grubby mitts on it.

The rear-end of an AT35 D-MAX will reveal just how much wheel poke you've got going to following cops, nice but naughty!

Nup, ridiculous rubber is neither legal nor smart!

Footnote

Since 16” are out the window now thanks to bigger brakes, it’ll be 17” that will prove most useful for your typical dual-cab ute or medium sized wagon.

To arrive at the best compromise between urban driving, 4WDriving, towing and universal availability, plus meet my benchmark for tyre strength, you will likely want this one in an all-terrain…

LT265/70R17 121S

This is likely the size that'll be the greatest all-rounder for most folk and their mid-sized 4WDs

------

Visiting the following links will give you an insight into Loaded4X4 and the products we sell:

Product Reviews

Dynamic Tune Suspension

About Our Suspension

Products Page

 

About Us

---- Post Script

Yeah, but 18s look better - Steane

Prev Post
Next Post

Thanks for subscribing!

This email has been registered!

Shop the look

Choose Options

Edit Option
this is just a warning