Tyre size does matter

By: Steve Cassano


Increasing the size of your standard 4WD tyres has benefits such as increased clearance, offroadability and traction, but it also poses challenges that you can overcome with a bit of clever thinking.

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Tyre adjustments are in the top three most popular modifications among 4WDers.

Standard 4WD tyres and standard SUV tyres from the factory floor are a compromise; they’re a balance between performance, handling, durability, safety and price.

These standard tyres don’t really excel at any one particular criteria, especially when you start to use your 4WD and camper on those rougher and more demanding journeys.

Choosing an alternative tyre can be quite confusing and certainly frustrating for many. There are so many choices. With the huge selection on the Australian market, there are never a lack of opinions from other 4WD owners during campfire discussions.

Rather than writing an article based on my opinion, I’d instead like to outline the impacts of each option on both your 4WD and you the owner, to help you make the choice yourself. Working through these considerations before jumping onto the big tyre spend will likely save you some heartache when you're on the track with an offroad camper hitched behind.


TYRE MEASUREMENTS

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A tyre will have its size stamped on the sidewall of the carcass. The most common standard used is a combination of metric, imperial and ratio as a percentage.

For example, 285/70/17 translates to a tread width of 285mm, a tyre to fit a 17in wheel rim and the 70 represents the tyre’s profile as a percentage of the width.

It’s worth noting that there are some tyre brands that use imperial only. For example: 33/10.5/17, which translates to a 33in diameter, 10.5in wide tread and to fit a 17in wheel rim.


WHY INCREASE TYRE SIZE?

Increasing the size of your tyres could include expanding the tyre’s diameter and/or the width (tread) or a combination of both attributes. In the vast majority of cases choosing to increasing diameter is tied in with an increase in tyre width. The major benefits are:

  • It’s an easy and reasonably economical means of increasing clearance under the vehicle. Increasing the clearance helps to prevent parts of the undercarriage, chassis and body of your 4WD or camper from scraping or getting hung up on track obstacles.
  • Increasing the tyre’s diameter improves the 4WD’s approach, departure and the ramp over angles. This will improve the 4WD’s capability when it comes to overcoming track obstacles such as steep slopes, rocky terrain and soft sand.
  • More rubber will be striking the earth and thus aid with traction. This extra width, in conjunction with a longer footprint, helps you to negotiate softer terrain such as sand, especially when combined with lower tyre pressures. This will help the vehicle to ‘float’ over the sand rather than dig in, to ensure continual momentum.
  • Installing moderately larger tyres tends, in the majority of cases, not to interfere with a vehicle’s geometry. This is very dependant on the model of your 4WD and chosen size.
  • They look damn good. Nothing else makes a bolder statement that you’re a fair dinkum 4WDer than a set of big tyres filling the wheel arches, especially when paired with an aggressive tread.

WHY NOT TO INCREASE TYRE SIZE?

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It’s worth noting that the greater the increase in tyre size, the greater the effect it will have on your vehicle. Below are the major disadvantages:

  • Tyres may not fit seamlessly. A minor increase in tyre diameter, say half an inch, should have a negligible impact on most vehicle’s characteristics. It’s very dependant on vehicle model. The tyres need to clear areas around the front steering, suspension components and wheel arches on full articulation, in particular while turning lock to lock. Some vehicles, especially independent variants, have little room for tolerance, mainly around their front struts.
  • Don’t forget the wheels. Will the tyres fit your existing wheels? All reputable tyre manufacturers clearly state the minimum, maximum and preferred wheel specifications for each of their tyres. So check online before parting with your hard earned.

TYRE SIZE SOLUTIONS

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PROBLEM

You could fix clearance issues using one of these two methods, or a combination of both:

SOLUTIONS

  • Get yourself a new set of aftermarket wheels with different specifications. This may include a different offset, in order to position your wheels/tyres away from vulnerable components.
  • Modify or install a suspension lift including properly sized bump stops.

PROBLEM

Your speedo will be out. Because you’ve increased the rolling diameter of your wheel from stock, this changes the gearing and the vehicle’s speedometer, odometer and on-board computer could be inaccurate. As a result, the speedo may show 100kph when in fact you’re travelling faster.

SOLUTIONS

  • Consider a physical apparatus known as a speedometer gear within your driveline that measures the speed. Commonly used for older model 4WDs, you'll find this is a relatively cheap fix at around $50.
  • Next is a software alteration that is available for many makes, including Jeeps, for about $200. This uses a device that plugs into the vehicle’s OBD port, allowing you to enter the required information to correct the speedo reading, along with other features.
  • Some 4WDs use a WIFI OBDII interface, for about $20, in conjunction with a smart phone app (e.g. Ford’s fordscan for a few dollars), to allow you to make changes to tyre size and other useful features.
  • Other speedos, due to their complexity, are only able to be recalibrated with a visit to the dealer’s service department.
  • Finally, there is a device commonly called a ‘speedo corrector’ for under $50 that allows changes to tyre sizes and gear ratios. It requires some electrical expertise, so may not be suitable for all 4WD owners.

PROBLEM

Increasing your tyre diameter is like changing the driveline’s gearing, so this will impact the vehicle’s available power and torque. It could be frustrating to drive a vehicle that seems to have been sucked of it’s power where before it flew along nicely. You’ll really notice it when climbing longer, steeper hills. 

SOLUTIONS

  • Regear the driveline. The most common method involves changing the crown and pinion gear within both differentials. As an example, my vehicle came with 32in tyres and 3.73 differential ratios. For me to retain the same performance and to run 35in tyres, I’d use the calculation (35/32) x 3.73 = 4.08. Then round it up to the most common gear, in this case 4.11. 4.11 would therefore be my best choice. However, whether you're hard rock crawling, long distance touring, towing a rig or you’re just hunting for best economy and durability will all make a difference to your choice. A full set of gears can set you back around $2000 for supply and installation, so please do your research before committing.

PROBLEM

Increasing your tyre size puts more stress on your vehicle, especially around CV or universal joints, axles, prop shaft, clutch and auto transmissions.

SOLUTIONS

  • You could replace some or all of these components but start saving your dollars. To do so can be a costly exercise.
  • Alternatively you can take the simple approach of driving with a mechanically sympathetic attitude. Maintain and treat your vehicle appropriately and you will help to minimise failure of your vehicle in the future.

PROBLEM

Don’t forget about the spare. Spares are kept in all sorts of places such as under the cargo floor, on the tailgate, under the rear and even inside the cargo area.

SOLUTIONS

  • Before proceeding, make sure you can reposition it in it’s standard position as some vehicles have little room to do so. Not much you can do if your new spare doesn’t fit, unless you...
  • Invest in an alternative spare wheel carrier. These can be expensive up to the $2000 mark and beyond. They can be heavy and frustrating to operate and not always available for your 4WD.
  • You could throw it up on your roof rack, but that there inherits its own problems.

CONCLUSION

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Whatever you decide to do, really think about the practicalities of choosing larger tyres. There are dozens of factors to consider, including how you use your 4WD, whether your tyres are a commonly available and even the legalities associated with changing tyre sizes. And then, with everything set, you can confidently head off to whichever beautiful destinations are on your radar.

Check out the full article in issue #130 of Camper Trailer Australia magazine. Subscribe today  for all the latest camper trailer news, reviews and travel inspiration