One of the most common modifications done to a 4WD is to fit bigger tyres to it. This is done for a number of reasons, but the primary one is to gain more clearance. You won't get far with the bottom of your 4WD dragging on the ground. A lift kit will pick your chassis and body up, but the lowest point of your vehicle is always going to be your differential (except on independent suspension vehicles). Contrary to popular belief, a lift does not give you more clearance. The only way to gain true clearance is to install bigger tyres.
Why fit bigger tyres?
There are a few benefits of fitting bigger tyres, which I will go into below. Remember though, every 4WD modification comes with a list of downsides too; don't forget about them!
I mentioned above; the only way to truly gain more clearance in your 4WD is to install bigger tyres. Actually, I lie; you could install a set of portal diffs from Mark's adaptors, but they are extremely expensive and out of most people's budget.
If you add tyres that are 1 inch, or 25mm bigger in diameter, the height of your vehicle goes up by 1/2" or 12.5mm. This is important; your vehicle only goes up by the radius, not the diameters difference. Most people go for 1 or 2" bigger tyres, giving you 12.5mm or 25mm of additional clearance. It's not much, is it?!
The bigger the tyre, the more rubber is in contact with the ground. This results in greater traction, as well as greater flotation. This is why vehicles with taller tyres perform well on the beach. More surface area touching the sand means you have less chance of sinking!
Tougher looking vehicle
I nearly didn't put this on the list, but the reality is, some people really care about this. A 4WD that has one or two size tyres up looks better. If this is what you are doing the mod for though, I'd suggest you skip it; just get a set of muddies! I prefer function over form, but not everyone agrees with this!
How much difference does it really make?
Lets look at this from a number of perspectives. For every 25mm bigger diameter tyre size you increase, your vehicle only gains 12.5mm of extra clearance. Say you go from 31" tyres to 33" tyres, you will gain 25mm. Does that 25mm really make much difference? Some say it does, and some say it doesn't. I would say the right Tyre pressures and tread pattern would make more difference.
If you go up more, from 31" tyres to 35" tyres, you gain 50mm clearance. This is quite a bit more, but again, nothing that Lockers wouldn't make up for.
It's interesting to look at the drag marks left by 4WD's; check it out next time you go off road. It gives you a good indication of whether your tyres are too small for the terrain. There's nothing wrong with dragging a diff slowly and carefully over terrain, but you don't want to smash it up against anything. This is where diff guards come into their own.
What about the 4WD you have?
This is the factor that makes all the difference. Take a 1990 model Toyota Hilux, and compare its clearance to that of a 2010+ version. They are chalk and cheese. Older vehicles, with solid axles tend to have more clearance from the factory than modern vehicles. Does this then suggest that you don't need to go up size tyres? Perhaps!
How big are your diffs?
When I had my Hilux, I compared the clearance from the ground to the lowest point to that of a GQ Patrol. The Hilux had about 25mm extra clearance right from the get go, purely because it had smaller differentials hanging down. Being oversprung too, they have a huge amount of additional chassis and body clearance over a GQ Patrol. The point I am making is it very much depends on your vehicle, and where you drive as to whether the bigger tyres are worth it.
Am I allowed to run bigger tyres?
Please, please don't skip this. You are responsible for driving a vehicle on the road that is safe, and legal on the road. Despite this, many people choose to ignore what the law says, and run what ever tyre size they want. You can't legally do this, and the ramifications are very, very serious. I'm not talking about a slap on the wrist and a yellow sticker from the local policeman, I'm talking about potential jail time and mammoth medical bills to pay if your proved to have been driving an unsafe vehicle, or if your insurance company finds out and just walks away, leaving you to sort it out.
In WA, you are limited to a maximum of 50mm bigger diameter increase. However, this is just the beginning. If you go up 50mm in diameter, you have lifted your vehicle 25mm and as a result are only allowed to install a 1 inch lift kit (or have the roof height go up by 25mm further). The total height that your vehicle goes up in WA must not exceed 50mm, unless you get engineers approval. This includes tyres, suspension lifts and body lifts. You can find out more about this at Is your 4WD legal?
The only exception to running tyres bigger than a 50mm increase on your 4WD without engineering is if there is a different model 4WD in the same year range as yours that comes from the factory with larger tyres, and has exactly the same setup, just cosmetic differences. You can read about this here - Can you go more than 50mm diameter increase with 4WD Tyres?
Are wider tyres better?
This is a can of worms. One that I am loathe to touch on, but I will anyway! When you see 3 tonne Land cruisers just idling down a super soft beach on the factory skinnies (or cheese cutters) it gives you an indication of the capability of a tall, skinny tyre.
On the other hand, a 10.5 or 12.5" wide tyre also does extremely well off road. Personally, I wouldn't go wider than this. Remember that your traction and floatation come primarily from the length of the surface area touching the ground, not the width. Wide tyres might seem like they would spread the load even more, but then you have to bulldoze sand, mud, snow etc out of the way, just so your tyre can continue moving.
This is very much a personal thing, depending on your driving conditions and requirements. To me though, there's not much point going over 12.5" wide.
What are the disadvantages of bigger tyres?
I will start off by stating our 80 Series Land Cruiser has 285/75/16" tyres on it. This is one size up, and gives us a small amount of extra clearance. I could have gone bigger tyres, but there are many, many reasons I didn't. There are plenty of negatives to fitting bigger tyres on your 4WD, which I will go into below.
The most important thing to know when you get in a 4WD with different tyres is that the speedometer may not be accurate. If you purchase a second hand 4WD and the owner doesn't tell you about it, you run the risk of getting a fine, or worse!
Your speedometer, trip meter and odometer will be out by the percentage of size increase. If your diameter increases by 50mm (31" to 33" or 33" to 35") your speedometer will be out by roughly 10%. This varies considerably from car to car, but its something to pay attention to.
By law, your speedo can only be out by 10% (seems a bit ridiculous that new cars can come out with 10% variation), but its something to be aware of.
Less power and torque
The tyres on your 4WD are essentially the last gear in your gear train. It's not too different to changing a sprocket in a chain drive; the output power and torque will change. By fitting larger tyres, your vehicle will do less revolutions per kilometer, but it has to work harder to do so.
Again, this is very much vehicle dependent, but for most vehicles there will be a noticeable difference in power and torque. In a diesel 4WD, sometimes this is less obvious as they seem to just tractor along, but it is evident. My 2.4 petrol Hilux used to lose a substantial amount of power every time I threw the 33's on. The 80 series turbo diesel loses a bit of power and torque, but its not as obvious.
The easiest and most accurate way to see what difference has been made by changing tyre sizes is to check via an EGT (Exhaust Gas Temperature) gauge. A bigger tyre makes your engine work harder, and this translates into hotter exhaust temperatures. Our 80 series went up by roughly 20 degrees throughout the range when we installed the larger tyres.
Possibly the primary reason for restrictions on fitting bigger tyres is the reduced braking capacity. As the tyre diameter increases, your braking capacity decreases. This is a serious concern; your brakes need to work as well as possible. So long as you stick within the legal requirements of your state though, you won't have an issue.
Increased stress on your driveline
Everything that indirectly attaches to your tyres is put under more stress when you fit bigger tyres. This includes your CV's, steering box, steering shafts, axles and differentials. Many people will not break components on standard tyres (especially CV's) but as soon as they go up a few tyre sizes the breakages happen on a regular basis.
Increased fuel consumption
Some people will argue with me here, saying it makes no difference. It may make very little difference if you are fortunate (generally the vehicles with plenty of power), but if your engine has to work harder to turn the new wheels you are going to use more fuel.
When you work out your fuel economy, be sure to factor in the tyre size difference. If you do 90km on the trip meter, and your tyres are 10% bigger, you've actually done 99km.
My Hilux went up at least a few litres per hundred kilometers, which was expected because the poor engine was already working hard as it was. The difference in our 80 series is slightly less noticeable, and I think at 100km/h it actually uses less fuel do to the revs being more centered in the torque curve.
Please don't fit bigger tyres without accepting the risk that your fuel economy may get worse!
You know those yellow stickers that get handed out regularly, which basically say your vehicle is not roadworthy? Bigger tyres are one of the easiest ways to attract attention to your 4WD, and are often the reason for yellow stickers given to 4WD's.
However, a yellow sticker is the least of your worries. If you are driving a vehicle that is not compliant with the local regulations, your insurance company can walk away from any claim you make. If you rear end a Ferrari, or you seriously injure yourself, you could end up with a massive bill that you have to pay.
Even worse, if you are found to have caused an accident that injures (or kills) someone because of the bigger tyres (or other illegal modifications) you could even go to jail. It's not worth it - do things by the book!
There is a reason that 4WD's come out with the tyre size that they do. You might be able to fit the next size tyres on your 4WD, but go a couple of sizes up and things will start to get tight. The first thing you will notice is that the tyres foul on the inner guards (usually at the front) or the bull bar. If it is only minor scrubbing you may be able to get away with it, but remember to consider when the suspension flexes upwards it will move closer to your guards.
The only way to fix this is to modify the 4WD, which isn't impossible, but its a steep slope that is hard to stop yourself on in both time and money.
Gateway to more modifications
Bigger tyres usually result in further modifications. People find they aren't happy with the power difference, and start looking for mods to make their vehicle make more power. If the tyres don't fit properly, the next step is suspension or body lifts, or to trim the guards. Believe me; once you start this slippery slide of 4WD modifications, there is no getting off! I don't say this to put you off installing bigger tyres, just to make you aware of the potential expenses down the track!
Higher center of gravity
The ultimate 4WD has a low center of gravity, and plenty of clearance. For every size tyre that you go up, your center of gravity also goes up, making it more likely for your vehicle to roll if something was to go wrong.
Alternatives to fitting bigger tyres
Ultimately, bigger tyres are fitted to make a 4WD more capable. The thing is though, there are many ways you can do this. For example, a locker will make your vehicle immensely more capable. If the decision came to it, I'd keep my vehicle legal and install a locker instead. Having installed ELockers not too long ago, I know they have made our 80 series a lot more capable than the tyres ever did.
Another alternative is to change the tread pattern. Muddies will give you significantly better traction than road or all terrains. However, match the terrain you drive on the most to the tyres you buy.
As always though, this all comes down to your individual circumstances. Where you drive, how often you drive, how far you want to go etc!
What size tyres should I run then?
The tyre size you end up with should suit your driving. A 6 inch lift and 35 inch tyres might look good, but it's not really suitable for touring around Australia. How often do you need bigger tyres? Could you get away with something else?
I usually suggest one or two sizes bigger than factory (if this is legal) with some decent tread (muddies or all terrains).
Above all, stick within the legalities; its not worth the risk.
Looking for something else to read? Have a look at 42 things you must know about 4WDing.