A lot of people seem to be obsessed with fitting bigger tyres to their 4WD. So much so, that in almost all groups on Facebook one of the most common questions is what’s the largest tyre size I can fit?
That’s all well and good, providing that you understand by fitting bigger tyres you are going to have a number of downsides. By all means, there’s nothing necessarily wrong with installing larger tyres as long as you understand what cost they come at.
Fuel consumption increase
Bigger tyres weigh more, and take more energy to turn. This in turn, means that your fuel economy suffers; you need to use more fuel to turn the tyres. A small change in diameter may not affect the economy too much, but go more than 30mm and you will see an increase in fuel consumption.
Of course, this depends on your vehicle; those with bigger motors seem to notice less of a drop. Sometimes, it may make no difference due to bringing the RPM down into the peak torque region, but most of the time you’ll get worse fuel economy.
Fuel range decrease
While you might accept the fact that its going to cost a bit more to drive around due to increased fuel consumption, a lot of people also forget that on the other end of the stick, you also lose the range that you once had.
If you are using 2 litres per 100km more, that’s going to cost most vehicles about 140km a tank in less fuel range, which when combined with other fuel economy losses (bar work, lift kits etc) can bite hard into your ability to tour without jerry cans!
Braking capacity decrease
I believe the primary reason you are limited to a 50mm increase in tyre size without engineering in Western Australia is due to the reduction in braking efficiency when you install bigger tyres. Your brakes will not stop a vehicle with bigger tyres as quickly as they would if it had the standard, smaller tyres on due to the extra rotational mass. This in turn means you need to brake earlier, or upgrade your brakes.
More stress on your driveline
Turning bigger rubber puts more stress on your driveline. You are more likely to break a CV, or smash the teeth off your crown wheel. You could say that larger tyres allow you to crawl over obstacles rather than use momentum so the trade of can be worth it
By far and away the most important factor that people overlook or ignore is the legalities. Getting pulled over and given an unroadworthy sticker is inconvenient, but the least of your concerns. Any insurance you take out relies on you driving a roadworthy vehicle, and if your car isn’t roadworthy due to having tyres that are too big, your insurance company can decline or reduce any claim you make.
Imagine that; run into a nice sports car, your insurance company goes sorry mate, your tyres are too big, you’ll have to cop that damage yourself. When we are talking thousands of dollars worth of damage, is it really worth it? Up to you, but I know for me, its not. Want to know how else you can make your 4WD illegal? Have a read of this; 32 ways to make your 4WD illegal.
Speedo and Odometer changes
Your tyres are essentially the last gear in the system, and it determines the final drive ratio. Your speedometer and odometer are calibrated off the factory tyre size and are allowed to be up to 10% out, but reading under only. For a lot of new vehicles, when you are doing 100km/h on the speedometer you are actually only doing about 91 – 95km/h.
When you install larger tyres, you do less revolutions for the same distance, and your speedo will go out. The 265/75/16’s that we put on our Dmax (54mm bigger) make the speedometer read exactly correct.
This can be useful, but if you go larger (like 35 inch tyres) then you’ll have to drive at a slower speed on your speedo in order to be under the limit. Likewise, your odometer will be out, and you should accommodate this into your service schedule (getting it done at 9000km instead of 10,000km for example).
The most common issue with running larger tyres is that they rub on something. This can be minor and just make a nasty noise, or it can damage your components if its bad. The most commons scrubbing place is the panel that your accelerator backs onto, and the same on the other side. This happens when you turn to go around a corner, and your suspension flexes up.
You can get diff relocation kits to move it further forward, or body mount chops, but both require engineering to keep it legal.
Greater centre of gravity
When you install larger tyres, it picks everything up and raises your centre of gravity. Your differentials go up, your chassis goes up, and the body goes up. A small amount is going to make limited difference, but if you install heavy roof racks, or a roof top tent, or you are loaded to the hilt it can make a big difference off road.
Keeping your centre of gravity down low makes the vehicle handle better, and the chances of it ending up on its lid lower.
Do you really need larger tyres?
At the end of the day, this is a personal choice. If you understand the consequences and go ahead with it then its on you. Bigger tyres are certainly better in many ways, and we went for a mild increase on our current touring vehicle.