40 Inch Tires 20 Inch Rims – Going off-road means you need big tires and the bigger the better, right? Well, maybe not. There are a few things you should keep in mind before sizing up.

First, we must confess that this is a bit of a generalization. What we’ll discuss here may not be exactly what your truck or SUV needs to make that jump to a 40-inch tire. For example, your rig may come with axles strong enough to turn a 40 without snapping the splines or itself in half. You might be able to take the fenders off your Jeep, but the guy on the F150 can’t.

40 Inch Tires 20 Inch Rims

40 Inch Tires 20 Inch Rims

If you’re just a mall tracker, you might be fine with most of the stock stuff, but if you’re going off-road, you’re going to need upgrades. So this is going to be a guide of things to consider before plunking down some cash on these great tires. We won’t touch on anything specific to a single vehicle.

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Let’s start with what everyone points to as the first thing to change or at least modify: your axles. First, why? Why should you consider your axle when switching to larger diameter tires? A lot of it has to do with the diameter and extra weight of the tires. Yes, you need to re-gear (and recalibrate the speedometer) to overcome the overall gear ratio increase. A 33-inch tire will have a spread of 103.7 inches and will rotate 630 times per mile.

A 40-inch tire, on the other hand, will have a spread of 125.7 inches and will rotate 517 times per mile, meaning you go further with each rotation. By switching to a taller tire, you’ve basically increased your overall gear ratio and will show a slower speed than what you’re actually doing. So if you were doing 65 MPH on a 33×12.50, you’d be doing almost 80 MPH on a 40×13.50 while still showing 65 MPH.

“…if you’re going with big tires, chances are you’re thinking of going with bigger axles at the same time.”

This means that not only would you have to recalibrate your speedometer, but you would also have to re-gear to keep the engine RPM close to the same for the corrected speed. Fortunately, there are many online calculators to help you determine what equipment you need to increase your tire size, as well as hand-held tuners that allow you to recalibrate your speedometer for your tire size and gearing.

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However, these will not be the only problems with your axles. Because of the increased weight, you’ve also increased rolling mass and rolling resistance. This means you’ll need more torque, and you’ll do it by adding power or lowering the gear ratio, or both. This increases the stress on the shafts and usually leads to failure of splines and shafts on straight shafts and also to failure of U-joints or constant speed joints on independent shafts. So if you’re going with big tires, chances are you’re going with bigger axles at the same time.

You’ll probably want to invest in new driveshafts, as these will be the next weakest links when it comes to transferring torque to your axles. Most truck and SUV transmissions and transfer cases can work well with large tires, but you’ll want to inspect them more often or consider switching to heavier aftermarket versions. You won’t necessarily need an Atlasor, or even an NP205, but you’re definitely looking for upgrades to your chain-driven New Process transfer case that will allow it to handle more torque. However, if you have a regular NP241, get something better or at least an NP241HD.

You will need a lift, even if you already have a 2 or even 3 inch lift, you will need to go a little higher to clear the tires. This is where an IFS suspension starts to lose its benefits as you raise the truck higher, it will continue to ruin the handling and feel of the truck or SUV. You will also wear parts much faster due to the stresses and increased load on the lift as well as the tires.

40 Inch Tires 20 Inch Rims

You’re looking at a custom suspension, regardless of whether you stick with your IFS or swap the front end for a solid axle. For leaf spring rears, you’ll be able to find re-arched springs for decent lift without having to resort to a huge block for spring axles (where the leaf sits on top of the axle). For the spring, you’ll need to make it wet or you’ll have arcs so big they won’t make sense.

The Pitfalls Of 20 Inch Tires

If you don’t lift, the body will need major modifications to make a 40 inch tall tire work. If you don’t want to cut sheet metal, your only other option will be to replace it with fiberglass parts made for prerunners and desert trucks. If you cut metal, most states will require you to have something to cover the tires and have clearance lights on the fender extensions to remain legal. Easy to do in a Jeep, not so easy to achieve in anything with a regular body.

You’ll also need to make sure the wheels you’re using give you adequate back clearance (travel) to clear these very wide tires. The rear can usually be more aggressive than the front and can get away with higher rear clearance (lower travel). The fronts, however, need to be spaced so you can turn properly and not rub the frame or suspension components. Again, in many states, there are also legal issues with tires rubbing against the body and chassis.

Running 40-inch tires isn’t easy, in fact, it’s probably only best for off-road teams and showboats. It’s not impossible to run such a large tire, but there are lots and lots of things to consider before doing so. This is just a short list, as there are explicit things you need to do with specific vehicles to run such large tires. The best advice we can give you is to do your research. Look up who has done it with a vehicle like yours and see the trials and tribulations they had to go through to make it work. Then decide if you are willing to do the same.

If not, there is nothing wrong with running 35s in your truck, SUV or Jeep and they are very capable. Just ask the guys running the Ultra4 Every Man Challenge.

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40 Inch Tires 20 Inch Rims

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