First of all, if you are in need of a solid off-road tune I have a few on my storefront! - My gamertag is the same as my username.
Disclaimer: I am not an expert tuner by any means, this is aimed for those who know little to nothing about tuning and want some advice for offroad tuning. If you happen upon this and want to make a correction or add your own advice feel free! This should be a combined post from as many outlets as possible to help out our fellow Forza goers.
What I have found works best for off-road and cross country tuning is to shoot for the A-S1 classes. This is mainly because you need a few required components to make your offroader as fast as possible. These required components usually put your near the B or A class (rally tires being the biggest factor here) so once you add power and decrease the weight you’ll be in the high A or already in S1 depending on the upgrades.
The car: You can pick any car you’d like for the most part because it is so easy to convert them into a rally build. I try to pick cars that are already known for their offroading capabilities (ex. Bowler, Jeep, Subaru, Alfa, Rally truck, Ford, etc).
The main components: To perform best, you want to shoot for as little weight as possible and a good amount of power. The Jeep Willy is a great example of this once you throw in a new engine. First, you will want to convert your car into AWD if it is not already. AWD, I have found, is a necessity for off-roading and cross country races. Next you are going to want to throw in a new clutch, transmission, driveline, and differential. These are equally as important as rally tires, suspension, etc because tuning the gears and differential of the car will improve its time exponentially. The next important parts are going to be the rally tires, rally suspension, brakes, sway bars, and wider front and rear tires. The rest of the components are important but can be added at your own discretion. Obviously, putting more power in and reducing weight will improve the car but know what class of car you are shooting for and adjust appropriately (more power and less weight the higher the car class you go). More power becomes a lot more important if you are in a lower car class. Having 150hp versus 350 for an A class car is obviously important.
The tune: The first thing you will want to do is lower the tire pressure, the lower the tire pressure the more ground (jumps, rocks, dirt, etc.) your car will absorb. Gearing is a very important aspect of the tune and should be fiddled with until it is just right. For off-roading purposes you should shoot for acceleration over power so you can get off the line and through the corners quickly. Top speed is not always the most important factor in rally and off-road courses. However, you do not want to have too short of gears either or you won’t be able to downshift in corners properly. Next, you will want to check that your ride height is at the maximum height (look below in comments this may not be needed) and that your anti-roll bars, springs, rebound, and bump stiffness fall on the soft side of the spectrum. If you happen to put some type of downforce components on your car, aim for the speed spectrum. I’ve found this to be more important than cornering for off-road venues. I like to have my brake pressure near 110-115% for reasons I honestly don’t know, it just feels right. Lastly, play around and fiddle with the front, rear, and center differential pressures. These can significantly help your build or completely break it. My advice would be to keep it near default at first and slowly change these components as you figure out what fits your car best.
Again, you can see that I am no expert. If you disagree with my advice or would like to add more please comment below. I have found that these tips have helped me tremendously in how I build my offroading vehicles and have given me many first place finishes. I hope that you find this helpful if you are truly interested in building this type of car. If you have any questions or want help please let me know!