I spent the weekend reading posts about 37” tires, and as you might imagine, a search resulted in hundreds of results with a lot of different points of view. As common as this question is, there didn’t seem to be a definitive thread that addressees all the relevant factors, so perhaps this one will be useful to others. This represents what I've learned so far. Please add to it with your real world experience.
Let’s make some assumptions to start:
We're focused here on the most common 37x12.5 tire size (which are typically closer to 36.5" tall.) Wider or taller will follow the same principles, but get even more challenging to fit.
We’ll be assuming a basic 2.5”+ OME lift as a starting point. Obviously usable suspension travel is important, but w/in a reasonable range, static ride height really doesn’t matter when it comes to “will 37’s fit” and many people get hung up on the false assumption that more lift is the key to fitting bigger tires. We'll save optimal suspension height and gear ratios for the hundreds of other threads on those topics.
"Fit" means that the suspension can fully flex w/o damaging the tires or the rig. A very small amount of tire rub on the smooth surface of the frame or fender liner at the limits could be okay. But it needs to fit for rock crawling, not just in the mall parking lot.
No cutting the sheet metal! Chopping the fenders will obviously make space for bigger tires, but for the purpose of this exercise, we’re trying to avoid that. And while removing the flares would be a way to add a little more clearance to the outer lip of the wheel arches, let's see if we can do this w/o removing them, but some minor trimming there is okay.
Now, how to make it happen:
Center the tire side to side in the wheel well: How far out the tires sit relative to the axle is a key factor. Most people report that 0 offset rims (like stock), and especially positive offset rims, result in the inside of the tire rubbing against the frame and fender liner. Neutral, 0 offset rims aren't a deal breaker, but adding a little negative offset, either natively in the rim's design or by adding wheel spacers to a positive offset rim, solves that. However, pushing the tire out too far results in trouble on the opposite side of the tires, with the tires not tucking inside the lip of the fender. With a 37x12.5" tire, -6mm to -12mm of offset (1/4"-1/2" additional backspacing past the rim's center line) sounds like the sweat spot. A 1"+ spacer on a neutral offset wheel (-25mm combined offset) seems to be too much with the outside edge of the tire hitting the flare and likely the lip of the fender. I'm planning on a 17" rim either 8.5" or 9" wide with -6 to -12 offset. What rim size have you all found to be optimal with fenders and flares intact?
Centering the axle under the truck is also important, or the tire on one side may fit while the other side rubs. Most people account for that as part of a larger lift w/ adjustable panhard bars and/or a correction bracket, but they often aren't done w/ smaller lifts. Either way, if you're getting rubbing on one side but not the other, that will be something to address.
Limit the tire's up travel: If the tires can't move up, they're not as apt to rub. We obviously need some up-travel, so something like a 4"+ bumpstop extension would be a major compromise, even if it helps the tires to fit. A basic 2.5" OME lift doesn't require bumpstop extensions, so anything you add to prevent tire rub will be a compromise that reduces your up-travel. Longer travel shocks typically require more bumpstop due to the longer compressed length, so it's only anything past that point that creates a compromise that's driven by the tire fitment. On my truck, I'm hoping to not exceed a 1" bumpstop extension in the front and 2" in the rear (I already have 1" in the rear to accommodate my Fox shocks), but I'll confirm exactly what's needed when everything else is dialed in.
Center the tire front to back: This would require aftermarket control arms to change the vehicle's wheelbase. Any changes needed would be relatively small, but this might make the final difference in dialing in fit. I didn't see much talk about people needing to alter the rear axle's position forward or back, so the rear doesn't seem like a common issue. Many aftermarket front radius arms are ever so slightly longer than stock, and DVS even offers a version that is 1" longer specifically to fit 37s. If you push your front axle that far forward you probably create rubbing on the stock bumper, but that isn't a problem with most high clearance aftermarket bumpers. I haven't added castor correction to my truck yet and could go with the Delta 3" Long arms instead of the Land Tank 3" castor correction plates I'd previously been considering. Will the long Delta arms make much of a difference in tire fit?
Move the body farther away from the tires: A body lift will create tire clearance similar to a bumpstop extension, but without limiting the suspension travel. These aren't as common on an 80 series, but are a viable option. I'm not planning to do this on my truck, but I mention it here for completeness.
Miscellaneous trimming/adjustments: While my goal is to not cut the sheet metal, I expect I'll likely need to trim a little off the inside of the rear fender flare. At a minimum, there's a small section in the rear corner that seems to protrude a little too much (my truck is a 94 w/ the fiberglass flares.) I'll also be saying goodbye to the mud flaps. Any other minor clearancing or adjustments that would likely be needed?
Okay, with all of that said, is there anything that wasn't accurate or that I may have missed? Is dialing all of that in enough to safely fit 37's?
Thanks,
Chris
Let’s make some assumptions to start:
We're focused here on the most common 37x12.5 tire size (which are typically closer to 36.5" tall.) Wider or taller will follow the same principles, but get even more challenging to fit.
We’ll be assuming a basic 2.5”+ OME lift as a starting point. Obviously usable suspension travel is important, but w/in a reasonable range, static ride height really doesn’t matter when it comes to “will 37’s fit” and many people get hung up on the false assumption that more lift is the key to fitting bigger tires. We'll save optimal suspension height and gear ratios for the hundreds of other threads on those topics.
"Fit" means that the suspension can fully flex w/o damaging the tires or the rig. A very small amount of tire rub on the smooth surface of the frame or fender liner at the limits could be okay. But it needs to fit for rock crawling, not just in the mall parking lot.
No cutting the sheet metal! Chopping the fenders will obviously make space for bigger tires, but for the purpose of this exercise, we’re trying to avoid that. And while removing the flares would be a way to add a little more clearance to the outer lip of the wheel arches, let's see if we can do this w/o removing them, but some minor trimming there is okay.
Now, how to make it happen:
Center the tire side to side in the wheel well: How far out the tires sit relative to the axle is a key factor. Most people report that 0 offset rims (like stock), and especially positive offset rims, result in the inside of the tire rubbing against the frame and fender liner. Neutral, 0 offset rims aren't a deal breaker, but adding a little negative offset, either natively in the rim's design or by adding wheel spacers to a positive offset rim, solves that. However, pushing the tire out too far results in trouble on the opposite side of the tires, with the tires not tucking inside the lip of the fender. With a 37x12.5" tire, -6mm to -12mm of offset (1/4"-1/2" additional backspacing past the rim's center line) sounds like the sweat spot. A 1"+ spacer on a neutral offset wheel (-25mm combined offset) seems to be too much with the outside edge of the tire hitting the flare and likely the lip of the fender. I'm planning on a 17" rim either 8.5" or 9" wide with -6 to -12 offset. What rim size have you all found to be optimal with fenders and flares intact?
Centering the axle under the truck is also important, or the tire on one side may fit while the other side rubs. Most people account for that as part of a larger lift w/ adjustable panhard bars and/or a correction bracket, but they often aren't done w/ smaller lifts. Either way, if you're getting rubbing on one side but not the other, that will be something to address.
Limit the tire's up travel: If the tires can't move up, they're not as apt to rub. We obviously need some up-travel, so something like a 4"+ bumpstop extension would be a major compromise, even if it helps the tires to fit. A basic 2.5" OME lift doesn't require bumpstop extensions, so anything you add to prevent tire rub will be a compromise that reduces your up-travel. Longer travel shocks typically require more bumpstop due to the longer compressed length, so it's only anything past that point that creates a compromise that's driven by the tire fitment. On my truck, I'm hoping to not exceed a 1" bumpstop extension in the front and 2" in the rear (I already have 1" in the rear to accommodate my Fox shocks), but I'll confirm exactly what's needed when everything else is dialed in.
Center the tire front to back: This would require aftermarket control arms to change the vehicle's wheelbase. Any changes needed would be relatively small, but this might make the final difference in dialing in fit. I didn't see much talk about people needing to alter the rear axle's position forward or back, so the rear doesn't seem like a common issue. Many aftermarket front radius arms are ever so slightly longer than stock, and DVS even offers a version that is 1" longer specifically to fit 37s. If you push your front axle that far forward you probably create rubbing on the stock bumper, but that isn't a problem with most high clearance aftermarket bumpers. I haven't added castor correction to my truck yet and could go with the Delta 3" Long arms instead of the Land Tank 3" castor correction plates I'd previously been considering. Will the long Delta arms make much of a difference in tire fit?
Move the body farther away from the tires: A body lift will create tire clearance similar to a bumpstop extension, but without limiting the suspension travel. These aren't as common on an 80 series, but are a viable option. I'm not planning to do this on my truck, but I mention it here for completeness.
Miscellaneous trimming/adjustments: While my goal is to not cut the sheet metal, I expect I'll likely need to trim a little off the inside of the rear fender flare. At a minimum, there's a small section in the rear corner that seems to protrude a little too much (my truck is a 94 w/ the fiberglass flares.) I'll also be saying goodbye to the mud flaps. Any other minor clearancing or adjustments that would likely be needed?
Okay, with all of that said, is there anything that wasn't accurate or that I may have missed? Is dialing all of that in enough to safely fit 37's?
Thanks,
Chris
