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315’s on an OBS?

by Steve Walker

Can you really fit 315’s on the rear of an OBS with stock leaf springs and no fender mods?

So you want to put the widest tires possible on your OBS truck (or Tahoe, or Suburban),  but you are dead set against cutting any sheet metal or major mods to make them fit? You just want to bolt them on and go. But your truck is lowered and you want the tires to tuck inside the stock wheel openings? It can be done, and we did it.

I started with our project OBS. It’s a ‘92 Silverado with the typical suspension mods. BellTech flip kit and C-notch with the stock leaf springs in the rear, DJM 2” drop spindles and 3” lowering springs up front. Your basic 5/7 drop. 

For the last several years, this truck has had 20×8 and 20×10 American Racing Torq Thrust wheels with 245/45’s up front and 275/40’s in the rear. Again, a wash/rinse/repeat fitment that has proven very popular with OBS owners everywhere. 

The truck has a healthy LS swap under the hood, and I’ll admit it, I like to do burnouts. This truck does them very well. I will also admit that the alignment hasn’t always been perfect. Consequently, the old tires were garbage and ready to be put to rest in the tire graveyard. With the 275/40’s on the rear, it always looked as if the truck needed more rubber. It also looked like it would be a tight fit. 

Before committing to the purchase, I spoke with a few folks who were running 315’s on the rear. One said he trimmed and hammered the outer wheel lip for clearance. One had a narrowed Ford 9” rear axle and wheels with custom offsets. A couple guys claimed they went as far as machining the mounting pad on the wheel center to help tuck the wheel in. I didn’t want to do any of those mods. Not that I’m afraid of a little work, but this truck isn’t the one I’ll be doing that much work to. 

The 20×10 and 275/40 combo that was on the rear was already a tight fit. The offset was ET6, meaning the wheel mounting pad surface is placed 6mm toward the outside of the wheel if you draw a line between the outer edges of the outer lip and inner barrel. There was no way a tire with hypothetically 40mm more width was going to fit. 

The search was on for a budget-friendly wheel that would fit. That’s when I came across U.S. Mag’s 20×10.5 Rambler U111 wheel with an offset of ET20. This offset would effectively move the entire wheel/tire package inward 14mm (versus the previous setup), effectively giving us that much more fender clearance, while still providing room on the inside, between the tire and leaf spring. If our measurements were correct, this would be just the ticket. We promptly placed an order for a staggered set of Ramblers in 20×8 and 20×10.5. 

After a quick search on the Google machine for 315/35/R20 tires, we came across Cosmo’s MuchoMacho. If we weren’t sold by the name alone, the price seemed almost too good to be true. Further research revealed that MuchoMachos are favored not only by car and truck enthusiasts, but by professional drifting teams and endurance racing drivers across the country. It seemed like I stumbled upon something here, so we got on the phone with Cosmo tech, and asked them to tell us more. The technology behind their product line is impressive and they explained to us that the price isn’t indicative of the quality. The value comes from extremely modern production techniques, smart marketing, and extensive market research. Everyone in the CK Mag office liked the aesthetics of the MuchoMacho and the tread design looked as if it would provide a quiet ride. And at the price, I could do burnouts all day long and still afford to buy another set of replacement tires. I was sold and promptly ordered a set in 245/45ZR20 and 315/35ZR20.

I dropped the wheels off with my nephew, Seth Peppers, a technician at Germain Mazda in Columbus, Ohio. He wrestled the tires onto the wheels and balanced the assemblies to perfection. No unusual amounts of weight were needed to balance any of the four tires. 

Back at the shop, the install was practically flawless. U.S. Mags provides a full set of small diameter, splined, chrome lug nuts and includes a unique socket for installation. Nice touch. Another nice detail was that the wheels are hubcentric for this application. I’m a big fan of hubcentricity, as it eliminates vibrations and takes stress off the studs. The front wheels bolted on without a hitch. Sweet. As I started to put the rear wheels onto the studs, I began to have doubts about my math. The leaf spring-to-tire clearance was questionable. After lugs were tightened and torqued and the truck was lowered and off the lift, I checked clearances everywhere. As I suspected, the fit was tight but acceptable, with about 1/4” between the leaf pack and the tire sidewall. All other clearances on all 4 corners were perfect. A test drive confirmed that all was well. The ET20 offset was the key to success.

After putting some miles on, everyone who has driven the truck really like these tires. I was pleasantly surprised with the MuchoMacho’s ride quality and minimal road noise. Additionally, these sticky tires make my truck handle better around corners. We haven’t had a chance to drive them in the rain yet, but the tire smoke they produce has exceeded our ratherlofty expectations.

More information:  us-mags.com    cosmotires.com

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