February 2007 4x4 Tech Questions - Nuts & Bolts

Big Rubber Wanted

Question: I couldn 't help but notice that pretty much the entire Ultimate Adventure team was crawling around on 39-inch Krawlers. I've looked for these tires on many different Web sites and in stores with no luck. Why can't I buy these things?
Brian
via nuts@4wheeloffroad.com

Answer: I finally got the father of the Krawler and BFGoodrich Tires' senior product category manager of light truck tires, Gary Enterline, to give me an answer. The 39-inch Krawler should be available by the end of 2006 or early 2007. I have no official numbers, but would expect the prices to be in the mid $400 per tire since the 37s are in the high $300s, but hopefully not. Also I do have info that a new mud-terrain is in the works and looks very similar to the Krawler (see spy photo above), though with its own unique look.

How low is too low?

Question: I have a set of Dana 70s with 6.17 gears and was wondering about the pros and cons of this low a ratio. I have a mildly built 350, a factory four-speed, an NP205, and no lift in my '77 Chevy. Tires are debatable; 35s right now but the wife wants 44-inch Boggers. I do about 50 percent street driving (small town Midwest) and 50 percent sand and lake driving. I am not worried about highway driving, just trips around town. I'm just wondering about speed, rpm, and having enough power.
Ben S.
via nuts@4wheeloffroad.com

Answer: I run 7.17 gears in the Dana 70 under my Toyota with a tired four-cylinder and 39-inch tires and I love it. The benefit of low gearing is that it puts more power and torque into each rotation of your tires. However, sand, like mud, often requires wheel spin and this gearset will result in very high rpm as the wheel speed increases.

It may be a bit low for your truck, especially if you only run 35s, but lack of power shouldn't be a problem. To determine engine rpm and mph with these gears you need to use the equations below.

Thus your rpm with your 35-inch tires spinning at 50 mph, using 6.17 gears, would be around 2,900 in high gear. Since your tires will most likely be about 33 inches tall when aired down, that rpm will increase to about 3,150. This may be fine, but if it were me I would look into a taller tire; a 44 wouldn't be impossible though it will add weight. I think a 40- to 42-inch tire would be great with your truck, powerplant, and terrain, and you may be one of the few guys in the nation whose wife wants him to buy bigger tires so you might as well do as she wishes and go for the big rubber.

There is one downfall of such low gears and that is the pinion head size. It will be much smaller than the pinion of a higher (lower numerically) gear ratio. This smaller head can be a weak link, but I wouldn't be worried about it, especially if your main terrain is sand which doesn't have extreme traction.

More Tire Questions

Question: I've been wheeling for the last couple years in a stock '95 F-150 which is also my daily driver. Most of the wheeling I do is in the mud, but there are a couple of trails with rocks and hillclimbs, and we also have a decent-size sand pit. I already installed a lift kit but I never really thought about tire brand. I want a 35x12.5 mud tire and I've been able to narrow it down to three that I think might be the best for my situation-BFG Mud-Terrain, Super Swamper TSL, and Super Swamper Thornbird. I figure that you guys are the experts so could you please tell me your feelings on these tires?
Henry
via nuts@4wheeloffroad.com

Answer: There are a ton of tires in the sizes you picked, as can be seen in our many advertisements, but I'll stick to the three you have here to give my opinion. The BFG's are a great all-around tire with great on-road handling and a respected off-road performance. The TSLs are excellent off-road tires with adequate on-road performance and a super-tough construction, a bit louder riding than the BFG, but also a more aggressive-looking tire in my opinion. The Thornbirds have a unique look, yet the performance is not on par with the BFGs or TSLs from what I've seen. I would be happy to have either the TSLs or BFGs on my truck.