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- Off-Road Adventures, Gearhead Dept.
- 610 West Walnut Street
- Compton, CA 90220
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The Gearhead on Gears and Gas Mileage
January 2005 Gearhead’s “Thirsty Bronco” inspired a boatload of questions that involved the effect of gearing and big tires on fuel economy. Chris had combined 3.55 gears with 35-inch tires on his 1995 Bronco, and his mileage had dropped to eight mpg. First, big tires and lifts will cost mileage. Period! No way out of that-but putting big tires and a lift with the stock gears often costs more mileage as well as a loss of performance. Taller tires feel just like changing to taller gear ratios. Your rig will think it has 2.84 gears with the 3.55/35’s combo. Around town, it will grunt at stoplights like an old man climbing out of an easy chair, and, depending on the tire/ratio mismatch, it may not be able to use overdrive on the freeway and will grunt on any hill steeper than a speedbump. Grunting equals poor fuel economy and poor performance. There are two useful formulae here: equivalent ratio and effective ratio. Equivalent ratio formula puts your overall gear ratio back to the same place it was with the stock tires.
Equivalent Ratio =
(New Tire Diameter/Old Tire Diameter) x Original Gear Ratio
Plug in Chris’ numbers:
(35/28) x 3.55 = 4.4375
Matching that up to the closest available ratio equals 4.56:1. The effective ratio below tells you the overall ratio of the new tire and gear combo, and it’s similar to the equivalent ratio formula.
Effective Ratio =
(Old Time Diameter/New Tire Diameter) x Original Ratio
Again, plug in Chris’ numbers:
(28/35) x 3.55 = 2.84
We can make this easy by following a few rules of thumb: Rigs with 31-inch tires should have 3.73 gears. Rigs with 33-inch tires should have 4.10s. Rigs with 35-inch tires need 4.56s, 37s and 38s need 4.88s, 39-40-inchers need 5.13, and 44s need ratios around 6.0:1. You can fine-tune even closer by using the formulas if your rig’s performance, with stock tires and gears, made you happy.
Axle Buster Lift
Our 2001 GMC Yukon XL has a six-inch lift with 35-inch tires. Twice in the last five months, while in four-wheel high turning moderately uphill at a slow rate of speed, the inner-axle CV joint snapped. Are you aware if this is a chronic problem? What can we do about it?
- Russ
- San Diego, California
This shouldn’t happen when everything is set up properly. When you turn the wheels, the axle shaft is pulled outboard. The inner CV has a certain amount of internal inboard/outboard travel built in (called the plunge depth). The angularity of the CV axle is increased with most lifts, and that uses up some of that travel (moving the spider outboard) and can cause the plunge depth to max out. Suspension movement can increase the CV axle angularity to further complicate the issue, whether it moves in or out. In your case, it sounds like the CV reached the end of available plunge depth and broke. Your lift kit came with spacers that go between the axle flange and the CV joint flange and are designed to center the plunge depth on the inner CV joint to prevent what you experienced. It complicates the issue if you cranked up the torsion bars for an extra free lift. The instructions usually will deal with this issue to some degree. Check plunge depth by removing the inner CV boot and observe the position of the tripod spider inside. At normal ride height, it should be centered in travel. Turn the wheels to full lock and cycle the suspension fully up and down to make sure the spider stays within it with the torsion bar adjustments, limiting up or down travel as needed or changing the thickness of the axle spacer as needed.
Bogging Carter
We purchased a 1972 CJ-5 with 50,000 miles. It’s in good condition with a sound 232-cubic-inch six-cylinder engine. Our main problem is the carburetor. Where the road is rough, the carburetor bogs and makes it hard to get through. Besides going to an expensive fuel injection, is there anything we could do other than replacing the carb with a rebuilt?
- David Huddleston
- New Braunfels, Texas
To some degree, this is normal for carbureted engines on rough ground or at steep angles-hence, the wonderful fuel-injected engines. Still, the Carter YF carb on your 232 is usually okay in all but difficult situations. If my books inform me correctly, your carb has a nitrophyl or plastic float. In old age, these can start to soak up fuel and get heavy. Brass floats can leak and do the same thing. This allows more fuel in the float bowl than is needed and makes the carb more prone to flooding and/or running rich. Sometimes the float is just a bit heavy-it’s not a problem all the time, and it sneaks up on you. Eventually, the carb will flood continuously. A carb overhaul that includes a new float or a rebuilt carb should get you back to normal operation. Carbs can be tuned to better deal with bumps and angles by installing special float bowl needles and seats, adjusting the float level lower, using spring-loaded floats and regulating fuel pressure. I don’t know of any improved parts for the YF, though a carburetor specialist (those that aren’t dead of old age) could probably improve it somewhat over stock.