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Trading Towing Ability for MPG
I have a ’92 GMC Sierra 1-ton dually with a 454 V8, 4.10 gears and a 10,800 pound towing rating. I would like to change the gears to increase my gas mileage but still be able to pull 7-8,000 pounds. What gear ratio do you recommend, or would I be better off to leave it as-is.
Al McCallister Belding, Michigan Via snail mail
You could easily tow 7-8,000 pounds with 3.73:1 ratios, but the increase in mileage would be negligible to non-existent. First off, any changes (including he ones I mention below) in gearing would only be effective on the highway. In town, you wouldn’t see much of an increase. It’s a big engine and a heavy truck, after all. You didn’t say which tranny your truck had, or even if it’s a 4x4 or a 4x2, but either way, for ’92 both types had a 1:1 top gear. That limits you in many ways and I think your answer lies not in axle gearing but tranny gearing. With axle gearing only, you are stuck with those tall gears regardless and you have to get to about 3.0:1 axle ratios to make any meaningful increases in economy, but that would hamstring towing ability. The better way is via an overdrive tranny, which leaves you with good pulling gears but an overdrive top gear for economy. You can get there several ways. First, you could install an NV-4500 manual overdrive trans which is reasonable, plentiful, adaptable and has a .73: 1 overdrive (which would give you a 2.99: final drive in 5th). It would also be possible to bolt and wire up a 4L80E, four-speed electronically controlled automatic, which was available just the year after your truck was built. It has a .70:1 overdrive that would give you a 2.87:1 final drive. Finally, Gear Vendors offers an overdrive unit with a .78:1 O.D. and would give you a 3.2:1 final drive. In each case, I’d guess you’d pickup up 3-4 mpg on the highway. You’re probably underwhelmed with that and realize it will take many years to recover the costs of anything you do. If you plan on being buried in your ’92 dually, you could probably recover the costs. Otherwise it’s cheaper just to pay for the fuel.
TTB Tradeout
I have had a long desire to do away with the IFS suspension in my ’93 Ford F-250 4x4 and put a F-350 4x4 solid front axle in it’s place. I have had a chance to look at an F-350 and it looks possible. Is it a bolt-up or is it past a back yard tinkerer?
Dean Baker Naples, Utah Via snail mail
It’s an easy swap. The axle you need is an ’85-97 Ford F-350 Dana 60, complete with the driveshaft (or you can have one built- the F-350 unit is shorter and uses a double Cardan) and the steering linkage. Some sources say the Pitman arm is needed but mine say not. The F-350s also had a sway bar which can be installed or not. The TTB pivot bracket must be unbolted, of course. To ’93, the Ford D60s used king pins and later versions used ball joints. You will find two gear ratios commonly, 3.54 and 4.10. The swap will give you a couple of inches of lift and it’s possible to use the F-350 springs too if you need them. According to my Ford Data books, in the earlier years, the F-250HD (full-float rear axles) and the SRW F-350s used the same front springs for the same engines. Diesels had higher rate springs.
Bowtie Dreamin’
I’m 18 and looking to buy a full-size Chevrolet straight axle truck, ’73-87 4x4. Which year is my best choice? I’m looking for something with a 350, standard cab, and short box that can be lifted 6-8 inches for 36 inch tires.
Craig Rose Shawano, WI Via snail mail
In the half-ton short box realm there isn’t a whole lot to choose between the trucks in the era you mention. Plus, they are so swappable that what you don’t like can be swapped with something you do. You might think the short bed Stepsides are cool. I do! Many people like the look and trim of the ’70s era better than the later rigs as well, but that’s eye-of-the-beholder stuff. If you’re going to run 36s, you’re just over the half-ton drivetrain limits if you are going to ‘wheel it and should either swap in a one-ton Dana 60 front axle, or plan on beefing the D44 or 10-bolt with alloy shaft and Ox super U-joints. In back, I’d recommend either a semi-float 9.5-inch 14-bolt in back, which is more than adequate 95 percent of the time and in the Dana 60 semi-float ballpark, or a full-float 14-bolt with its beefy 10.5 inch ring gear. The stock 12 and 10-bolt front and rear axles are safe for 35s MAXIMUM, and safer with 33s. In the earlier years you’ll see TH 350 automatics or CH-465 4-speed manuals. Both are keepers in terms of beef. Later trucks used the TH-700R4 4-speed auto, which has been known to be cranky but is fixable and has an overdrive top gear for economy. T-cases can be either an NP-205 (very rare in half-tons, early sixes only) NP-203 full-time 4wd units or NP-208. Of the three, the NP-205 is the beefiest, but the NP-208 has the best low range and is plenty strong. The NP-203 is strong and can be converted to part-time operation. As to engines, if you want a 350, the 4-barrel is the one and only and there isn’t a lot to choose between them in that era. A throttle body 350 would be nice, but that comes a year after the last solid axle in ‘86.
Spare Tire Dilemma
I have a 1997 Dodge HD2500 4x4 X-tra Cab with a limited slip and am running 33 inch BFG tires. Can I use the factory 245/75R16E spare tire if I have a flat on the road. I’ve never worried about it, since living on an island, you can only drive so far!
Rolando Oahu, Hawaii Via e-mail
There is somewhat under three inches difference in tire height that could indeed eat up you limited slip and could even ruin a standard diff over a long run. I’d say you have two choices; pony up for a 33-inch spare or be prepared to swap tires front to rear if you get a flat in back. With the front-end unlocked, a tire difference won’t matter.
Suburban Axle Woes
I recently purchased a ’92 Suburban 1500 with 3.73 gears for a tow rig. I have had it less than two weeks and already the 10-bolt rear end is going out. I have a 3.73 ratio 12-bolt from a ’76 Chevy and I believe it is a stronger axle than the 10-bolt, but it’s 1-1/2 inches narrower. I’m worried if I just replace or repair the 10-bolt I’ll have the same problem later. I plan to use this truck for a daily driver and to tow my Jeep to the trails and mud holes. Should I just replace what’s here or install the 12-bolt and use wheel spacers?
Dave Grauerholz Nora Spring, Iowa
Dave, you didn’t say what problem your axle had, so I can only respond generally. First, it’s a myth that the truck version of the 12-bolt is better than a 10-bolt, especially in your Suburban’s era. As far as the housing goes, they are dead even. Both have 3-inch, .250” wall tubes. Both have similar cast iron center sections. As to axles, yours should have 30-spline shafts, as does the 12-bolt (pre-’89 10-bolts have 28 spline shafts). The 10-bolt actually has a larger pinion shaft and pinion bearings and the carrier bearings are spaced farther apart, offering a bit of extra strength. The 10-bolt’s original 30-spline carrier is a weak link, but you can buy better replacements or install a limited slip with a better carrier. Don’t use a locker for towing. Overall, a properly rebuilt 10-bolt, used within it’s limits, is a good piece. That said, if you feel you need an upgrade, go with a semi-float 14-bolt. It has a 9.5-inch ring gear, 33-spline axles 3.34-inch axle tubes (usually about .270” wall). It’s rated for 5700-6000 pounds, versus 3700-4000 pounds for the 10-bolt. It can be found in six-lug form under some of the light GVW 3/4 tons and some of the extended cab, longbed 1/2-tons. You may also find them under 3/4-ton Suburbans. There are a few different widths (Subs are often the narrowest), so you’d want to measure before buying. It would bolt in using the 14-bolt U-bolts and spring plates, but you might need to shorten the driveshaft a skosh. s