Looking for input. I have a 70 gmc half ton with a 307, factory 3 on the tree, 3.73 rear end. This is just to drive to shows and summer cruising. I'm thinking about going to a 3.08 rear end. I'd like to lower the RPM's when running 55mph but I don't want to sacrifice to much torque and struggle with uphill grades. I live in northern IL. so it's not terribly steep grades. Thoughts? Recommendations?
The 3.73 is probably the best possible gear- - - -don't mess with success. A 307 has the same stroke as a 327- - - -3 1/4 inches, and it can live comfortably at 6000 RPM or more if necessary. My race engines ran 6K-7K RPM all the time. Depending on tire size, you're probably going to be under 3K at highway speeds, which is just loafing along for that engine. If you want a Cowboy Cadillac that's only going to carry a Yeti cooler to tailgate parties on weekends, go for the 3.08. If you want to do real truck things, stick with the lower gear.
Swapping the direct drive 3 speed for one with a Borg-Warner overdrive would get you the best of both worlds. Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Thanks for the input Hotrod. It is by the way going to be used as a cowboy cadillac so I still may pursue the gear change down the road. I'm just finishing up the restoration and got it on the road for the first time recently. Once I get more drive time on it I'll get a better feel for whether I want to really make the effort of a gear swap. I don't have a tach installed yet but by ear and feel I'd say I'm running around 2300-2400 RPM at 55. 28" tire diameter
The obsession with running engines just barely above idle speed while highway cruising began back in the late 1980's when a bunch of government bureaucrats with no engineering and/or manufacturing experience started telling car manufacturers how to build their vehicles. Setting outrageous standards for both exhaust emissions and fuel economy forced them to build cars and trucks with completely inappropriate gearing so they could meet the requirements of chassis dyno simulations that had no resemblance to real world operating conditions. The penalty for failing these tests was severe- - - - -the manufacturer's entire production run of all vehicles- - - -not just the ones that failed the test- - - -could not be sold anywhere in the USA. Standards set by California were even stricter. Naming the California standard setting group C.A.R.B. (California Air Resources Board) was no accident, I believe!
Those kinds of requirements compel manufacturers to redesign everything- - - -combustion chamber shape, compression ratio, camshaft timing and lift, ignition timing, computer programming, etc. Trying to make a 1970's engine without computer controls function at those artificially low RPM numbers is not only virtually impossible, it's downright foolish. Gas mileage will drop precipitously because the engine is lugged down all the time and the power valve in the carburetor will stay open virtually constantly- - - -spark plugs will foul out quickly, and there's a distinct possibility of piston and ring damage from detonation. Performance will also suck like a Dyson ball vacuum cleaner because the engine will be running far below its efficient torque range.
Other that those minor concerns- - - - -there's no problem with running a stupid-high rear end gear! Jerry
"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt! There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway Love your enemies and drive 'em nuts!
Here's another perspective on numerically low rear axles ratios. My Stovebolt is a restored '36 Chevy pickup. I converted its rear axle ratio from 4.11 to 3.55 using 1950 Chevy car gears in a '37 Chevy car rear axle. The '36 has a bone stock original 207 cubic inch engine that easily pulls the 3.55 gears, including on a long 11% grade near my home with the transmission in 2nd gear on that hill. Your V8 is much more powerful.
My other toy is a '32 Ford roadster street rod. It has a very mild 350 engine (Crane cam with 218*/230* duration @ 0.050" specifications, Edelbrock 9:1 aluminum heads, Performer intake manifold, Performer 600 cfm carb, block hugger headers), 200 4R trans with 0.67 OD ratio and a 3.80 rear gear. In OD the effective gear ratio is 2.55 and the roadster engine pulls that ratio easily at any speed over about 50 MPH, although it did not pull the 3.40 rear gear ratio I initially tried in OD. Cruising along at highway speed in OD now the roadster gets about 19-20 MPG and mashing down on the accelerator triggers a downshift, or even a double downshift that puts the engine in its torque sweet spot that really pushes us back in the seat. In 15 years and 23,000 miles of driving that roadster I've never regretted the highway friendly gears.
So those numerically low gear ratios are definitely feasible in light vehicles like our "Cowboy Cadillac" Stovebolts.
