Hello all. I know most of you will probably tell me to swap a 700r4 or 2004r trans in, but I would like to keep as much original as possible. I have a 1970 Cutlass Supreme 2bbl 350 rocket with 2.56 gears in it. Now I know nobody makes gears for the O type rear axle, at least no one I've heard of, but I was considering taking out the O type, setting it aside and hiding an 8.5 in it. Now I know I will have to find a rear axle with the 2.41 carrier in it, but I was wondering if anyone made anything higher than that. Maybe in a 2.08 area. I currently average 15 mpg city with a little highway and have gotten 19.5 doing all highway. Now I'm not complaining about that mileage at all, but I would love to maximize it and potentially see 22mpg on a roadtrip. I have a friend with an edsel that has gotten 25mpg with 2.07 gears I think. I know a qjet would likely yield better mileage as well, but I'd like to keep the two barrel alive. If anyone knows of anything, either in a junkyard or from a manufacturer, let me know!

 

I'm pretty sure you won't find gears taller than the 2.56s for any rear that fits under your '70 Cutlass. Even if you could, I doubt it will increase your MPGs. My '68 350-2bbl S came w/ 2.78 gears and it routinely got over 20 MPGs on the highway, once as high as 23. I felt that w/ those gears the RPMs was in the sweet zone for the engine at 70 MPH and it got its best mileage. When the national speed limit was reduced to 55 back in the '70s, it was tough to get over 20 MPG at 60 MPH. Thus, I suspect anything taller than 2.56s will result in your engine bogging a bit on the highway and it not being at its most efficient RPM. There is more than reducing RPM to getting optimum mileage, its a delicate balance of RPMs, wind resistance, rolling resistance and engine efficiency. I think the Olds engineers hit the sweet spot w/ the 350-2bbl + 2.56 or 2.78 gears.

 

GM made 2.41 gears for the 8.5" corporate axle. The 2.41 and 2.56 carriers are unique 2-series carriers. The 2.52 first gear in your TH350 will really suck with 2.41 or lower numerical gears. You'll be hating life. This is a really bad idea.

 

Add taller tires, skip gear change. How much difference would you expect for a .15 change in ratio?

The answer you don’t want - keep the 2.56 though likely too tall (2.78 questionable, 3.08 just right IMO) but you could try it, add 200-4R and switch to well calibrated Qjet. You have your cake and get to stomp on it too.

How much gas money can you save vs that spent on mods vs miles traveled per year?

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None of these changes will ever be cost-effective if the goal is gas mileage. Taller tires will make even less of a difference than going from 2.56 to 2.41. An OD trans with a much wider gear ratio span is the ONLY way to make an effective change, and the 200-4R actually helps at both ends. The 2.74 first gear makes those 2.56 rear gears more livable, and the 0.67 overdrive is the equivalent of changing the 2.56s to 1.72 gears. It's definitely easier and probably less expensive than a rear axle swap.

 

You may get to the point that the motor will be lugging to try and maintain a certain speed and gas mileage will drop. The sweet spot for gas mileage has something to do with torque or horsepower of motor. Can't remember what it was as it was years ago when helping my dad build a street rid and he wanted gas mileage and not fast 1/4 times.

 

A 2.56 is plenty low for gearing, around 2200 rpm at 60 mph, correct? If you want to drop rpm, are you still running 14 inch tires? Step up to a tall 15" tire, like 235/70R15 or even a 255/70R15. Another option is a TH350C trans. The lock up converter will gain more than going to 2.41 or similar to the super tall rear tires, about 300 rpm with the converter locked up. It can be controlled with a toggle switch, other than an electrical plug, it looks the same. I currently have a 2004R with 2.78 gears, doing a 3.90 posi swap in spring. Honestly unless you run a really short tire, it runs too low on the highway. With the 275/60R15 tires on, under 1400 rpm at 60 mph! I actually got better mileage with a short 25" tall tire doing 70+ mph at 1700 rpm vs 1500 or 1600 rpm. A couple of different ways to skin a cat.

 

Take your car to a chassis-dyno shop that speaks Quadrajet (it will be a bit of a search). Have them calibrate the carb correctly at cruise load and rpm.

That could be another lower-cost way to increase fuel economy.