I am currently replacing my donated rear end housing with a new one from currie. I presently have 3.0 gearing with posi that has quit working. While I am having another rebuilt the builder (Bill Thomas Enterprises in San Bernardino) suggest that if I go with 3.25 or 3.5 gearing I might actually increase my mileage . It does seem backwards to me, but he is quite knowledgable compared to me.


What are your opinions? I would really like to get it right.

 

What is the rest of your drive train? If you have a direct drive automatic without lock up torque converter you will likely get better mileage with the 3.0 gears. If you have a smaller engine and a manual trans with a tall overdrive then 3.5 or lower is for you. Total package needs to work together, tell us what you have.

 

If your gearing is tall enough that you are into the power jets (or power enrichment circuits) any time you need some power, your shop could be right. A vacuum gauge would show that while driving around. Agree with 38Coupe, you need to look at what kind of driving is typical, what conditions you want to optimize for (highway cruising or "suburban" type driving), tire size, engine build, etc.

 

Sorry, I have a 460 with 500 ft lbs of torque, peak torque at 3800 rpm(529) C-6 beefed up. With the 3.0 gears I was running about 2700 rpm's at 70mph with 26"tires.I'm getting 6 or 7 mpg. I'm not complaining but if I can improve on that by changing ratio it would be good.

 

That much torque is not in need of more gear at cruise. I'd be very surprised if turning up the rpms on a 460 enhances your fuel mileage.

 

That was what I thought too, but just looking for confirmation before I pull the trigger.


I forgot to mention my stall converter is about 2200.

 

When my eng. builder dynoed it he tied? the secondaries open. How much did that hurt mileage?

 

Those are some short tires! I agree, you should be around 2200 at 70 for mileage. I doubt they make gears to get that low (2.50 or so), an OD or taller tires would do the trick. The tires won't get you all the way but it's quick and relatively cheap, and reversible.

BTW are you sure you are measuring miles correctly? i.e., the odometer is correct?

 

I don't know how he could do that, unless you mean he adjusted them so they are partly open all the time? Not good for mileage for sure. Does it idle fast?

 

No, it is a slow idle, about 600 rpm's in gear with the air on, and maybe 700 with the air off.


I don't know what he meant by tying them open, but I'll check with him tomorrow. I know he did it for performance enhancement.


As far as the accuracy of the speedo it is dead on. it is electronic and calibrated on a measured course, and compared to the reading on our new car.


tires are actually 26.5""

 

You might consider a ratio in the the high 2's. That would more likely enhance your gas mileage than going the other way. The 460 is a high torque engine and could easily pull a 2:75 ratio or something in that range.

 

B/B Ford,
Try this link to a differential gear ratio calculator. It will help you do the calculations.

Differential Gear Ratio Calculator

I agree with Ross that a taller tire might be a better solution. And the fact that it's reversible is a plus.

Phil

 

Rear gear and tire diameter will affect the cruising RPM. Cruising RPM should be matched to the engine build, specifically the cam power range. If the engine runs most efficiently at 3000 RPM you dont want to be cruising at 2000 RPM.
That said if your engine idles at 600 RPM then it has a stockish cam, so the engine should be making big torque from off idle up. I would think you have a dual plane intake and 750 cfm or close carb. Your 3.0 gears should be working fine for what you have. Is your timing curve correct? Vacuum advance hooked up? Carb setup correct? I would expect at least 10mpg out of a stock BB unless there are tuning issues.

 

The cam is a FF 280H-10 Comp Cams


running 9.0 to 1 compression


I suspect what he did to the secondaries was for performance and sacrifices mpg.

 

I suspect you have a vacuum secondary equipped carb and he is trying to circumvent that with a trick on the linkage. I'm just speculating of course. There are more proper ways to dial that function and he would be aware of that.