What rear end ratio for Big Block on Freeway?

[WinDancer]

Big Block guys, what rear end are you using with factory size tires to cruise the freeway comfortably at 70 mph?
Thanks,

[rwgregory]

Gear it tall and use your big block grunt!! Start with 3:08 and get an overdrive if you can find one. I have seen big block passenger cars with 2:76 ratios stock from the factory.

[kevncar]

Very curious on this one as well. I've got a 454 out of an 81 dually goin in my 67 with a th400 and a GV overdrive. I want to have that BBC torque fun, but at the same time be able to drive it to work (40min one way) occasionally.

so keep the suggestions coming. good thread WinDancer

[mnunn454]

I'd attack it with math.

Figure what minimum RPM is most efficient for your engine/cam/exhaust setup to turn at (let's assume 2200 for the moment).

Measure the diameter of the tires you have and figure the circumference (I'll assume 30").

Then determine the high gear ratio through the tranny (I'll assume it's a TH400 that has a 1:1 high gear whose converter can't lock-up so the ratio through the tranny is about 1.03:1 because it slips about 3% at highway speeds).

Take the speed in miles/hour (70) and divide by 60 to get (1.17) miles per minute. Then multiply that by 5280 to get (6160) feet per minute. Next, divide that by 2200 to get the number of feet your truck needs to move per engine revolution (about 2.8 feet per engine revolution).

Now take the rear tire diameter in inches (30) and multiply by 3.14 to get its circumference (94.2"). Then divide that by 12 to get (7.85) feet.

Now divide the tire's circumference in feet (7.85) by the number of feet your truck needs to move per engine revolution (2.8) to get the total driveline reduction needed (coincidentally about 2.80).

Next, divide the total driveline ratio needed (2.8) by the tranny ratio in high gear (use 0.7 if your tranny is a TH700 or TH200, 0.8 if it's a late model manual with overdrive, or 1.0 if your tranny is an older manual or an automatic Powerglide/TH350/TH400).

Assuming you have a 1:1 tranny ratio, 2.8 divided by 1.0 means you'd need a 2.8:1 ring and pinion. If you have the 0.7 overdrive, then 4.0:1 is about right.

Since there isn't a 2.8:1 (or a 4.0:1) Chevy R&P option, you'll have to choose either a 3.08 or 2.73 (or 3.73 or 4.11 if O/D tranny) depending on your needs/preferences.

If your tires are smaller than 30" in diameter, then the 2.73 (3.73 for O/D) may well be the better choice. If the tires are taller than 30" and/or if your engine needs to turn faster that 2200 to be efficient, then the 3.08's or even 3.23's (4.11 for O/D) may be the better choice.

You do the math using your own real numbers. You should get similar results.

Now that the "top end" is set, think about the bottom end. 2.73's will yield great mileage, but are going to take off very slowly from a traffic light or when pulling a heavy load. You may want/need a tranny with a lower (numerically higher) low gear. If so, rerun the math above with the new tranny ratio figures. For example, a TH700 has a 3.06:1 low gear and a 0.7 high gear. That combo with a 3.73 R&P and 28" tall street tires should offer offer you the best of both worlds.

One thing to watch out for though. "If" it's an automatic, you don't want the engine RPMs to be running slower than your torque converter's stall speed (unless it has an electric lockup function and it's enabled). Otherwise you'll cook the tranny.

[one9six9]

WOW! Can you say that again?

Keith

[KFreddy]

454 w/ 3.08, TH400, 255/70-15 tire about 29.1"

@50 MPH, RPM is about 1775
@60 MPH, RPM is about 2150
@70 MPH, RPM is about 2500
@80 MPH, RPM is about 2850

I believe the general consensus is that the closest factory tire equivalent is a 235/75-15 about 28.9" so this is a fairly good camparison between the two tire sizes, RPM would be off by a negligible amount with the slightly smaller tire by 15-30 RPMS.

[Jim_PA]

I have a set of 2.92's I can sell you

[Longhorn Man]

hey mnunn454... were you using a cheat sheet or anything for that?
I only ask, becouse that's impressive for a normal car guy to know, let alone put into aplication, mathc like that.
Hell, you had be confused after the first couple lines.

[larry may]

Originally Posted by Longhorn Man

hey mnunn454... were you using a cheat sheet or anything for that?
I only ask, becouse that's impressive for a normal car guy to know, let alone put into aplication, mathc like that.
Hell, you had be confused after the first couple lines.

Troops like that put us on the MOON first..lol

[Bill72BB]

I'm running a Dana 60 with 3:54 and 265/75/16 tires .No problem on the Kali freeway doing 70 .

[Longhorn Man]

Originally Posted by larry may

Troops like that put us on the MOON first..lol

well put.
Even if there was a cheat sheet in front of him... I'd probably still been totally confused and screwed it all up.
Then again, for all i know, HE screwed it up. But I doubt it.

[WinDancer]

WOW is right but I expected nothing less from this board.

You guys are terrific with info and sharing your knowledge!

mnunn454, thanks for the info. Anyone know what the stall speed is on a regular th400?

I have noticed that if you let off the gas that the transmission kind of floats off somewhere and isn't a solid lockup like with other vehicles I have owned.

Is there a way to just have it engaged all the time or does that require a tranny change? Is there a way to check for the electric lock-up function?

Jim_PA is that a standard ratio for our trucks? If so and it is in good condition can you get a shipping cost to 98589 and PM me with it? Don't want to be doing business in this thread.

Thanks again,

[Longhorn Man]

your truck does not have a lock up converter... so the drop in RPM that you describe, is normal.
You ask for standard ratio.... what engine/tranny did your rig come with originally? That dictates the ratio.

[WinDancer]

Originally Posted by Longhorn Man

your truck does not have a lock up converter... so the drop in RPM that you describe, is normal.
You ask for standard ratio.... what engine/tranny did your rig come with originally? That dictates the ratio.

I was trying to ask if it was one of the standard ratios available for our trucks. So there was maybe some hope of making it work with my existing rear end and parts. I have 5 lug axles and rear leaf springs. The ratio in mine is a 3.73, and way too low to make me happy.

How do you know it doesn't have the lock up converter? Because of the freewheeling? The other suggestion was to check the wiring for it? So maybe I do have one and it isn't wired in? Where do you guys get all this stuff in your heads? I am 58 and seems like I lose access to a little more info every year

Thanks!

[WinDancer]

I did a search of these forums and came across this article which explains torque converters and stall speeds so well I even understood the theory

http://www.hardtail.com/techtips/sel...converter.html