Ok
So I have worked on machinery of all kinds since I could hold a hammer (about 34 years or so). I can run a Bridgeport, rebuild a logan 9" lathe without any books, but I have never quit been clear when it comes to automotive gear ratio terminology.
My truck is a 68 ford with a 9" 10 bolt rear and I need to replace the Ring and Pinion. Basically rebuild the thing since it sounds like Captain Kirk's old Warp Drive when I start going over 55mph,which leads me to believe 55 is warp one(ha ha oh well it was funny when It popped into my brain).
Anyway I have a low ratio I believe to be 3.50 and I would like to change the rear a bit to give better fuel mileage as well as avoid a blown rear.
From 1957-72 the 9" 10 bolt Ford rears came in five ratios
3.00
3.25
3.50
3.70
4.11
given that I am running a 3.50 rear I've at the middle of the set.
As I said I was fuzzy on my understanding so please correct me if I am wrong.
3.50 = 3.5 revolutions of the ring gear to 1 revolution of the Pinion gear, ie the drive shaft.
If that is then 3.00 would be 3 revs of the ring to 1 rev of the Pinion, or 4.11 = 4.11 revs of the ring to 11 for the pinion.
BTW : I recently pulled the 360 engine, had it bored 60 over, swapped over to a 390 crank, and added 428 cobra rods and pistons. The whole kit was sent out and balanced and essentially is 407 cid. A very strong "Torque Monster" as my machinist Rich called it. It will have a 600 four barrel Holley, oversized and cooled trans and engine pans, electric fan on a four core Radiator, and MSD ignition (Distrbutor, coil, and spark box). Removing the fan frees up to 15 HP and that would allow me to add AC without any adverse drag. For towing I could just pull the belt to gain a little extra power. The truck is a 2wd f100 with a factory C6 automatic, dual tank towing package. I have towed a large trailer in the past and had no problem.
As I said the gears are worn and hum louder and louder as you speed up past 55. I need to replace them.
I do understand that a larger diameter tire achieves a lower engine rpm thus better fuel economy. The circumference laid out would add several inches per rotation and thus greater MPG and a higher top speed but a loss of traction performance side to side. Its one reason the military uses tall narrow tires. Mile after mile the inches add up. That’s fine when driving under 50 most of the time. I'm willing to rotate off the street tires to put on a taller set for hauling when I need it.
I am aware that It is an involved swap. But the housing is fairly generic and the conversion involves parts and adjustments not machining. <O></O>

Now the truck is going to be used to haul an auto trailer, but from time to time it will be used to take the family camping. So I was wondering if going too light could cause a problem when towing as well?
As I said before the gears are worn and need to be replaced either with 3.50 or better. With the cost of fuel rising Id like to get somewhere between 15 to 20 mpg unloaded if I can. I am looking to possibly replace the stock Fuel tanks with Plastic replacements, as the steel ones are about 40-50 lbs each.<O></O>

Could someone please explain which in your opinions would probably be the best choice for my application?
John

 

For the rear gears, you're looking at the ratio of input to output, which happens to also be the ratio of the number of teeth on the ring gear to the number of teeth on the pinion.

So, 4.11:1 = 4.11 pinion turns to one ring turn, but also equals 4.11 ring gear teeth for every 1 pinion tooth.

 

Nope, it's the other way around. 3.5:1 means 3.5 turns of the pinion (driveshaft) makes the ring gear and axles turn once. Higher gearing is numerically lower, meaning a higher speed for a given rpm, while lower gearing is numerically higher, meaning lower speed for a given rpm.
With that size engine, getting to the high teens is going to be tricky. As long as it has the torque, you probably would be best with a 3.25 gearset. With the c6, it may be a little slow off the line towing, but should handle highway speed ok.
Edit to ad:
Your speedometer will be off, since it's driven off the driveshaft.

 

The gear set from any 9 inch will work, which gives you a lot of choices. Look on eBay - they stretch from 2:50 to 8:88 and you can have special gear sets cut. People go for the numerically high gear sets to offset the effects of extra tall tires.

I disagree when people suggest an overall lower ratio for an automatic transmission - they forget the torque multiplication you get with a torque converter. Ford equipped trucks with automatics with a higher overall gear ratio than those with clutches. But then I am not a racer, and build my trucks as daily drivers.

The ratio tells you how many times the driveshaft will turn for every turn of the wheel - a ratio of 3.50 to 1 means that the driveshaft will turn 3.5 times for every complete revolution of the wheel.

With this, and knowing the diameter of the wheel and tire, you can use math to figure what RPM your engine will be turning at any speed. Your C6 has a top gear of 1 to 1, so just ignore the transmission for now. However, with any non-lockup automatic you do add some (a few hundred?) RPM do to slippage in the torque converter. I would ignore this too.

What I do is pick the tires I want to run, the speed I like to cruise, and the most economical RPM of my engine. I then solve the math for the ratio I want. For my 59, it came out 3:10, and I found a set of 3:12 gears on eBay. The ring and pinion cost me about $50 with shipping.

I also added a limited slip carrier.

For a check on suggested ratios for any size tire look at:
http://www.sierrabronco.com/TireGearRatio.htm

or maybe better:
http://www.jawsgear.net/store/catalog/calc.php

I like to figure the RPMs in the gears - that shows where you should downshift when towing. I keep my engines between 1750 and 3000 RPM as much as possible.

My C6 has a first gear ratio of 2.46:1, second is 1.46:1 and third or high gear is 1 to 1. Plug those numbers into this calculator:
http://www.car-videos.com/tools/speedrpm.asp

For a good discussion on the 9 inch:
http://www.kevinstang.com/Ninecase.htm

Hope this helps, ask more questions if you have them. Post your tire diameter amd desired RPM for what speed and I will figure your ratio for you.

 

I finaly got my dyno up on my PC and also lifted the bed off my Truck yesterday. There are no tags on the housing or tubes but once I get the thing cleaned up I hope there are stampings hidden under the dirt and grease. Not that that realy matters as the old gears are worn so replacing them with ones matched to the modified engine is realy the way to go.

So heres How the engine specs out
Bore 4.110 Cylinders 8
Stroke 3.78 Compression ratio 9.50
Cam Type Hydraulic @ +10 Deg
Exhaust Headers Wet Sump O/P windage
Intake Type Dual Plane Aluminum
Stock Heads In 2.02 Ex 1.60
Electrical Alternator
HF Water Pump F/P Electric W/Cool tank
Wrist Pins Stock Press Fit Stock Rods
Spark Advance Electronic No Fan (electric)
Cubic In 401.4 Liters 6.5
Est. Shift Point @ 2750 rpm
Est. Peak HP 339.7 @ 3500
Est. Peak Torque @607.7 ftlb @ 2000 Big block Ford 390
NOS not used Duration @ 0.050 192 Lift 0.247
Estimated Launch @ 2350 rpm Cam Centerline 116 deg
Trans gear Ratio 0.73 to 1
Tire Size 255-70R15 T/Width 7.2” Diam 29.1” Revs.pm 694 Diff 3.11
RPM Hp TQ
1500 171.4 600.1
2000 231.3 607.7
2500 289.1 607.3
3000 318.3 557.3
3500 339.7 509.7
4000 317.2 416.4
4500 284.1 331.6
5000 211.5 222.2
5500 181.4 173.3
6000 141.5 123.9
6500 120.6 97.4
7000 82.3 61.8
7500 24.1 16.9

The Differential recomended is 3.11 and I think once the trucks done I'll go to have the speed shop down the road do a Real dyno on it. There were a couple of factors that the program wouldn't take into account: the 428CJ Rods are heavier then stock, the program doesn't allow for Carb changes and adjustments either. The program suggests a 650-700 4bbl carb Flat no options.

Does anybody Know of a better dyno than Drag plus or Virtual Engine? they seem to be lacking and leave no room for adjustments in certain areas.

John

 

I went into my shop and dug into the book pile and found I had a copy of HPBooks "Auto Math Hanbook" by John Ludlow.
following the pencil metod the engine does work out to a 401.4 displacement and basiclly matches the specs for a 406fe. Est. Peak Torque @607.7 ftlb @ 2000 rpms. and an Est. Peak HP 339.7 @ 3500 rpms.
I'm still trying to work out the rear ratio and running through a 1.1 ratio on the C6 Automatic the ideal economy comes from 65 mph with the rear set at 2.50 which runs 2071 rpms with 29.1 inch tires (255-70R15). From what I see She'll be soft on the launch witch the desktop dyno suggests is at 2350 rpm. and the book math agrees.
And I get no results close to 3.11 with the paper method for the gears.
any thoughts?
John

 

By the way according to the feul economy calculator I have the 2.50 gears should fetch 10.55 mpg in 1st gear at 28.1 mph, 15.21 mpg in 2nd at 47.3mph , and 21.17 mpg in 3rd at 69 mpg. For an big block that sounds sweet but how sluggish is it going to be for excelleration?
3.00 gears might be a better bet for excell with 8,79 mpg in 1st gear at 23.4 mph, 13.54 in 2nd at 39.4mph , and 18.64 mpg in 3rd at 57.5 mpg.
any thoughts?
john

 

if theres no tag there should be a stamp on the passenger side axle tube facing the back. it should be the gear ratio. just to the right of the center chunk.

 

Jphnny, seems like you got the idea now. The thing to remember is that you are looking for the best compromise between power and economy. I agree that an end ratio of 2.5 would be the best for economy, but, from the sounds of your build, you are also wanting a lot of useful power. Your the one that has to be satisfied with the performance of your truck.

Actually, ford sold a lot of trucks with the 2.88 gear set and a big V8 and automatic, with those looking for good acceleration ordering 3.00. The real problem is that we do not know where the "sweet spot" is on your engine. With your build I expect it is a bit higher RPM than the 2100 you are figuring. If it were me I would be looking at a 2.88 to 3.0 gearset. I wouldn't go any lower than 3.25 even if I wanted a 1/4 racer.

As for the RPM at launch, it is not something I know anything about but that is why they designed high stall torque converters - so you could rap up your motor and be spinning fast at launch.

I told you 3.11 worked out best for me, but that is in a lighter truck with a six, 4 speed and an old man towing smaller trailers.

But than gear sets are not that expensive, nor, for that matter, 3rd members. Lots of people use math to get close and then try different ratios to see which one suits them best. Just remember to change speedometer drive gears when you change ratios and tune your selection as needed.