lnetter
9 inch rear ends
New User
Joined: Aug 2001
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9 inch rear ends
.....I'd say first, figure out what you have ratio wise. Do you have an open diff or a limited slip one? I'd guess if you have a 9 inch its probably open. I have a F100 w/ a Dana rear thats limited slip and two 9" rears that are open. To figure that, block front wheels on level ground to prevent rolling, and jack up both rear wheels off the ground. Now w/ truck in neutral and front tires chocked, turn one of the rear wheels.... Does the other one turn with it in the same direction? If so you have a limited slip. Does the other wheel not turn or even maybe turn in reverse? Then you have open diff. Now... if you have limited slip, the calculation is easier. Make a mark on your tire at say 12:00 and make a mark or visualize one on the driveline perhaps by the U joint. Rotate the tire once.... how many times did the driveline rotate for that one turn of the tire? Two of my truck have 3.50:1 rears that means that for one turn of the tire, the driveline went around 3 1/2 times. I have another truck that I've swapped a 3:25 into, that is one turn of the tire, 3 1/4 turns of the driveline. Now with the open differential, I believe you have to imobolize one of the tires by lowering it to the ground and doubleing the # of driveline turns. Since this is rough measuring I think it helps to turn the wheel more times to average out the results..... What I mean is for example.... a 3:50 rear ratio, for one turn of the wheel the driveline would turn 1 3/4 (half of 3 1/2) or for 2 turns of the wheel, 3 1/2 turns of the driveline, or 3 turns of wheel 5 1/4 turns.
.......
BBT
.......
BBT
The best way to calc. gear rayio is to pull the gears and count teeth.
Next best is:
To calculate gear ratio for a non-limited slip
diff. is to leave one wheel on the ground (jack up the other),
turn the tire five revolutions, count the number driveshaft turns,
( include fractional turns ) then divide by 10.
this reduces your error on the estimated fractional turn by a factor of 20.
sample numbers:
a 3.11 would give 31.1 driveshaft turns.
( I'de proably extimate it at 31.125 (31+1/8)
and calc it as 3.1125 then round it to 3.11 )
a 3.12 would give 31.2 driveshaft turns.
( I'de proably extimate it 31.25 (and make a note it was slightly under)then come up with 3.125 and know from my note I need round down
to the next probable size which would be 3.12)
( even I can tell those apart )
For a quick rough estimate just turn one tire one turn ( lock the other tire)
then divide driveshaft turn count by 2.
( works ok if you just need anything close.)
it usually doesn't matter if you go a step up or down
unless you're racing, pulling heavy loads, or geared too high
or low to begin with.
just a note: every vehicle I've ever owned (6) has had the speedometer read 5-10 percent high with the original speedo gear and recommend tires.
Senior User
Joined: Oct 2012
Posts: 422
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From: Arlington
and Big Brown Truck has not made a post since 2004
Elder User
Joined: Sep 2014
Posts: 862
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From: like subarctic, brrr man!
Well, since it's dug up...
Find out what you have for speedo gear now. My '78 np205 has the speedo gear (don't know what) and a changable gear inside of it, so count that inside gear too.
And then make a new thread
Find out what you have for speedo gear now. My '78 np205 has the speedo gear (don't know what) and a changable gear inside of it, so count that inside gear too.
And then make a new thread
Cargo Master
Joined: Jun 2014
Posts: 2,215
Likes: 323
From: Central Florida
I'd say that BigBrown has hit the nail on the head. The 460 is probably the biggest part of the fuel consumption problem that you described.
One might do better with working on engine efficiency starting with a thorough tune-up. In addition to the tune-up, change the thermostat from the stock 200º unit to a 180º unit.
Assuming that this is not a 69-71 vintage 460 (check to see what the VIN says with an online VIN decoder), the camshaft timing is retarded by 7º in order to meet the 70s smog requirements. This should be be set back to 0º and, so, replacing the timing chain with a 69-71 unit will significantly improve engine efficiency. A good set of headers and adding a 1" riser under that carb if it doesn't already have one will top off the list of high return mods.
This will yield more horsepower and improve fuel efficiency without breaking the bank but the MPG gains will be modest, very modest. It will never come close to the 300 CID 6 of this era.
One might do better with working on engine efficiency starting with a thorough tune-up. In addition to the tune-up, change the thermostat from the stock 200º unit to a 180º unit.
Assuming that this is not a 69-71 vintage 460 (check to see what the VIN says with an online VIN decoder), the camshaft timing is retarded by 7º in order to meet the 70s smog requirements. This should be be set back to 0º and, so, replacing the timing chain with a 69-71 unit will significantly improve engine efficiency. A good set of headers and adding a 1" riser under that carb if it doesn't already have one will top off the list of high return mods.
This will yield more horsepower and improve fuel efficiency without breaking the bank but the MPG gains will be modest, very modest. It will never come close to the 300 CID 6 of this era.
New User
Joined: Feb 2015
Posts: 15
Likes: 0
not to steal the thread, but i have a quick question. i have a 76 f-100 with a 9 inch rearend, on the tag it show 300 gear ratio. my question is does the 300 give you more pulling power? better fuel mileage? and is it the higher the number the faster take off speed thanks in advance.
Lead Driver
Joined: Apr 2011
Posts: 6,399
Likes: 40
From: El Dorado, Arkansas
not to steal the thread, but i have a quick question. i have a 76 f-100 with a 9 inch rearend, on the tag it show 300 gear ratio. my question is does the 300 give you more pulling power? better fuel mileage? and is it the higher the number the faster take off speed thanks in advance.
A 4.11:1 ratio is good for pulling (or drag racing) but, will limit the top speed of the truck and the engine will be turning high RPMs at highway speeds (if no overdrive transmission).
A high ratio is numerically low --2.75, 3.00:1.
A low ratio is numerically high --3.70, 3.89, 4.11.
A non-overdriven transmission will be 1:1 in high gear. A vehicle with an overdrive transmission will have an equivalent drive ratio. This is figured by multiplying the transmission's overdrive ratio by the rear end ratio.
Example: a .67:1 overdrive ratio x a rear end ratio of 3.70:1 would have a final drive equivalent of 2.48:1 (.67 x 3.70 = 2.48). A final drive equivalent of 2.48:1 would be even higher than a rear end ratio of 3.00:1 (with no overdrive).
However the biggest difference is a vehicle with a 3.70:1 rear end ratio, with an overdrive, is going to have much better pulling/accelerating power than a vehicle with a 3.00:1 rear end ratio and no overdrive. At highway speeds, the vehicle with the 3.70 and an overdrive will cruise at lower RPMs, due to the overdrive's final drive equivalent.
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Posting Guru
Joined: Jan 2005
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From: Douglas, Georgia
Yeah, sorry bud, I don't care what gears or how many you have, a 460 in a pickup truck is going to be hard to get anything more than 12 mpg.
geez, who pulled up this thread? This kid isn't in college anymore, he's probably retired by now.....
geez, who pulled up this thread? This kid isn't in college anymore, he's probably retired by now.....
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