I have a 1979 F-150 with a 9" rear and want to change the ratio from what it is (I don't know), to a 2.75 what I have sitting in a box. I want to be able to go down the highway at a faster speed with less RPM. What parts do I need to switch from a 3 or 4 series gear down to a 2 series? Or can the chunk just be pulled out and the gears swapped? Thanks for any help.

 

Check the gears that are in the rear first, because most of the stock 9" rears (trk), came with 2.75:1.
Also the beauty of the 9" is that you can swap gears from 2 series up to, if not including 5 series.



Dre'
1978 F-150, 460ci(750cfm,Edelbrock Performer,Dynomax Ceramic Coated headers,etc.)Auto-C6,3:70.1 - 9" Posi.

 

Thank you, but I think someone changed the gears at sometime. I have talked to some speedshop guys and they say I need a new carrier gear so the new ring and pinion will match up, because the new 2.75 ring gear is narrower and the new pinion will not match up correctly? Do you know about this, or is there any truth to this? Thanks again

 

There is an article in the tech articles that details how to rebuild a 9". You may want to just buy a bearing kit or rebuild kit from Summit. It would include new carrier and pinion bearings and everything else you would need. If you've never set up the gears on a rear end, you might want to find someone more experienced so you don't get gear whine or something worse. You can figure out your ratio now if the third member is still in the truck. Jack up one wheel and put the truck in neutral. Spin the wheel one turn and count how many times the driveshaft spins. It should get you close.
Dave,
79 F-150 4x4, 390 w/C6, Edelbrock carb, 33X12.50 never will be finished.

 

Hey guys can you splain this seris thing, I had never heard of that.

I thought that the only concern when changing 9" centers was the axle spline count, 29 vr 31.

John

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66F100s Rule
In the cool still quiet of night you can hear chevies rusting away.

 

[font size="1" color="#FF0000"]LAST EDITED ON 28-Jun-01 AT 03:40 AM (EST)[/font][p]I've never heard of that with the 9", just the Dana's with anything above or below 3.73 I think

 

[font size="1" color="#FF0000"]LAST EDITED ON 28-Jun-01 AT 12:32 PM (EST)[/font][p]The series ideal he is referring to is usually used my GM to define weather it is a 2.xx:1 (series 2)
3.xx:1 ( series 3) etc. ring and pinion.

In my head I knew what he was referring to, so I just replied accordingly.


Also ford's 9" gears are cut in a manner where the thickness of the ring gear changes to compensate for the carrier, so you can use the same carrier with different ratio gears.



Dre'
1978 F-150, 460ci(750cfm,Edelbrock Performer,Dynomax Ceramic Coated headers,etc.)Auto-C6,3:70.1 - 9" Posi.

 

I've changed a couple of my 9 inch rears. Hows your back first of all? 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.

Now.... now that you know what ratio you have, you can decide how much you want to change it. The 9" ones commonly are 2.75, 3.00, 3.25, 3.50, 3.73 The other thing you need to know before swapping rears, is the spline count on your axles. Ford trucks commonly have either 28spline or 31 spline axles. You must use the correct rear for your spline count. To know for sure if you don't trust your diff tag (if you still have it) is to pull the axle out and count the splines. If you do that , you probably are commited to changing rears because of the effort you've gone to. Know don't assume.... I got a 3.00 rear out of a 74 F100 that was a 31 spline variety. Both my 76 and my 79 F100 and F150 had 28 spline axles. Also if you happen to have 31spline 3.50 gears, according to the Seattle Area salvage yards, those are selling for $500+ because of the demand for them from 4X4 folks. Now if you are looking at nine inch carrier that is out of truck, you can know its ratio by counting the teeth on the ring gear and dividing the # of teeth on the pinion gear. It helps to have a grease pencil to make a mark one the gears to keep track of... For instance the rear out of my 79 that was suppose to be a 3.73.... and I was putting in the 3.50 from my 76 turned out upon post swap inspection to be a 3.50. Anyway I counted it kept coming up w/ 35teeth on ring and 10 teeth on pinion. The consolation was that the "new"3.50 gear was in far better condition. In considering what to do w/ my 31 spline 3.0 gear, I found that I could order new axles for truck that would work for about $230. That one for now lives on my garage floor. As far as installation, you may be told that the rear end gaskets are no longer made, to use RTV to make your own gasket. I've done this twice. And after second time I found a parts store that has FelPro gasket that supposedly wasn't suppose to be availible. Also I couldn't find the soft copperlike crush washers that are on every stud under the nuts. I ended up finding a like washer in the plumbing dept of a good hardware store. I'd highly recomemd renting or borrowing a slide hammer to assist in breaking axles free from carrier to be pulled out. One last consideration... you can effectively change your gearing by going to different tire size as well. The tire store that sold me my 31" tall tires over stock 29" size told me it was like changeing my 3.50 to a 3.25. Be aware that when you change your rear ratio either by tire size or actual gears, that your speedo won't be acurate w/o changeing the speedo drive gear to one that is correct. Thats' another topic.

BBT

 

AMEN!

 

BigBrown, it's nice to get detailed explanations about specifics some of us don't know about. I appreciate all the extra effort folks like yourself go through to help educate. Many thanks
'79 F100 300-6 C4

 

Thank you very much BBT. The information you gave is very useful. I did not know you had to double the driveshaft rotations when figuring the ratio on an open differential. I will go do that, just to see where I am. Then get this 2.75 ratio in the rear. The trip on the highway from college to home is killing me.

Space
79 F-150 460

 

Another thing to consider is this. If you put a high ratio in your truck, how hard is the engine gonna work to get up to speed? I always figured that lower RPM=better gas mileage till I had a vaccume gauge to watch. The vaccume gauge measures manifold vaccume. Thats how much air the cylinders is sucking through the carb. With my old 300 and 3speed I noticed that sometimes shifting early and kinda lugging the engine would suck more gas than letting it rev faster but not work as hard. What sized engine do you have? How much improvement are you hoping for? Do you ever have to deal w/ slippery conditions? A high gearing has more tendency to spin tires rather than have good traction. I have a 79 F150 that has a 302 w/ stock 2v, 3.50 rear gears, 31" diameter tires, and a manual Overdrive transmission. I get around 14mpg generally in city driving mostly. Before I had an overdrive transmission, it I had trucks w/ a 302/C6 and another w/ 390/4sp w/ the same rear gears and tires. Those got about 11mpg. My Dad has a 95 F150 w/ Fuel Injected 300 and 5sp overdrive w/ 3.55 gears and 29" tires. With all the advantages of the computer controlled engine and 5 speed he averages maybe 18mpg. You may find a 2.75 is a pain in the neck around the neighborhood and at lights and whatnot. I have a buddy w/ a very high geared Chev truck w/ 3speed and small 250ci 6. You have to slip the damn clutch in 1st for half a block to take off from light. BTW the overdrive I have is essentially a 4sp toploader, where there isn't any 3rd gear anymore, 3rd gear is 1:1 like 4th would be, and 4th or OD is .79:1. It drops my 60mph Rpms from 2300 to 1850. If you are interested, look for these in 78-79 Fseries and vans. Not all floor shifted manuals in this era are this overdrive, many are granny low 4sps. There were some later years of Ford Trucks w/ the OD tranny but I understand that they have aluminum cases and aren't so strong. My overdrive is very much like a Mustang toploader or a 3sp truck manual w/ external shifter bolted to tailshaft w/ small connecter rods to shift tabs on the side of the tranny. Mine is called a RUG because it has RUG cast into the case, the later aluminum ones (starting in 80 I believe) are called SRODS and had a shifter that went directly through the top of the tranny like the Granny low 4sps have.

BBT

 

All I am looking for is better milage on the highway. Going home 35 miles from school on the highway, I burn about a quarter tank of fuel. I have no tac, but by sound at 60mph I am easily spinning more than 3000rpm. I am driving a 79 F-150, with a 460, C-6, and a 650cfm 4bbl carb. I like to cruise around in the truck because it is like new and I recieve many compliments on how nice it looks, but it kills me to take it anywhere I have to be on the highway for more than a few min. So one more question, I should be able to just swap gears from what is in there to the 2.75 ratio I have, or do I need a new carrier gear? Or is it just a clean swap? I am not going to do it, but I want to know what I need, so I don't get jerked around at the shop when it comes time to put them in with bogus parts.

Thanks once again

 

Spaceman, if you're turning 3K + rpm at 60, you've got something closer to a 4.56 gear in it (unless your tires are 12" tall :-)) If thats the case be thorough when inspecting the case, short gears like to put a lot of stress on bearings. Pay close attention to the webbing around the pinion gear pilot bearing, it almost always cracks in high stress applications (like a big-block powered, low geared truck ;-))

Your best option might be to just get a complete third member from a junkyard with the gears you want and drop it in, and like BBT said, know the spline count of your axles.

BTW, very good posts BBT. One little cheat I'll throw in is turn the brake drum around and start the lug nuts a thread or two, and presto, instant slide hammer.

Cheers,

BFM

 

460 huh? I've heard it said they get 9mpg empty or loaded to the hilt, uphill, downhill....... 9mpg period. I'm sure a shop is gonna want to take steps that cost you money. When rebuilding 9inches, taking the pumpkin apart, there are all kinds of fine tolerances that you probably can't do w/o tools you probably don't have. If you want to try and get the better mileage, just pull your axles, find out the spine count. Go get the appropriate 2.75 gear 28 or 31, whatever your axles are. I think full sized FoMoCo cars are a common source. I believe that the 9inch was discontinued in the very early 80s to be replaced w/ the 8.8" rears used in trucks now. I'm guessing you will be able to spin the tires mercilessly w/o a light foot w/ a 460. You will also have to change out your speedo drive gear to get a correct speedo reading and maybe even get an adapter. Do you have a Speedo shop in your area w/ a Chassis Dyno to test? Good luck finding a pro-shop to do the work w/o wanting to charge you several hundred dollars. My guess is that a pro-shop would insist on rebuilding w/ new ring gear and pinion gear and whatnot and charging $500 (thats a guess). If you are trying to save money, I'd consider not doing anything except perhaps driving a more economical vehicle. I'd say getting good mileage out of a 460 is a tough goal.

BBT