2.75 to 3.50 3rd member swop issues

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Superbond

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I took her out for a short ride today after upgrading my 2.75 3rd member to a 3.50
Have some issues! Brakes smell and smoking. I get that. I adjusted them so the drums went on with a little friction. Heat gun says 228 and 198 degrees both drums.
The speedometer is way off. Like 20mph! I get the rpm’s being off but the speedometer?
Do I have to change that small nylon gear at the end of the speedometer cable and if so to which one?
 
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So
I took her out for a short ride today after upgrading my 2.75 3rd member to a 3.50
Have some issues! Brakes smell and smoking. I get that. I adjusted them so the drums went on with a little friction. Heat gun says 228 and 198 degrees both drums.
The speedometer is way off. Like 20mph! I get the rpm’s being off but the speedometer?
Do I have to change that small nylon gear at the end of the speedometer cable and if so to which one?
I put in the same 3.50 TL from the 2.75 open (and FMX to C4 with shift kit) and I went I believe 23 tooth... can't remember exactly but it was as close as I could get from any suppliers after researching ratios. Anyways, I am over recording on the speedo 5mph and I can live with that vs the other way around. 🤷🏻‍♂️
 

Ron Tanzi

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So
I took her out for a short ride today after upgrading my 2.75 3rd member to a 3.50
Have some issues! Brakes smell and smoking. I get that. I adjusted them so the drums went on with a little friction. Heat gun says 228 and 198 degrees both drums.
The speedometer is way off. Like 20mph! I get the rpm’s being off but the speedometer?
Do I have to change that small nylon gear at the end of the speedometer cable and if so to which one?
Yes, you have change your speedometer drive gear. You can calculate the correct gear by inputting the the 3.50 ratio you have installed and your rear tire diameter. The formula is easily available on line.

Ron
 

Hemikiller

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Speedometer cable is driven off the output shaft of the transmission. The faster the shaft spins, the higher the speedometer will read. You need more teeth on the driven gear to slow it down.

The correct speedo driven gear is found with 5th grade math. Divide new gear ratio by old gear ratio, multiply the result by the number of teeth on the driven gear.
 
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Auto or standard makes a difference, the direction of the pitch of the teeth are different. Don at OMS has several to choose from. The most likely tooth count on the drive gear, if it is the original transmission, is 7. If rear tire diameter/roll out is other than stock it will make some difference on speedo accuracy. Chuck
 
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Speedometer cable is driven off the output shaft of the transmission. The faster the shaft spins, the higher the speedometer will read. You need more teeth on the driven gear to slow it down.

The correct speedo driven gear is found with 5th grade math. Divide new gear ratio by old gear ratio, multiply the result by the number of teeth on the driven gear.
👏🏻 I need a 24 or 25 tooth gear to be dialed in "exactly". Do you know if someone offers it? Thnx
 
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The largest Ford had was 21 tooth, the biggest aftermarket is 23 tooth. If 23 teeth doesn't get you close enough you may have to get a ratio adapter, or a GPS to mechanical adapter.
 
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1972 Mustang convertible, 351C 2V with FMX (#'s matching)
Currently equipped with 351C 4V 4BM, .060 over, roller cam, Sanderson block huggers, -AC
The largest Ford had was 21 tooth, the biggest aftermarket is 23 tooth. If 23 teeth doesn't get you close enough you may have to get a ratio adapter, or a GPS to mechanical adapter.
Roger that and I can live with being actually 5 under the posted speed limits. May have already helped me save some cabbage on speeding tickets...
 
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So
I took her out for a short ride today after upgrading my 2.75 3rd member to a 3.50
Have some issues! Brakes smell and smoking. I get that. I adjusted them so the drums went on with a little friction. Heat gun says 228 and 198 degrees both drums.
The speedometer is way off. Like 20mph! I get the rpm’s being off but the speedometer?
Do I have to change that small nylon gear at the end of the speedometer cable and if so to which one?
Hello Superbond,
Sounds like nice improvement with the gear swap. Your indicated rpm’s won‘t be off from a gear swap, as you already know you are now spinning the engine faster for any given speed. My tach indicates about 3300rpm at 65mph with C6 trans and 3:50 gear. I used mph app to very speedo and timing light with built in tach to verify indicated tach rpm. Good luck dialing in speedo.
 
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skerwath

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I went from 3.0 to a 3.73 posi in mine. Before you drive it, double-check your geometry. Snapping u-joints isn't fun. I got an awesome deal at Denny's Driveshafts in NY. The Currie pinion was just short enough, that when I gassed it trying to get up the driveway, "pop" and clang, clang, clang. It's all set now, and talk about sinking into the seats...... It's a poor man's version of launch control. I scared my wife, my in-laws thought they were at Disney, and my son's think I am a rock star. Best improvement I ever did to her, besides the head rework. My speedo is about 10 over my actual speed. I have yet to swap my gear out.
 
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My all time favorite vehicle is our 1969 Shelby GT500
Our 69 Shelby GT500 has a very steep 3.0:1 rear axle ratio (Factory A/C required a high rear axle ratio of 3.0:1 or 3.25:1). At 60 MPH the tach is reading just under 2,000 RPM. We also have a 1973 Mustang Convertible with a 2.79:1 rear axle ratio. It is also turning its engine at a little under 2,000 RPM at 60 MPH. Our 1973 Mach 1 now has a 3.5:1 TractionLok rear axle, which had the engine running at just under 3,000 RM at 60 MPH with the original C4. We now have an AOD transplanted into the Mach 1. Now, at 60 MPH the engine RPM is also just under 2,000 in 4th OverDrive gear - making for a very pleasant cruising experience.
 

Superbond

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Thanks guys for all your information and advice. After researching and to the best of my scenario. I’m thinking the 23 tooth gear. I am going to pull out the original one in my FMX and add 5 mph per tooth and see what I get.
Mrgmhale what upgrade transmission did you put in you 73. Out of what year and make car did it come from? Thanks all
 

Capeman1954

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Hello Superbond,
Sounds like nice improvement with the gear swap. Your indicated rpm’s won‘t be off from a gear swap, as you already know you are now spinning the engine faster for any given speed. My tach indicates about 3300rpm at 65mph with C6 trans and 3:50 gear. I used mph app to very speedo and timing light with built in tach to verify indicated tach rpm. Good luck dialing in speedo.
you have to change nylon gear that goes into the transmission.
anything 3.90 or above has to use a special speedo cable , speedo gear adaptor and other item (at least that was the way it was in the old days when I ran a 4.30 Detroit Locker in my Mach 1
 

Hemikiller

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Standard drive gear tooth count machined into the transmission output shaft on all automatics is 8 teeth. There are some exceptions for F/L/M big car/T-bird applications with a 9 tooth output.

Standard driven gear on a 71-73 with an automatic and 2.75 rear axle, except with E70-14 tires, was 17 teeth. E70-14 tire cars received an 18 tooth.

Going to a 3.50 from a 2.75 would require a 21 or 22 tooth gear. (3.50/2.75) x 17 or 18

Tires that are taller than stock will reduce the tooth count on the driven gear.
 

Spike Morelli

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I had that same dilema. I went from a 3.00:1 open rear end, to a 3.50:1 Trac-Loc. My Mach also came with an FMX, and there are just so many speedo gears made for the application, none that are for the 3.50:1 set up. My solution, as there is a speedometer shop locally here in town, was to get their opinion as to what to do. Without hesitation, the owner said " No problem, we'll just put a "Ratio Box" inline on your speedo cable." It's a simple, small little devise that they can put any number of up, or down, gearing ratios into to calibrate your speedo to the new gears . I needed a new speedo cable anyways, so they made up a new speedo cable for me, a bit shorter by a few inches to compensate for the added overall length when the devise was added inline. It's been on the car for many years now. They also put my car on their rollers to check the speedo calibration afterwards, and also will take into account any tire size variation. Neat deal. So, try calling your local speedo shop and get hooked up.
 
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Superbond

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It turns out that it is shifting ok.
I think the first time I took it for a test drive after installing the 3:50 3rd member and brakes all around. The back brakes were not adjusted correctly. Now the brakes have been adjusted. The shift pattern seem good.
There is one thing that has me confused. After about a 40 mile cruise today, at the end of the drive I started smelling brakes! I checked the drums with a heat gun and it was saying 245 degrees on the left back brake and 197 degrees on the right back brakes. I had just adjusted them so they weren’t rubbing at all. Now they seem to be! Confused!
 

giantpune

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So
It turns out that it is shifting ok.
I think the first time I took it for a test drive after installing the 3:50 3rd member and brakes all around. The back brakes were not adjusted correctly. Now the brakes have been adjusted. The shift pattern seem good.
There is one thing that has me confused. After about a 40 mile cruise today, at the end of the drive I started smelling brakes! I checked the drums with a heat gun and it was saying 245 degrees on the left back brake and 197 degrees on the right back brakes. I had just adjusted them so they weren’t rubbing at all. Now they seem to be! Confused!
Did you make sure to use the self adjusting kits on the correct sides? One side has reverse threads. They are made to tighten when you are traveling backwards and mash the breaks.

Also, I have seen stuff like that happen when using incorrect or out of spec aftermarket parts. O'reileys auto parts is the king of out of spec break parts around here.
 
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What's the condition of your backing plates, specifically the raised flat areas the shoes ride against? If they are worn with a ridge in them, or worn through, they can hold the shoes against the drums.
 
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