granny 4 speed/ O.D. conversion.
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From: waco
granny 4 speed/ O.D. conversion.
I have a 73 model F 100 that has a <a href="http://motorhaven.autoanything.com/">390 </a>with a np 4 speed. Would it be possible to put a 3 speed with overdrive out of an early 80's model truck in my truck? If so, would it stand up ok behind a 390? Last but not least, what parts would I need to make this swap? If any one has a better solution please let me know. My engine is basicly stock except for an edelbrock manifold, a <a href="http://motorhaven.autoanything.com/">Holley</a> 600 carberator, headers and dual exhaust. It is a swb truck, so it doesn't do any towing or hauling except for an occasional engine block. I'm mainly looking for a little better fuel milage without spending an arm and leg. I've already added an msd ignition module, coil, and duraspark distributor. Thank you for any advice on this matter.
Please don't cuss chevys........I've made a lot of money fixing those raggedy things.
Please don't cuss chevys........I've made a lot of money fixing those raggedy things.
Cargo Master
Joined: Aug 2000
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From: SW Michigan
granny 4 speed/ O.D. conversion.
Doc, I am familiar with the trans you're thinking of, and no, it won't live behind a 390. Not for very long at least. What size tires do you have, and what axle gears ? Would you consider changing them ? What mileage are ya getting now ? DF
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From: Copley, OH
granny 4 speed/ O.D. conversion.
If you've got an NP435 now, swap the rear gears to a 2.73. That will give you some decent highway mileage and make your granny 1st a usable gear. The only downside is that you won't have a synchronized 1st, so you can't drop into 1st in motion without double clutching.
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From: waco
granny 4 speed/ O.D. conversion.
I currently run 255/70r15 tires on 3:50 gears. I get about 10 to 12 miles per gallon depending on how young I feel.
I was already considering going to 3:08 gears but Strange ranger suggested 2:73. Which would be better? Also, would it be easier to change the rear gears or the whole rear end? Old ford's are plentiful down here so a whole rear end would be cheap and easy to come by. Thanks again guys.
I was already considering going to 3:08 gears but Strange ranger suggested 2:73. Which would be better? Also, would it be easier to change the rear gears or the whole rear end? Old ford's are plentiful down here so a whole rear end would be cheap and easy to come by. Thanks again guys.
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From: Copley, OH
granny 4 speed/ O.D. conversion.
With your current gears you're turning about 2775 RPM at 65 in 4th. 3.08s would drop that to about 2400 and 2.73s would drop it to around 2125.
The other thing you care about is the 1-2 shift. Right now 4000 in 1st is about 14 MPH and in 2nd it's about 28. I'm guessing you hardly ever use 1st. If you do the gear swap 3.08s take those numbers to 16 and 32 respectively and 2.73s take them to 18 and 36. You'll be using 1st all the time with the 2.73s and probably with the 3.08s.
BTW you've got a 9" right? For the 9" the gear ratios are 3.00 and 2.75, which are close enough to the 8.8" rarios of 3.08 and 2.73 that the numbers work out pretty much the same.
The other thing you care about is the 1-2 shift. Right now 4000 in 1st is about 14 MPH and in 2nd it's about 28. I'm guessing you hardly ever use 1st. If you do the gear swap 3.08s take those numbers to 16 and 32 respectively and 2.73s take them to 18 and 36. You'll be using 1st all the time with the 2.73s and probably with the 3.08s.
BTW you've got a 9" right? For the 9" the gear ratios are 3.00 and 2.75, which are close enough to the 8.8" rarios of 3.08 and 2.73 that the numbers work out pretty much the same.
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From: waco
granny 4 speed/ O.D. conversion.
I don't mean to sound dumb, I don't know what the difference is between a 9 inch and an 8.8. You are right, I never use first gear with the 3:50 gears. Which ratio in your oppinion would be best for my application? Do you think it would be best to change the whole rear end or just the gears? Thanks again for all your help.
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From: Copley, OH
granny 4 speed/ O.D. conversion.
The 8.8" and 9" are completely different designs. The 8.8" is in current use. It is accessed only from the rear and has to be rebuilt in place in the axleThe older 9" which is what should be in your truck is stronger and better in almost every way. It has a carrier which drops out the front of the axle and can be rebuilt on a bench and re-installed. On a 9" you don't need to change the whole axle, you can just swap out the carrier or you can just change the ring and pinion in your carrier and re-install.
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From: waco
granny 4 speed/ O.D. conversion.
I am eternaly grateful for your wisdom Strange Ranger. You have probably saved me a ton of money and countless hours of failed experiments. I have the 9" rear in my truck. All the goods go in from the front. I don't remember seeing a ford rear end where the stuff goes in the back. It must be made like a chevy rear end.
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