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Discussion Starter #1
I know everyone's set ups might be different, so I'll tell you mine.
383 sbc, 500 hp, 700r4, 3000 stall converter, 28 inch tall rear tyre.
I'm in the process of looking to replace my diff. I will be using a f#*d 9 inch diff. I'm thinking 3.70 gears would be the best choice for me. My converter will have the lock up in 4 th gear only.
For all of you that have done a similar conversion I'd like to know if a 3.70 diff gear would be ok .
Cheers.
 

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Trifive Automotive Electrical Wiring Expert
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What do you want to do with your car? Drag race, circlet track, road course, or cruise. The 700R4 has a low first gear. 3.70's will be good for about 3 feet of rubber before it shifts.
 

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I have a similar combo 383 /700r4 have 3.73 gears and 28" tall tires. Love it great on long road trips too.
 

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Mine is 383, 460 hp, 3.50 gears. Launches really hard, and does 70 mph in 4th gear at 2,000 rpm. Perfect for me.


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I'm running a 700r4 with 3.73 Ford 8.8" axle, and 29" tall pie crust slicks. Believe me it goes much further than 3ft. before shifting. Never checked the measurement, but I'd guess it shifts from 1st to 2nd at around 50' and rpm's at 70 mph are around 2100-2200 rpm.
Go any taller rear gears, and your engine wont run well at freeway speeds, as the rpm range wont be up on your cam I'll bet.
 

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Discussion Starter #7
What do you want to do with your car? Drag race, circlet track, road course, or cruise. The 700R4 has a low first gear. 3.70's will be good for about 3 feet of rubber before it shifts.
It’s a street car. Mainly for cruising. A few hard take offs that’s about it. 3.5’s might be a better option maybe. I know the 700r4 has a low first gear. That’s why I’m asking to see what other people that have all ready done it think.
Mine is 383, 460 hp, 3.50 gears. Launches really hard, and does 70 mph in 4th gear at 2,000 rpm. Perfect for me.


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my engine guy recommended 3.50 gears. I was worried it would bog down off the lights as the 3.70 would be a happy medium.
How far do you get out of first gear?
 

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I'd be more concerned with higher rear gears when it hits that 1.62 2nd gear. A 3.70 rear gear will be a better choice once it shifts to 2nd and hits the tall 2nd gear.
 

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LOL, believe me it doesn’t bog down. It launches really hard. Frankly, I’d be concerned that Winding it tighter in 1st gear, with 3.70 gears could blow the engine. Not really sure how far out it goes in 1st gear. Bottom line is it’s fast with 3.50 gears, and a lot of fun to drive.
 

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No reason to ever be concerned an engine would blow up before it shifted, unless you're manually shifting your trans, or it's out of adjustment. Yours might not bog when hitting the 1.62 second gear, but some might. Mine doesn't for sure, as it barks pretty good when it hits 2nd, even with cheater slicks, and 3.73 gears.
 

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Personally I think the 3.70 gear is the right way to go with the combination presented, and I'd even consider a 4.11.

If you're concerned about over revving the engine, that's what they make rev limiters for. They are quite common and reasonably priced. Thing is, people use much steeper gearing combinations at the track and have no problem shifting on time.
 

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No reason to ever be concerned an engine would blow up before it shifted, unless you're manually shifting your trans, or it's out of adjustment. Yours might not bog when hitting the 1.62 second gear, but some might. Mine doesn't for sure, as it barks pretty good when it hits 2nd, even with cheater slicks, and 3.73 gears.
Personally I think the 3.70 gear is the right way to go with the combination presented, and I'd even consider a 4.11.

If you're concerned about over revving the engine, that's what they make rev limiters for. They are quite common and reasonably priced. Thing is, people use much steeper gearing combinations at the track and have no problem shifting on time.

I agree completely 3.73 is a great street gear especially with a 28" tall tire. .
 

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My 3:55s make my right foot happy to push the peddle down off of a stop light, and on the highway trips the engine is happier than it was with the two speed. The 700R4 was a great change for me.
 

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My 3:55s make my right foot happy to push the peddle down off of a stop light, and on the highway trips the engine is happier than it was with the two speed. The 700R4 was a great change for me.
what size rear tire ??
 
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383 / 700R4 2400 RPM convertor / Ford 8.8 w/3.55 gears / 27" tall Nitto 555 255/45ZR18 tires. 3.55 is a good compromise for me. Anything more than half throttle in first gear results in tire smoke. That big drop between first and second is not a problem. If I were doing more highway driving at 75-80 I would probably try a 3.31 gear. If you have a motor making some decent power, a 4.10 gear would make first gear pretty useless unless you had a decent amount of tire on the rear.
 

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With a 27" tire a 3.50 rear gear is probably OK, but with taller 28"-29" tires a lower 3.73 is a better choice. Most engines aren't that efficient cruising down the highway at under 1800 rpm's, so if you have taller 28"-29" tires and cruise freeways of 55-60 mph the engine will be under optimum rpm range. Fine if your freeways are 70 mph versions, but a lot of freeways aren't.
Especially if you've got a high hp engine with a bigger camshaft. Those engines prefer a higher rpm cruise speed, and would be better served with a lower rear gear if the tire size warrants it.
 

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With a 27" tire a 3.50 rear gear is probably OK, but with taller 28"-29" tires a lower 3.73 is a better choice. Most engines aren't that efficient cruising down the highway at under 1800 rpm's, so if you have taller 28"-29" tires and cruise freeways of 55-60 mph the engine will be under optimum rpm range. Fine if your freeways are 70 mph versions, but a lot of freeways aren't.
Especially if you've got a high hp engine with a bigger camshaft. Those engines prefer a higher rpm cruise speed, and would be better served with a lower rear gear if the tire size warrants it.
I couldn't agree more.
 

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I've got a friend who has a 1968 Nova and I swapped in a T56 six speed in it. He originally had a 3.73 Ford 8.8" rear I put under it, and thought it would be a great highway cruiser. But he quickly discovered his tall tires made even 5th gear a bad choice at 55 mph. He was usually running 4th gear on the freeways, and occasionally running 5th if we hit a 70 mph freeway area. But still couldn't use 6th ever. He had the gear set swapped out to a 4.11 rear gear, which allowed him to use 5th gear at 55-70 mph, but said he needs to still go over the posted 70 mph speed on the freeway to not lug the engine in 6th gear.
He's got some pretty tall rear tires I think are 29"-30" so it really affects the engine rpm range.
 

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I have 87 GMC 4x4s 700s one with 3.42 and the other 3.73 and hands down I prefer the 3.42 . remember the first gear in 700 is 3.06. My 56 has a 57 Olds 9.3 rear with 3.42 perfect. But truthfully when I ran an early Chevelle 12 bolt 3.55 posi life was also good. I have just been blessed with more affordable options than most, I just wanted to be era correct which is not spelled Ford.
 
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