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Discussion Starter #1
I'm considering a 700r4 with a 3.23 rear end gear ratio in my '78 standard El Camino with 350v8, 4bbl. I think this combination will make a noticeable difference from my stock TH350 and 2.41 rear end.

Is anyone running a 700r4 with 3.23 rear end? With a 25.5 inch tire, how many RMP will that combination produce at 70mph? (i can't find a calculator online that will marry transmission gearing with rear end gearing)

If you're running that combo and do not like it, what would you do differently?

Thank you
John
'78 El Camino
 

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Hi John.
I did the formula for a 700r4 with a 4.11 gears and it will run 2600 at 70mph. I will find the formula and post it.
 

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I'm considering a 700r4 with a 3.23 rear end gear ratio in my '78 standard El Camino with 350v8, 4bbl. I think this combination will make a noticeable difference from my stock TH350 and 2.41 rear end.

Is anyone running a 700r4 with 3.23 rear end? With a 25.5 inch tire, how many RMP will that combination produce at 70mph? (i can't find a calculator online that will marry transmission gearing with rear end gearing)

If you're running that combo and do not like it, what would you do differently?

Thank you
John
'78 El Camino
I have a 4L60E which has the same gear ratios of a 700R4 and a 3.55 rear. My tires are 27.5 and it's running around 2350-2400 ish at 70. You will probably be around the same with a bit shorter tire. My 5.3 is really happy around that RPM on the hwy. Rear end gears are always a compromise. I had 3.08 gears before and I did like seeing 2000 Rpms at 70, just seemed kinda cool to be turning that slow but the motor didn't really like it. The best part is you will really feel the difference accelerating!

Joe
 

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Discussion Starter #5
Thank you for all these great replies! Yes, I really want some more torque off the start, but still have low rpm at highway speeds, which is why I’m thinking 700r4 with 3.23 rear end. Thank you for the calculator any4xx!
 

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Anyone know how to calculate the ideal cruising RPM? Is it 2,100 rpm? 2,300 rpm? 87ls5.3 (above) mentions that his elky didn't really like cruising 70 mph at 2,000 rpm? How can we learn, or calculate, the ideal cruising RPM? (for a stock 350v8, 4bbl)
 

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Anyone know how to calculate the ideal cruising RPM? Is it 2,100 rpm? 2,300 rpm? 87ls5.3 (above) mentions that his elky didn't really like cruising 70 mph at 2,000 rpm? How can we learn, or calculate, the ideal cruising RPM? (for a stock 350v8, 4bbl)
Only 100% sure way is to drive it and let it tell you where it’s happy. Just pick a high gear and a target RPM and cruise for a while. I just went through this when selecting a gear set for my car. The engine builder told me that 2,000 RPM would be the lowest he’d recommend I cruise at. After playing around for a while I found that the car was just a bit happier at 2,100 RPM and didn’t like 1,900 at all when under a load. I wouldn’t have thought such a small difference would have mattered so much, but it did.

Carburetor, weight, cam timing, compression ratio, torque converter, etc. all play a part in finding that sweet spot. Once they’re all together They’ll let you know where they’re the happiest.
 

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heres another
GTS GearCalc

x2
just cause your cruising at 1500-2000 rpm don't mean the motor is happy,,
probly get better mileage at 2500 rpm.. every motor is different..
I have no problem cruising a motor at 2500rpm,,
or more I don't spend hours on the freeway usually..

3.73 with a tall tire is the lowest numerally i'll go with a .70 OD..
that's still a 2.61 final drive ratio, and 2200 at 70 mph..
 

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I just updated the a 350 on a 700r4 with a 3.66 so Im running about 2300 rpm highway. I do like it but I think I will go down to a 3.23 are maybe 3.31
I am running stock size tires

 

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350 crate, 700r4 3.23 17" wheels (not sure overall dia)
2600 at 80mph on I-95 (with the a/c)
if you want street performance part time and highway cruising, get a lockout switch.... luv mine
 

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I left my rear stock because I like the gas mpg and I do not tow anything. I installed the L98 with the 700R4 from a 89 IROC Z-28 in my 78. It had a 305 2bbl in it originally
 

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I have 4L60E behind my LT1 engine (300HP 350) 3.42 rear axle and 28" tires - about 2300rpm on the highway. You'll be fine with the 3.23 gear, since the 700R4 has such a steep first gear.
 

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i use the Gear Ratio Calculator calculator when trying to figure out balance. i think you may want to go with 3.55 or 3.73 depending on what highway speeds are like where you live. i feel like with 3.23 gears it would likely put you too low rpm wise and it would make the trans jump in and out of overdrive more than normal.
 

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I'd keep the 2:41 cog and get a TH-400 and have 2.75 first gear set installed. Then bolt a Hurst Autostick I shifter. Nothing like it!! They bolt to the tranny and have direct linkage like a stick. I manually shift mine all the time, love it!!



130505
 

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I have a 65 and when I bought it, it had the peg leg stock 3.36's and a 700r4. With those gears it was around 2100 but I am running 215x65x15. Believe they are 26" tall. Low enough I went to 3.73's. Now it's about 2400 at 70. I had a 97 Mustang with the same .070 last gear and liked it. So felt this would be the way to go. Really like it with the El Co. If you do the math it is like having a 4.56 gear in the first 3 gears but still like a 3.08 or less in OD. Just hauls. I think you will be underwhelmed going with a 3.23. MPG is really not much different. Maybe 1 mpg.

I'm considering a 700r4 with a 3.23 rear end gear ratio in my '78 standard El Camino with 350v8, 4bbl. I think this combination will make a noticeable difference from my stock TH350 and 2.41 rear end.

Is anyone running a 700r4 with 3.23 rear end? With a 25.5 inch tire, how many RMP will that combination produce at 70mph? (i can't find a calculator online that will marry transmission gearing with rear end gearing)

If you're running that combo and do not like it, what would you do differently?

Thank you
John
'78 El Camino
 

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Discussion Starter #20
i use the Gear Ratio Calculator calculator when trying to figure out balance. i think you may want to go with 3.55 or 3.73 depending on what highway speeds are like where you live. i feel like with 3.23 gears it would likely put you too low rpm wise and it would make the trans jump in and out of overdrive more than normal.
thank you crazywolfie! you raise great point. Is there a way to adjust (or even calculate) the shift points? You're right - i don't want it shifting in and out of overdrive too often. do you know how i would calculate these shift points? i'm not even sure if the shift points can be adjusted, such that the transmission conversion work would include readjusting the shift points to avoid the important issue you raise. Any ideas?
 
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