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Discussion Starter #1
I'm trying to figure out tire size, gear ratio and cruising RPM. Corral.net calculates my hwy speed for stock 255-70R16 tires in OD to be:Engine RPM: 1800
Vehicle Speed: 75.5607956090749
Read End Gear: 3:73

That of course is with OD at .71:1. that's the closest ratio they have to my .69:1 (2% Higher gear ratio) that the 47Rh puts out. So 1700 RPM puts you right at 71 MPH. I believe the sweet spot for the 4bt is 1700-1800 RPM. Is this the correct RPM band for highest efficiency on the HWY. That tires size is stock. I could go with 265 70 R 16 i believe. That would put me at 1650 RPM for 70 MPH. I will be towing a boat some and i don't want to stall it down where i have to kick out of OD and be back to a 1:1 which is not very efficient unless you don't mind driving slow.

If anyone can verify ideal RPM, let me know. I would suppose stall on the TC would need to be 1700-1800 as well.
 

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Using a better online gear calculator (there are many) http://www.richmondgear.com/101032.html 32" dia. tires, 3.73 rear, .69 OD, 75mph = 2027rpm. If you drive mostly lower rpm and/or offroad, this rpm will be fine. If your truck see's highway duty more often or is a mileage queen, you may find those rpm's a little high. Yes 1750-1800 rpm is the 4BT's sweet spot (peak torque), and should provide close to the best mileage, although there are many variables effecting mileage and some swear even lower rpm = higher mileage, regardless of lugging the engine. As far as dropping out of OD while towing, not sure you'll be towing a boat at 75mph for any distance, but youll have to decide.

There's a few threads with references to 47rh TC stall speed, most recently; http://www.4btswaps.com/forum/showthread.php?23795-wiring-a-1995-47rh
 

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If I calculated correctly your 255x70x16 tires are 30" in diameter. That would mean at 75 mph your engine is turning 2158 rpm with a .69 od. If you had 32" tires and a 3.55 gear you'd be about 1900 rpm. Depends on how much time you plan to drive at 75. I my state most interstates are 65 with a very few remote areas at 70. 65 mph would put you at 1870 rpm which is spot on. If you're going to be crusing at mostly 75 then you need a higher gear or a second od. With that size tire you'd have to pull the gear ratio to about 3.11 to get 1800 rpm. Going to a 265 tire gains 1" more in diameter. I believe you'll find your 70 mph speed will be 1952 not 1650.

The calculation is as follow: Tire diameter=31"/12"=2.583'x3.1416(pi)=8.1158'per rev 1 mile =5280'/8.1158=650 revolutions to travel 1 mile x 3.73 gear = 2425 rpm @ 60 mph(60mph=1mile/minute)x.69od=1673 rpm @ 60 mph, 1812 rpm @ 65 mph, 1952 rpm @ 70 mph, 2091 rpm @ 75 mph.
 

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Discussion Starter #4 (Edited)
Yup, i had just plugged in numbers on the websites referenced. I had not crunched the numbers because i assumed they were right. I think the max tire height can be 31" so the 265 70's would do the trick. 1952 wouldn't be too bad for 70 mph. But, of course i do not have any experience with the engine. Most all of our Interstate I 59, I 20, I 65 are all 70 mph and the 4 lane hwy's (HWY 49) are all 65 MPH. I live 4 miles from interstate and most of my travel is 20 miles one way or the other. V actions and other road trips are 70MPH. The Branson trip has a lot of 55MPH hwy in the hilly areas. So i'll go with the 31" tires. I may put 3.08's in if it doesn't work out. I've read that the 1800 RPM at 70 is too boggy on the engine and makes it cut up pretty good if you try to accelerate. Keep the advice coming.

If I calculated correctly your 255x70x16 tires are 30" in diameter. That would mean at 75 mph your engine is turning 2158 rpm with a .69 od. If you had 32" tires and a 3.55 gear you'd be about 1900 rpm. Depends on how much time you plan to drive at 75. I my state most interstates are 65 with a very few remote areas at 70. 65 mph would put you at 1870 rpm which is spot on. If you're going to be cruising at mostly 75 then you need a higher gear or a second od. With that size tire you'd have to pull the gear ratio to about 3.11 to get 1800 rpm. Going to a 265 tire gains 1" more in diameter. I believe you'll find your 70 mph speed will be 1952 not 1650.

The calculation is as follow: Tire diameter=31"/12"=2.583'x3.1416(pi)=8.1158'per rev 1 mile =5280'/8.1158=650 revolutions to travel 1 mile x 3.73 gear = 2425 rpm @ 60 mph(60mph=1mile/minute)x.69od=1673 rpm @ 60 mph, 1812 rpm @ 65 mph, 1952 rpm @ 70 mph, 2091 rpm @ 75 mph.
 

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I wouldn't think you'd have to worry about lugging a 4bt at 1800 rpm unless you've got too big a turbo that isn't spooling. The engine reaches full torque at 1600 rpm and max hp around 2300 rpm. If you're going down the highway at 1800 rpm and hit the accelerator it's gona go. If the turbo has any kind of boost at all it should really go.
 

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Try for 1700 or more RPM at 65.

I agree with char1355's math too. You're not going to be [email protected] 75 with what you have. 1800 at 75 would be a poor setup.

Fuel consumption doesn't increase dramatically cruising at a slightly higher RPM, but driveability falls off quick if you try to cruise 1600 at 70 or something some such.

The short fat tires you have are not ideal for mileage. A narrower 31-32" tire would be better suited to the 3.73 and .69 ratios, such as a 235/85R16. If you had a different transmission with a .8-.75 OD ratio or if you were talking about a 5.9 Cummins then a 33" tire would be ideal.
 

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Discussion Starter #7
Yup, i checked with a guy i usually buy tires from and he said 265- 75 R16 would be the tallest i could make work on the truck. With my current set up 73 MPH (just over the limit without getting risky) it is 2003 RPM. 65 MPH would be 1784. I would suppose that to be okay without lugging it down.

Try for 1700 or more RPM at 65.

I agree with char1355's math too. You're not going to be [email protected] 75 with what you have. 1800 at 75 would be a poor setup.

Fuel consumption doesn't increase dramatically cruising at a slightly higher RPM, but driveability falls off quick if you try to cruise 1600 at 70 or something some such.

The short fat tires you have are not ideal for mileage. A narrower 31-32" tire would be better suited to the 3.73 and .69 ratios, such as a 235/85R16. If you had a different transmission with a .8-.75 OD ratio or if you were talking about a 5.9 Cummins then a 33" tire would be ideal.
 

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You can look up tire specifications [diameter, width, tread depth, wear, etc] at www.tirerack.com Pick a tire brand and size. Then look up the tire line specifications. Some dimensions for the same size vary between different manufacturers.

The online calculators are not precise calculators but are very close. There are un-calculated factors in an automatic transmission vehicle such as converter slip which are not entered into the calculation.

The Cummins factory peak torque for a CPL 857 or 858 engine is generally around 1800 RPM. Modifications such as increased timing and fueling tend to bump the peak torque higher in RPM's. Fuel mileage generally falls flat on its face in a stock engine operating above 2000 RPM's.
 

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Discussion Starter #9
Yeah, i've read about advancing the timing. I think i'm gonna leave it bone stock and change one thing at a time so i can tell its effect.



You can look up tire specifications [diameter, width, tread depth, wear, etc] at www.tirerack.com Pick a tire brand and size. Then look up the tire line specifications. Some dimensions for the same size vary between different manufacturers.

The online calculators are not precise calculators but are very close. There are un-calculated factors in an automatic transmission vehicle such as converter slip which are not entered into the calculation.

The Cummins factory peak torque for a CPL 857 or 858 engine is generally around 1800 RPM. Modifications such as increased timing and fueling tend to bump the peak torque higher in RPM's. Fuel mileage generally falls flat on its face in a stock engine operating above 2000 RPM's.
 

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Discussion Starter #10
So, what mechanisms are being used to cause the TC to NOT lock-up or UN-lock during towing???
 

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If you're going to use pressure switches ..instead of the factory Dodge TCU.. to control OD and lockup function, then those pressure switches are the sole "mechanism" in place that control OD & LU function. They rely completely on fluid pressures to latch/unlatch ground to the transmission's OD & LU solenoids, regardless of driving condition.

Approach incline, speed slows, trans pressure drops below OD switch latching threshhold - OD solenoid disengages, your back to a 3 speed 727.
Speed continues slowing, trans fluid pressure continues dropping to LU switch latching threshhold - LU solenoid disengages, your back to a slipping converter.

Many guys also run the OD & LU ground wires through a dash switch (manual rocker/toggle/pushbutton) as well, to gain more control over their functions, but it does reduce "user-friendlyness" of the system and could be dangerous to the unsuspecting occasional "guest" driver of your vehicle who doesnt realize they need to be the great wizard of Oz at the controls, just to drive your truck down the block for a gallon of milk.

Something that hasnt been mentioned yet is, the 47rh also has a TV cable, which I believe only controls valve body fluid pressures for auto-shifting 1st through 3rd gears, and possibly kickdown function as well (ie. 3rd to 2nd or 1st for passing), but someone with deeper knowledge of the 727/46rh/47rh can confirm.
 

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Discussion Starter #12
You are correct. I do have the TV cable which controls the governor on the tranny. It changes the shift point in relation to the position of the throttle if i'm not mistaken. I pan on hooking that up as it will "ruin" the tranny if it is not in proper operation as well. I've read a lot about the use of a potentiometer to take the place of a TPS also to control some aspects of OD i believe. I've still got a lot of reading and head scratching to do. It can be a complex issue i suppose.

If you're going to use pressure switches ..instead of the factory Dodge TCU.. to control OD and lockup function, then those pressure switches are the sole "mechanism" in place that control OD & LU function. They rely completely on fluid pressures to latch/unlatch ground to the transmission's OD & LU solenoids, regardless of driving condition.

Approach incline, speed slows, trans pressure drops below OD switch latching threshhold - OD solenoid disengages, your back to a 3 speed 727.
Speed continues slowing, trans fluid pressure continues dropping to LU switch latching threshhold - LU solenoid disengages, your back to a slipping converter.

Many guys also run the OD & LU ground wires through a dash switch (manual rocker/toggle/pushbutton) as well, to gain more control over their functions, but it does reduce "user-friendlyness" of the system and could be dangerous to the unsuspecting occasional "guest" driver of your vehicle who doesnt realize they need to be the great wizard of Oz at the controls, just to drive your truck down the block for a gallon of milk.

Something that hasnt been mentioned yet is, the 47rh also has a TV cable, which I believe only controls valve body fluid pressures for auto-shifting 1st through 3rd gears, and possibly kickdown function as well (ie. 3rd to 2nd or 1st for passing), but someone with deeper knowledge of the 727/46rh/47rh can confirm.
 
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