how many rpm's will i drop going from 3.91 to 3.55????????????????

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duster360

duster360

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Been thinking about pulling the 3.91 gears and going to 3.55. About how much of an rpm drop would i see at cruise speed.
 
Your RPM at any given speed will be 3.55/3.91 lower. Sheeze.

I know math isn't most of Alabama's strong suit, but really...sheeze.

3.91 is a good backroad gear if you don't plan on going over 50ish. My rod didn't have a speedo and I just multiplied the first 2 digits of the RPM in 3rd by 2. 2500 = 50MPH or so. Worked for me. Same 50 MPH with 3.55 would be 2500 x 3.55 / 3.91 = 2270 RPM. Cam isn't even working at that RPM.
 
airwoofer said:
Your RPM at any given speed will be 3.55/3.91 lower. Sheeze.

I know math isn't most of Alabama's strong suit, but really...sheeze.

3.91 is a good backroad gear if you don't plan on going over 50ish. My rod didn't have a speedo and I just multiplied the first 2 digits of the RPM in 3rd by 2. 2500 = 50MPH or so. Worked for me. Same 50 MPH with 3.55 would be 2500 x 3.55 / 3.91 = 2270 RPM. Cam isn't even working at that RPM.
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you provided some good information, but your insult was uncalled for.
 
duster360 said:
Been thinking about pulling the 3.91 gears and going to 3.55. About how much of an rpm drop would i see at cruise speed.
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Not a bunch. What size tire are you running. If you run a taller tire it will help some.
 
Reading my reply, let me back up; airwoofer is totally out of line, and rpms will slow down.
 
duster360 said:
Been thinking about pulling the 3.91 gears and going to 3.55. About how much of an rpm drop would i see at cruise speed.
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here is a nice little calculator to check with all different gear and tire sizes..
http://www.bigblockdart.com/forum/speedtire.php


going from a 3.91 to a 3.55 will make a little difference but not a huge one.. i did it in my dart a few years ago and it took the edge off some but the car still wasn't great on the highway. all depends what your looking to do with the car.
 
abodyjoe said:
here is a nice little calculator to check with all different gear and tire sizes..
http://www.bigblockdart.com/forum/speedtire.php


going from a 3.91 to a 3.55 will make a little difference but not a huge one.. i did it in my dart a few years ago and it took the edge off some but the car still wasn't great on the highway. all depends what your looking to do with the car.
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Just a local weekend cruiser mostly. I really like he 3.91 because of the way it pulls with them. But sometimes I would like to just take a 100 mile round trip without the having to drive so slow and take forever. I have a 3.55 suregrip lined up to buy. Just want to make sure I make the right choice. Having to decide between performance and economy sucks.
 
..i'm with E booger on this one.
3.55 seem to be the gear everyone is after but are not a good highway gear or quarter mile gear.
3.23 will help you slow the revs down better and still provide good performance.
 
i used to do 2-3 hour cruises on the highway with 3.91 and 3.55 gears.. sure the 3.55 took the edge off some but it still wasn't good for the speeds i wanted to cruise.. i guess if your only doing one or two long highway cruises a year 3.55 will be fine but for me i went to a 2.94 and really liked them. in the end i still wanted OD for the highway with the 2.94 gear..lol
 
Sounds like you almost wanted it at idle while cruising. LOL. I think the fact that I have been driving nothing but overdrive vehicles for a long time has spoiled me.
 
I go 80 with my 3.91's, the little 340 loves it and gas might hit $3 this fall

I would not go with 3.55's, you will lose more then you will gain. 3.21 or yeah, 2.94's would make a better highway gear for today's fuel prices. The 3.55's will still burn up the gas but it won't have as much kick when you nail it.
 
In 1970 Motor Trend tested the 6 pack 440 with 4.10 gears RR vs the 383 4 bbl with 3.55's, they went on to say quarter mile et is close but the 440 with 4.10 gears is much safer in the 90 to 110 freeway passing speeds and they were not talking KM/H

People used to drive at high speeds with 3.91-4.10 gears all the time untill 1973 and 55 mph came about, just burns up more fuel but the engine will last..least for the 3 years most folks kept cars back then
 
To see a change worth spending the cash you need to go with the 3.23's. We just made that change and are happy with them and we are running a 27.5" tire. It really depends on your combo, most sb's make enough steam to put a smile on your face w/3.23's @ almost $4.00 gal
 
abodyjoe said:
i used to do 2-3 hour cruises on the highway with 3.91 and 3.55 gears.. sure the 3.55 took the edge off some but it still wasn't good for the speeds i wanted to cruise.. i guess if your only doing one or two long highway cruises a year 3.55 will be fine but for me i went to a 2.94 and really liked them. in the end i still wanted OD for the highway with the 2.94 gear..lol
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I did just that today. With the overdrive and 3.73 at 75 I was turning about 2500. Now with the 2.94 at 75 I'm turning 1900. This is with a 26.5 inch tire. Should be able to cruise all day now lol
 
duster360 said:
Just a local weekend cruiser mostly. I really like he 3.91 because of the way it pulls with them. But sometimes I would like to just take a 100 mile round trip without the having to drive so slow and take forever. I have a 3.55 suregrip lined up to buy. Just want to make sure I make the right choice. Having to decide between performance and economy sucks.
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I run 3.55 gears in both my cars,but I really don't like driving them long distances. Some say 3.23's are the best all around gear.
I don't know,gears are fun from light to light, but that's about it.
Do you have a 8 3/4 ?
If so get a 2.73 or 2.94 peg leg and switch them out when you want to travel.
 
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