gear ratio / rpm question

[slugish]

Hi fellow trucksters. Ihave a 350 turbo 3 speed trans. with a 3.08 rear end. Ioriginally put a 3.73 in it but wasnt too good for cruising. I took it back to my trans. guy so he could change to a 3.23 (sort of split the difference )he said 3.23 ring &pinion wouldnt work so he put 3.08 in it . Of course I LOST a lot on the bottom end. Ican cruise 60-65 mph now at about 2000-2200 rpms. The best of both worlds would be a 700 4r trans. with 3.73 back in 9WHICH i still have) Money is the problem here. Can anyone tell me if a 3.42 would work & what rpm would it be turning at 60 -65 mph. Ithink there is a web site with a chart somewherethat would tell me. Igo to the strip sometime as well . any help appreciated.

[Texasdeere]

Here is a link, just punch in your parameters.

http://www.xse.com/leres/bin/gearrat...mference=91.11

[slugish]

Thanks for the reply. you are the only respose I got. Imust be a dummy . I dont know how to use the chart on the website you gave me . would appreciate help on how to use this .

[N2TRUX]

You don't say what size tires your running. That might be part of the problem too. I have had several trucks with 3.73 gears and loved them, but they all had OD transmissions. The 3.42 would be a good compromise if you kept the TH350.

If I were you I would try the 700r4 before I change gears. I put one in my 77 Cheyenne thinking I would go with 3.42's or 3.73's. It turns out that the 3.08's don't feel bad at all. Of course that might be because I put a Ramjet 350 in at the same time.

[Texasdeere]

You just need to change the axle ratio and your tire diameter at the botom and hit "calculate. 4th gear is represented by a 1.00:1 ratio which is the same as 3rd gear in a TH350 or TH400 trans. Overdrive in a TH700R4 is something like a 0.70:1 ratio if I remeber correctly.

[Texasdeere]

Here is some more info for ya:

The standard gear is: First Gear - 3.06:1, Second Gear - 1.63:1, Third Gear 1:1, Fourth Gear (Overdrive) - 0.70:1, Reverse - 2.29:1. ( A 4.11 rear end ratio effectively becomes 2.87 in Overdrive)

[hiway]

There is about a 400 rpm difference between each gear step for the truck rears. 3:08 to 3:42 will add about 400 rpm to your 60mph rpm's. 3:73 will add another 400 rpm over the 3:42's.

[piecesparts]

If you had a 700R4 with a 3.42 rear end, like my 91 Silverado has, you would end up with RPMs depending on your tire size. I just played this game with tire sizes and the calculation numbers below came out real close to my actual RPMs. The output trans ratio on my engine calculated out to be at 0.7426, due t o the overdrive, where a 350 would be at a ration of 1.0.

60 MPH 28" tire= 1824 RPM 27" tire= 1896 RPM

65 MPH 28" tire= 1977 RPM 27" tire= 2054 RPM

70 MPH 28" tire= 2128 RPM 27" tire= 2212 RPM

75 MPH 28" tire= 2281 RPM 27" tire= 2370 RPM

I run a 700 R4 tranny in my 84 GMC Sport truck with a 3.73 gearing and 27" tires ---I get 2500 RPMs at 70 MPH and 2700 RPMs around 75 MPH. This works well for my truck because it allows me to drive comfortably and accelerate strong. This also allows my cruise control to operate and not have to overwork itself in any hilss or grades. My Son's truck has the same setup and it works well in his truck also. He had a 3.08 gearing with the 700R4 conversion and it sucked gas hard, because the motor was always working to catch up to the load.

[Texasdeere]

Here is a link to convert tire sizes to overall diameter:

examples: 255/70/15 = 29" 275/60/15 = 28"

http://www.discounttire.com/dtcs/infoTireMath.dos

[slugish]

Originally Posted by N2TRUX

You don't say what size tires your running. That might be part of the problem too. I have had several trucks with 3.73 gears and loved them, but they all had OD transmissions. The 3.42 would be a good compromise if you kept the TH350.

If I were you I would try the 700r4 before I change gears. I put one in my 77 Cheyenne thinking I would go with 3.42's or 3.73's. It turns out that the 3.08's don't feel bad at all. Of course that might be because I put a Ramjet 350 in at the same time.

running 275/50/17 approx. 28in. Yes I would prefer a 7004r, but I spent right much to have 350 built opprox 7000-8000 miles ago 7it still performs good. It would cost me approx. 2400.00 to take the 350 out, have a 7004r built& have the 3.73 ring & pinionn installed. So you can see reason why Im leaning to just swapping the 3.08 for a 3.42 . Ataller tire would help with rpms while cruising but it would lift rear end up more than i like. thanks for your response

[slugish]

Originally Posted by piecesparts

If you had a 700R4 with a 3.42 rear end, like my 91 Silverado has, you would end up with RPMs depending on your tire size. I just played this game with tire sizes and the calculation numbers below came out real close to my actual RPMs. The output trans ratio on my engine calculated out to be at 0.7426, due t o the overdrive, where a 350 would be at a ration of 1.0.

60 MPH 28" tire= 1824 RPM 27" tire= 1896 RPM

65 MPH 28" tire= 1977 RPM 27" tire= 2054 RPM

70 MPH 28" tire= 2128 RPM 27" tire= 2212 RPM

75 MPH 28" tire= 2281 RPM 27" tire= 2370 RPM

I run a 700 R4 tranny in my 84 GMC Sport truck with a 3.73 gearing and 27" tires ---I get 2500 RPMs at 70 MPH and 2700 RPMs around 75 MPH. This works well for my truck because it allows me to drive comfortably and accelerate strong. This also allows my cruise control to operate and not have to overwork itself in any hilss or grades. My Son's truck has the same setup and it works well in his truck also. He had a 3.08 gearing with the 700R4 conversion and it sucked gas hard, because the motor was always working to catch up to the load.

Actually the 3.08 ratio with the 3 speed 350 is a pretty good combination . Maybe I should leave well enough alone. but I like to go to the strip ocasinaly with my son & Ican only get 15 seconds with the 3.08 7 Ive got a pretty warmed up 400 in it . Thanks for your response

[piecesparts]

Originally Posted by slugish

Actually the 3.08 ratio with the 3 speed 350 is a pretty good combination . Maybe I should leave well enough alone. but I like to go to the strip ocasinaly with my son & Ican only get 15 seconds with the 3.08 7 Ive got a pretty warmed up 400 in it . Thanks for your response

You are right the 3.08 and a TH350 work out just about right. However the 700 offered a lower first gear and the OD was a desire that we had. His 350C tranny was biting the dust so we put the same thing into his truck that I already had in mine and the combo of 700/3.08 was a true "gashog" in the making. We went maybe to far by going to the 3.73 rear end, but it was a step up to a posi set up and 12 bolt, which was easily obtainable. We did however, pick up 2.5 MPG by getting away from the 3.08 gears. I do have a set of 3.42 gears that I could install in his truck, but I guess I am getting lazy.

I believe that you will find that running the engine around the 2100 to 2500 RPM range will give you the best of everything.