373 gear ratio

[70orangeboy]

just wondering if anyone out there has switched their truck to the lower gear ratio. if anyone could post their opinions, results, etc. I would love to learn about it.

[boraxman]

I was thinking of going w/ 373 but did 342 instead. Glad I did. Unless you know what youre doing have it done.

[1969 GMC]

my GMC originally had 4.10's fore and aft, but the p/o replaced them with 3.73's. the truck pulls pretty good, but my engine ('74 350) isn't that powerful.

[chevytruckfan]

Make sure if you change the rear gear ratio that you also change the driven gear for the trans and speedo otherwise your speedo and odometer will be going faster than you actually are. Also I have a 3:73 in mine and am running higer rpms on the highway. If you drive in town alot then they are ok if you put on alot of higway miles go with the lower ratio. When I am on the highway I am wishing I have overdrive.

[skokie]

Originally Posted by chevytruckfan

Make sure if you change the rear gear ratio that you also change the driven gear for the trans and speedo otherwise your speedo and odometer will be going faster than you actually are. Also I have a 3:73 in mine and am running higer rpms on the highway. If you drive in town alot then they are ok if you put on alot of higway miles go with the lower ratio. When I am on the highway I am wishing I have overdrive.

I changed min out from a 3.73 to a 3.07 and the tranny gears are next. Gas mileage went up as well. Glad I swapped it out.

[1968GMC]

skokie, how much rpm did tou looze at 60 mph? I run about 3000 rpm at 60 with 373 gears. If I could looze 500 rpm by going with 308, I would do that instead of switching to a 700r4. I run a 350/350 combo.

[skokie]

1968GMC
I dont remmeber exactly but I think it was from 2100-2400. If it were me and I had the $$$$ I would get my 3.73 back and put in a 700r4. But since I dont have the $$ the 308 will do for now. Also this is a handly little tool for the rpms.
http://www.angelfire.com/fl/procrastination/rear.html

[Tx Firefighter]

Your RPM will decrease 18% for any given speed when you swap the 3.73 for a 3.08.

I swapped my 373 out for a 308. My truck gets 5 miles per gallon better now, AND, I can keep up with interstate traffic now.

[PICKMUP]

I did that same swap in a '78 Suburban 4X4 1/2T and went from 12 to 13 mpg. I can't imagine picking up much more in trucks as heavy as ours.

[KEH]

I have my differential apart and determined I have 3.73 gears. What would I need to do to switch to 3.07? Just a new ring gear and pinion? Thanks for the help.

[dubie]

I am getting about 14 MPG with my 350/3 speed/ 3:73 combo. At 60 mph I am hovering around 3000 rpm. The truck isn't a highway cruiser so the 3:73 works great for me. I will find out i June how it does on the highway when we travel 1 hour and 20 minutes to the neighbouring town for their car show. Gonna be white knuckled all the way there and back

[KEH]

I put on a lot of highway miles, that is why I want to go to the 3.07's. I hope it is an easy swap of gears? Can someone tell me what to do?

[GreyHoundSteve]

I have 4:10's in mine and am thinking i want to swap. Im in TX so almost anywhere i go im on the highway so im thinking a lower geared rear would suit me better. I have also had my 6 lug drums rebuilt and the 4:10's are brand new too. If anyone has a 5lug disc rear with lower gears pm me if your interested in a trade and some $.

[JimKshortstep4x4]

KEH,

Unless you are experienced at setting up rear end gears I would suggest either having it done by someone in the business or swap out the whole rear end. (When going from a 3:73 to a 3:07, a different carrier housing is needed).

Most of our trucks have the 3:07 in them but my short stepside has a 3:73 in it. Driving around town is fun with the 400 SBC in it but I don't take any long trips. The revs work on your nerves at highway speeds.

Jim

[KEH]

Jim, that is the same advice I received from Tx Firefighter. The high rpm's on the highway is driving me crazy, but I will have to live with it until I can afford to have the swap done by a qualified shop.

Thanks for the help.

[toddtheodd]

I guess I'm the only one in here. I'm actually planning on swapping my 3.08's for 3.73's. I could care less about gas mileage. As soon as I get a posi I'm putting her it together and 's till the tires pop.

[Ogier]

Hehe, I would love to get up to 4.11's but I will have to change my rear axle to do that. Right now, I have 5.33's I think.

Ugh!

[Guys 71]

i put 3:73's in my truck. ill like them a lot. i do need a little more power out of the motor but that is soon to come.

[Tx Firefighter]

My 70 shortbed was 3.73 and with the big block, there was no need for that much gear, so I did the swap. It ran as well with the 3.08 gears as it did before with the 3.73. Plus, the mileage went way up.

Todd, I think you may be a little dissapointed after the 3.73 newness wears off. On the highway at 70, that engine will be singing full song. It's always been too much for me.

[toddtheodd]

My chevelle has 3.73's in it. Never really had a problem with 'em. (It was the 2 core radiator with no fan shroud I had a problem with. Cracked the block. ) I used to run that b*tch at 75 all the time. That motor would scream like a banshee at a 100.

[Russell]

My truck has 3.73s and she pulls about 3 grand going about 65, 32" tall tire though. I plan to go with an electric overdrive, and run with a Posi 3.73 rear end

[69_hafton]

I've got the Eaton 4:11 posi but after swapping out the 3sp w/granny, for a a833 3sp w/od, I'm going to see what rear end gearing to go with. I was thinking of doing the 3.73 but it sounds like the 3.07 will be the one to go with, for highway use. I've got 31 x 10.50 tires right now, and that helps with the low gearing, but I'd like to change those later on to a normal size for a 2wd truck. I plan on keeping the posi unit in place, and just switching out the gears, or may let a shop do it and all the bearings when I get to that point - depending on free time. I think the 292 can handle the 3.07 gearing ok? If not, the engine will be my next project.

[Longhorn Man]

Here you go...
3.07 rear gears (3.08 was a car gear, not a truck gear), 28 inch tall tires (275/60 R 15), a non lock up TH350. Engine size and out put don't matter...



Low end was pretty pathetic though. It took quite a bit of work to do this;


While it is true, spinning the engine at a higher RPM on the freeway will shorten it's life, I don't feel it is all that much. Spinning 3000 RPM on the freeway isn't going to hurt a motor in good shape, and if you build it to do so, it'll spin 3000 and get decent MPG.
The 350 that I recently killed (overheated) was in my old 4.10 geared longhorn. That thing would cruise at 3200 RPM @ 70 MPH. i got a consistant 14 - 16 MPG on the open road.
Installed that engine and tranny into the 3.07 geared GMC, and MPG dropped to 10 - 12 MPG. Nothing chainged axcept the gears for the speedo. Install a cam that is designed to run at that RPM, and you'll have the best of both worlds IMO.
You won't be able to get 20+ MPG like a new truck should (with any luck they will anyways) but if you wanted 20+ mpg, you need a toyota truck(ish) to ride in.

[71-longbed]

i just went from 3.07 to 3.42 ... i haven't figured my highway mileage yet .. but my around town mpg's stay'd roughly the same .. just need to fix my speedo now .... according to calculators online ... with my 3.42 ratio , 3,000 rpm is at 72 mph ... becuase my speedos off .. the tach only sits at 2,500 rpm at 70 mph...
if i were you , i'd think about where ya drive your truck the most (town or highway) and build it according to that ... or go with a 700r4 and 3.73 .. best of both worlds

[chickenwing]

If you are hauling loads or trailering, I understand the 3:73, 700R4 setup to be the best thing going. If you get heavy or steep drop down to 3rd and it's like having the turbo 350 again. Goin long in your rear end you lose some haulin capacity. Wish I could quote gas mileage, but my speedo doesn't work.