Rearend ratio

[53burb]

I am trying to decide what gear ratio I want for the rear. I figure I am going to use either a 3.73 or 3.08. If I use the 3.73, I would eventually get a 700R4 tranny. If I get a 3.08, I would keep my 400. The difference in RPM at 65mph w/26" tall tires would be just under 400RPM. I know the advantages of the 700, lower first & second gears, better mpg w/overdrive, etc., but I just wanted opinions from both sides.

[BubbaGreen]

without doubt....go with 3.73, best damn ratio EVER!!!!

[roj2323]

pardon the dumb question if it is indeed a dumb question but why is "3.73, best damn ratio EVER!!!!" ?? i'll be running a 700r4 myself so i'm curious.

ondrew

[BubbaGreen]

Its my opinion, any one can differ. But here is my thoughts, these trucks came with either 3.08 or 3.73, if you want *any* kind of throttle response, 3.08 are going to take that all away. 3.73's will give a terrific response.
Now, if you add a 700R4, which has overdrive, you gain a lower first gear, followed by an economy overdrive, which it outstanding. This all adds up to great drivablity.
The only time I would suggest the 3.08s is if you're doing alot of highway only driving with a TH-350/400 tranny, as this would help lower engine rpms and maybe some gas mileage increase. I currently also drive a later model truck with 3.55s....and wish like H-E-*-* I would have ordered the 3.73 as this is a 4x4, which would require both axles changed. :-(
Again, I want to emphisize, IMHO...3.73 are the best all around ration to have. If you dont mind analyzing your overall plans...you may decide otherwise...ie, total offroading, you may prefer 4.11 or lower. You hold the power to chose what ever you wish.

[ChevLoRay]

I ordered a new Astro in '88. I spec'd it with 3.42 gears and posi. With the AOD transmission, I never was disappointed with the gas mileage, but I couldn't tell you what it was, now. It always had plenty of spunk, since we had 4-5 kids to haul around and I had an aluminum bass boat.

I was always under the impression that our trucks had 3.73 gears. I didn't check mine to see. When I got my '69 Dealers manual, it stated 3.08 gears were standard, and listed the option number for others. Mine has 3.08's and a TH400. After driving AOD trans for so long, I sometimes wait for it to shift again, but it won't. I wish it had a better bottom end, but that would just lead to burning more rubber....and it's hard to convince the cops that at my age (57), it was just an accident.

We had another '69, that had a TH350, in place of the TH400. It had a little more compression than mine does, and it would chirp the tires on a 1-2 shift, without any problems at all. I don't know which gears it had in it, either, but the option plate didn't show anything additional.

[67chevemall]

4 speed M-21's you want 3:08 for top speed. 3:73's are fast of the start
but once you hit the highway your done and so is your gas. Not to mention
wondering how long your moter will rev at 3500rpm before she blows up.
$0.02
Later

[Already Gone]

In my opinion 3:73's are too tall and you will get crappy highway gas mileage and the engine is reving to much for nothing. 3:08's are fine if you just want to ***** around and gas mileage is big thing for you. If mileage is your thing get 3:08's with a700R4 like I got in my 87 daily driver Burb. To me and what I'm going to eventually do to my is put posi 3:42's with a700R4 in my 396/350hp 72 long bed then you have in my humble opiion the best of both worlds. If you have megabucks you could go 3:42,s ,700R4 and one of the those Gear Vendors Under/Overdrive units that there is an article on in the Feb 2005 "Custom Classic Trucks" magazine. I think they put one of those on the Copperhead....

[wannabeburbowner]

I would stick with the TH400 and put 3.42 or 3.56 in it. Just my opinion, but with my experiences 700r4's are junk. TH400 is the best trans. ever built. I am stocking up on them for when everybody's 700r4's take a **** I got the perfect tranny to replace it with. I would go with a 2004r if you want an overdrive. I have that trans in my monte with 3.73 gears and it is great. Great acceleration, and tach about 2800 going 65 (give or take since my speedo isn't right with 275's on the back). There's my .02

[53burb]

Thanks everybody for your opinions. Right now I have a stock engine, but when I replace, it will have between 300-350HP. I figure to put in a 700R4, it would cost about $1500 total for the tranny, throttle valve setup, new yoke, etc. The 400 would probably be about $400-$600 total. Do these figures sound correct? I figure a stock rebuild on the 400 would be fine for the amount of HP the engine will have. I am not into driving fast on the highway or taking off quick in the city. The Burb will probably split duty between highway and city. So what are your opinions on spending an extra $1000 for the 700R4, is it worth it?

[Longhorn Man]

I'd say no it isn't worth it. 3.07 gears can be fairly fun in town even with a stock-ish engine. It won't be a race truck, but you can still make lots of smoke if you wanted to. Then on the freeway with 275/60R15's (28 inch tall tire) you'll only spin it about 2400 RPM....NOT all that bad in all reality.

[Huck]

Application, application, application--what are you going to use it for --primarily. What is important--yea I know we all want it all--. If you are towing--373s. Around town primarily=373. Lots of highway--308's. A lot of inbetween---342's are great. So it all depends on what you are trying to accomplish. 700R4s are as good as they are built and installed. If you get a good rebuilt one and install it correctly, they will run super. Most guys don't take the time or expense to do that and --well--they are not as forgiving as Turbo350's or 400's. My .02 and its free ~~hehehe

[69 longhorn]

mid 3's are a good all around gear. If you like a smooth ride , turn the D shaft a tad slower.....i am going with a .340 gear, on a 29" tire. with a .76 OD ratio, calc says 100 mph @ 3000 rpms thr Richmond is geared deep enough in low to still "dance" a bit tho....... 69L