373 gear ratio

[N2TRUX]

Don't change gears, get an OD trnasmission. I have found that with an OD transmission the difference between 3.08, 3.42, and 3.73 is only about 1.5 mpg but the difference in torque is huge....

[RON WOODGEARD]

Gears are one of the most important facters in what you want to do with your truck..

If you don't haul or pull and do a LOT of highway driving, then 307's are the way to go.. But you're going to suck at "off the line performance" Just remember tire size also has a lot to do with the final gearing to the street. Taller tires are going to give you a Higher ( lower numerically) Ratio, and short tires are going to do the opposite..

373's are a great performance gear for a Small Block.. 350's would be fine for a big block.. But basically, you have to tune the gears to your usage.. In Drag Racing, you want you Ideal RPM ( red line) to be just as you hit the traps ( end of the Quarter mile)

I have 4:10's in mine now, I put them in when my Truck was Raised 8" and I had 32" tires all the way around. Now I have 27" tires and it's lowered ( which the lowering doesn't have anything to do with Final Ratio) I live in a rural area, 35 miles from the closest little town, I'm sure I will be changing them out in the Future when I get it back on the road. But I like that Off the line snap, so I won't be putting in 307's.

One additional thing !!
You need to keep the motor you have in the working RPM that it's designed for.. And here's where some work can come in... figuring what RPM at what speed you'll get the best gas Mileage at.. I've always done this with a Vaccuum guage..

[67ChevyRedneck]

That's the second or third time someone has mentioned figuring mileage with a vacuum gage. I know what its for and I believe, can't remember exactly, but I think a 350 should pull between 12-16" at idle speeds. SO, what are good and bad readings going down the road??? If this really helps I might add a vacuum gage to my cluster....

[stickshift_or_walk]

Redneck,

In most carbs, the power circuit kicks in at around 7". That's when your mileage really goes south. If you can cruise above 9", you're in good shape.
And below 7", just enjoy the ride...
SSOW

[KEH]

Where should the vacuum gauge be attached; at the carb or the intake manifold?

[RON WOODGEARD]

Originally Posted by 67ChevyRedneck

That's the second or third time someone has mentioned figuring mileage with a vacuum gage. I know what its for and I believe, can't remember exactly, but I think a 350 should pull between 12-16" at idle speeds. SO, what are good and bad readings going down the road??? If this really helps I might add a vacuum gage to my cluster....

Take it at the Intake,,Manifold Port that goes to the Power Brakes. When your engine is operating at it's peak mileage it will also be at the peak Vacuum Reading.

I used to have a 64 El-camino with a 283-/308 gears/ Powerglide tranny.. I took a trip from Washington to California, I forgot the mileage that I was getting, But it wasn't too bad.. ( or so I thought) Going through southern Oregon Northern California, I got in behind a 18 wheeler that was running Fast. I pulled in behind and stayed there getting close to 100 MPH. for a long way ( No Andy, I didn't have a camera :p ) My mileage Jumped up a BUNCH ??? I had put in a Console from a 63/64 Pontiac Grand Prix.. It had the Factory Vacuum Gauge.
I took notice that the vacuum gauge was reading more vacuum ?? Then When I got back to Earthly speeds in Sacramento, Again watching the Vacuum gauge I noticed that the vacuum increased when I put the Tranny into low gear while driving in Town traffic at respective speed.

Moral of this story ? You need to find the operating Efficient RPM of your engine at what speed you intend to run it the most. One way is to install a Vacuum gauge, and going down the road a slower then normal Speed, change gears in the tranny and see where you get the highest reading and at what RPM. This is where you want it running, for where you do your major driving. Or take it out on the highway and stand on it in high gear watching the Gauge, again, when the vacuum reaches it's highest reading, (slower speed or faster) that's where you want your final rear end gearing set to at the speed your going to 'normally drive.'

There's a couple of ways to do this to set it permanently. But the most feasible is Rear end Gearing, and tire size.

PS: I traded the Camino off before I had a chance to change the gearing...

[RON WOODGEARD]

One more thing

For you guys running High gears ( 307s and the such) try dropping the tranny into a lower gear in town and see what your mileage does.

It'll be working your engine at a better efficiency instead of Lugging it..


Yea, I Know !! It doesn't set right in the head, but Do it.

[68LSS1]

My problem is I've got the 3.73's and I am planning to up it to 4.11's. And I still think 20 mpg is feasible. Gotta love 6 gears. Sorta wish 2 of them weren't overdrives but I'm sure my wallet will.

[68CST327]

I had to get on the freeway the other day, and I was spinning 4000 at 70 mph.

[70 shorty]

Hey Longhorn, 2 things. You got some balls to take a picture at 90 and also, you better get some gas soon, or you'll be walking!

[GreyHoundSteve]

To those who posted about wanting to go to 4:11's...... talk to me before you buy because i have a brand new set of 4:11's and i want to go down because i soend most of my time on the highway and dont really haul anything so if your looking to swap up to 4:11's pm me and we can work something out.

[red71cheyenne]

Can someone tell me what is meant by a new "carrier"? Do you mean an entire new axle with different pumpkin? I still don't get the whole rear end thing. My 700R4 with 3.73 still only gets about 14 on the highway and 8-10 in town. Thats with fuel injection as well. Have thought about going to 3.07, but don't know if it will make any difference. Did see a drop of about 300 rpm though at highway speed when I dropped the TH350. Jeff.

[N2TRUX]

Originally Posted by red71cheyenne

Can someone tell me what is meant by a new "carrier"?

Jeff the "carrier" is what you ring gear is bolted to. What they are reffering to is there were different series or sizes of carriers made. They were designed for higher ring gears (3.08 to 3.73) and lower ring gears (3.73 to 4.56) because of the "physical" size of the ring gear....

[red71cheyenne]

Ok, so just buying a new ring and pinion would not do it for you. You'd have to get the carrier as well right? Because 3.07 won't fit in the 3.73. Is that right?

[N2TRUX]

I'm not 100% sure. I know that there is a cross over point on the gear sets fitting the carriers. I think it's at 3.73 but won't swear my memory is correct.

I "think" you can order 3.73's for a small or large carrier, just have to know which one you have.

Can someone with a better memory help here....

[67ChevyRedneck]

I've always had gear questions too and no one has ever really been able to answer them. My truck is a '67 Chevy with a 350 mated to a 700R4 which spins a 3.73 rear. The carb is an edelbrock 650. My truck gets 15.5 mpg on the highway going about 75mph. At about 65 the tach reads 2000 and at about 75 it reads between 2300 and 2400. I have recently converted my truck to power disk brakes off a 71 GMC...and yes, GMC did put coil suspensions on trucks, which gave me awesome braking power and gave me the 5 lug pattern. Out back, she still has 6 lugs. I have access to a 5 lug 3.07 and a 5 lug 3.73. I think the 3.07 would be too high for the 700R4 but I would like to improve my mpg, her "around town" average is only around 13. The only other route would be to spend some big bucks on something in the 3.40's...is this even worth it? Is 15mpg the best these can do? I've seen a couple of magazine articles where these trucks could get 18 highway...are these people liars?? I like the way the 3.73 and the low first and second gears of the 700R4 combine to kick ass at stoplights! Should I just swap my 3.73 for another 3.73? I need to convert to a 5 lug rear anyway. Also, I DID NOT "hit" the gas at all on the tanks I was trying to figure out my mpg's.

[chickenwing]

If you plug in the numbers on one of the calculators I think you can proove this one yourself. http://www.quiknet.com/~ke6vut/rpmcalc.html I don't have your tire size or I'd plug in the #'s.

[Longhorn Man]

I say stay away from 3.07 gears with overdrive trannys. it'll be too much of a load on the engine.
As far as taking pics at 90 MPH, do you really drive with 2 hands on the wheel? I never do. So that leaves a hand free to do what ever. I took about 5 or 6 pics and that was the only one that came out. I started takin pics at 100 MPH. Just point and click...never looked through teh camera nor used the LCD screen. I have pics of a few of my vehicles pegged.

[67ChevyRedneck]

According to that calculator, which was cool...thanks chickenwing, I'd have about a 400 rpm drop going from a 3.73 to a 3.07...something in the 3.40's was only about a 150 rpm drop....soooo, if I want to keep some power, I guess just keep the 3.73. Doesn't seem to me that 400 rpm would be much more than an extra mile per gallon..... Also, newer Chevy trucks that have the 5.3L...is this a modern 327??... tend to come with a 3.73 gears....ALL Z-71's come with it. I think the only other gear is in the 3.40's and it's typically in the V-6's and in some regular car v-8's. Not 100% on this. So if Chevy is still goin with a 3.73 after all these years....why not keep it in the older trucks??

[67ChevyRedneck]

I meant regual "cab" V-8's, not "reguar car"

[67ChevyRedneck]

holy crap I can't spell today.....REGULAR CAB not REGULAR CAR!!!

[Tx Firefighter]

Originally Posted by 67ChevyRedneck

So if Chevy is still goin with a 3.73 after all these years....why not keep it in the older trucks??

Because all of the newer trucks have overdrive transmissions and the older ones didn't.

By the way, the 3.42 gear ratio was the most common from 88 through 98 trucks, whether with V6 or V8 engines. A 3.42 with an overdrive transmission is a pretty good combination.

[86 Red_Bowtie]

I have 3.73 gears in my truck and TH700 transmission and i am getting better gas mileage than i was with 3.08

[Super Duck]

the newer overdrives are (bigger) a 700r4 is a .62 if i am not mistaken and a 4l65e is a .68 or something close, the other thing to think of is tire size. I have 3.07s with a th350 and 255 60s out back. a taller tire will make it seem as if I had a 2.73 gear. so lots of factors

[Tx Firefighter]

I believe the OD ratio in a 700R4 is 0.70.