I am so confused. Please help.

[chadvicki]

I have the following: 72 C20 350/350 and QuadraJet Carb. I have a 14 bolt rear. 3 speed automatic.

On the freeway I am getting 3000 RPM's at 65mph. So my gas goes very quickly.

I called 4WheelsParts in Redondo Beach Ca. and asked if I could change the gearing to get better mileage. I suggested 3.08. He said no that would make you soooo slow off the line. I have done many searches here for other peoples mileage reports and setups and they have many of the same things I do except for the gearing and get better mileage. The motorand trans and carb are all new as far as mileage goes but not as far as time goes. It sat for a while during the rebuild.

How do I get my RPM's down for freeway driving and still not lose performance? I guess I have 4.11 gearing and he said that is good and to keep with that.

BTW I have 16" 265/70 R16 tires that are old and hard. Do I need to go up or down in tire size and rim size? I am getting new tires and wheels this weekend.

Please help. The mileage is killing me. **I know not to expect Toyota Prius MPG's.

Although it is kinda the polar opposites when you are next to one. One gets 50mpg and one gets 8mpg----Wonder which is which

Anyway thank you for your help, It is greatly appreciated.

Chad

[lilred67]

are you using the truck to pull or just ride around?????i would put a 3.42 gear in it and get better milage and still have some performance......imo

[qksilver]

Swap in a 200r, you would have overdrive and it would be a lot easier to swap than a 700r

[Aarons72s]

I had 3:08 in my orange and white, even though it was a half ton. I loved them. I got 3:73 in my green truck and I hate them. My orange and white was all stock and I even towed a parts truck home. Felt good to me. Didnt seem to lag as much as most people say. If you arent out for a all out race truck and mileage is one of your concerns then go for it.

[Sweet72]

Go with the 3.08:1 gears if you're that concerned with fuel economy. I have 3.07's in mine and it was a dog "off the line" with the little 307ci motor, but I hope things change a little with the 383. You could install a 700R4 OD trans and get a couple MPG too.

[Rick Mills]

Here's just a couple of thoughts. I'm running a 3:42 gear in my '90 4X4 and it get's 16 mpg on the open highway, even with a 4" lift kit and semi tall tires. My '70 BB is getting 12-14 mpg depending on the size of my foot. It has 3:50 rear gears and P-265-50-15 tires.
If you can fit 20" wheels & tires on your ride it would be like droping your rear gears to around 3:73 or less.
If you go with smaller tires, you may want something around 3:50. It gives fair off line power and low RPM's at 60-65.

[chadvicki]

Ok I went here http://www.bowtieoverdrives.com/cata...SUBCAT&CATID=C

Which Stage is recommended and what do you think a transmission place would charge to install it if I brought it to him? Is this all I would have to do? or gears also?

So for now I should look for:

1. gearing
2. tire and wheel size
3. tranny w/ od

Would this be the order to go? Which tire and wheels also please. I do not ever haul anything heavy in the bed nor do I tow anything. The most that goes in my bed is about 150#'s

Thank you Thank you Thank you to you guys for helping.

Christmas vacation is around the corner and I want to get this stuff done if I am going to do it, over the vacation before going back to work.

Chad

[chadvicki]

..

[skchevota]

If you go with a taller tire you would help your rpm problum, a 285/75/16 is 33" tall and about 10.5" wide, tis would drop you to 2700-2800 rpm, other wise a gear swap is in order unless you can aford an over drive trans. I would start with the tires if yours are dryrotted any way unless you plan to lower it or somthing. You got to remember that a 350 with a 4 barral in a 3/4 ton truck is never going to get great milage. with over drive and you current setup I would gess that you still wouldnt get mor than 12-14 mpg. then you have to figuer in the cost. If you are happy with your truck the way it is and didnot plan a trany swap that will cost you $700-1500. then just drive it for what it is, $1500 will buy you a lot of gas and with just a 4-6 mpg gain it would take some time to make up that differance before you start to save money. Now if you are doing it to replace worn parts or to restore you truck and you plan on spending cash on it any ways then by all means go with the parts that will give you the most return. Just my 2 cents

BTW 4 wheel parts is used to dealing with lifted trucks and big tires. In there world 4.10s are mild and 4.56 on up is normal. with your current tire size 3.08-3.42 would be fine, but what are you futer plans for your truck, taller,shorter tires, ect.. Its better to follow a plan and do it once than fly by the seat of you pants and have to change it again later.

[ghetto_superman]

just my .02:
the best of all worlds IMO would be some 3.08 or 3.42 rear gears with a 700r4 or a 2004r. those trannies have a lower first gear than any of the others, so you would still feel decent torque off the line- plus they have a higher final gear than any of the others. overall, a better range of gearing compared to a TH350 or 400. another option would be to bolt a gervendors overdrive unit on to the tailshaft of your existing tranny.. but not many folks are made of that kind of cash.. good luck!

[kwmech]

You have a few options. You can do a gear swap, with the 14 bolt you can even go to a 3.21. You can also install an overdrive, one being a gear vendor. 22 percent over drive---very strong unit---- will thoereticallly have 6 forward gears. This will drop you to about 2300 at your present speed. In the end it depends on the check book. Or you can go to an overdrive transmission. Personally I wouldn't reommend this in a 3/4 ton truck

[e rock]

#1) I would not ever take the advice of any employee of 4WheelRetards. Let alone let them install gears.

#2) The perceived overdrive effect offered by larger tires is more than offset by the additional rotational mass (i.e. sluggish off the line), rolling resistance, etc.

#3) I'd just hunt around junkyards or Craigslist for an appropriate 14-bolt that would be a bolt-in swap. To most people, your 4.11 14-bolt is worth way more than their 3.07, so at worst you should be able to break even (A C20 is leaf-sprung on the rear, right? I'm still learning...).

#4) An overdrive tranny would help with freeway revs, but as said above if you haul much, I wouldn't take the chance on reliability. Aside from reliability, the cost of a swap usually results in a net financial loss unless you put a LOT of miles on your truck.

HTH,
Eric

[sleepyboy]

I went from 3.73 to 3.08 gears with a 350 Edelbrock crate motor and Muncie 4-speed tranny. At 70 mph, rpm went from 3k to 2k. I am very happy I did it. I still haul a yard of dirt at a time on a trailer with no issues. No, it is NOT a 1/4 miler truck, but my life is highway miles anyway. It was worth the swap to me.

[ProStreet68SB]

IMHO A good gear splitter would be good: i.e. gear vendor. it will give you more off the line acceleration than you have and will drop the RPMs for highway cruising so you get better mpg.


PS68

[67ChevyRedneck]

A 3.08 rear end with a 3 speed transmission (non overdrive) will get less, but faily similar fuel mileage to a 700R4 with a 3.73.

These trucks typically came with a 3.08 rear if they were an automatic. Newer trucks have OD and to compensate for the "weaker" 4th gear (and to keep the RPM's up) they have steeper gears in the rear.

Yeah, you'll lose some acceleration, but you drop rpm's a good bit and improve fuel economy.

[panhandler62]

:tj:


Somewhat related.... I have the imfamous 3 + granny trans. I never use first gear since the truck will float the valves before I can pass a kid on a big wheel in first.

Could I just go to a lower ratio rear end gear and make a "real" 4 speed out of my stump puller?

[Texasdeere]

Originally Posted by chadvicki

I suggested 3.08. He said no that would make you soooo slow off the line.

That is total horse poopy. A 3.08 is not that bad unless you like drag racing. Keep the 3.73 and do a 700R4 conversion or put in some 3.08 gears. You will need a different carrier for those new gears.

Just my opinion.

[Gray Ghost]

The rear gear swap is a great idea. I have 3.08's on my C10 and it gets 14mpg when my foot is light.

On trannys....the 200 series should never enter your head, a 700 might be ok if you can find a good one and don't do too much towing. Both are very expensive to rebuild and neither are built for much HP or torque. That said, the 700 would do the trick and save you some gas paired with the 3.08's.

[piecesparts]

I am here to tell you that the 3.08 gears and a change to a OD tranny (700 R4 or 200 R4) is not the wisest thing that you could do. There is a breaking point for a GM V-8 motor as to the efficiency. That point appears to be around 2000 to 2100 RPM. I had 3.42 gears in my truck and added a 700 R4 tranny to it and the mileage worked out to about 15 MPG on the highway (this is with a stroked 383 motor). I put in 3.73 gears to get better acceleration and to manage the larger cam in the 383 better and I lost about 1 MPG total. I have no complaints there.

However; on my Son's truck we had a 350/350C combo and switched it to a 700 R4 and lost MPGs. He had the 3.08 gears. The truck was running down the freeway at 1900 RPM at 70 MPH and this created a condition og lugging the motor when approaching hills and the cruise control was overworking itself constantly. We changed his rear end gears to 3.73 and picked up 2.5 to 3 MPG by getting the RPMs up to about 2400 to 2500 at 70 to 75 MPH and gained responsiveness as an extra. Both of our trucks are SWB mid eighties trucks, but that would include about the same weight and size as yours. There are computations that you can use to get a plan for which size tires and tranny combo you would want to get to the right RPMs at whatever speed you drive the most. That is what I used to get the trucks wwe have figured out.

[mrein3]

Contrary to what many above have posted, you can tow all day long with a 700r4, just don't use overdrive. 3rd gear in a 700 is the same 3rd and final gear in a th350. If you are running a tranny cooler and watching your temp you can tow just as much as a 700r4 in 3rd as you can in a th350.

Overdrive on a 700r4 is .7 to 1. In other words you have a 30% drop off in your rpms when on the freeway in OD. (Not exactly but close enough for state work).

ALL factory rear end gears are 10% different then the next size. To go from a 4.11 to a 3.73 rear you will have 10% less rpms at the same speed as the guy with the 4.11. 4.56 to 4.11 is 10%. 3.08 to 3.43, 10%. 3.43 to 3.73, 10%.

Change two gear sizes is 20% (not exactly but again close enough for state work). Three sizes is close enough to 30% to just call it 30%.

SOOOOOoooo...
If you have 4.11s now and switch to 3.08, you achieve effectively the same change in highway rpms as getting a 700r4.

Switching from 28 inch tall tires to 33 inch tall tires will decrease your highway rpms approximately 20%.

What do you want to do with the truck? All this stuff costs money. If you drop $1000 on a new tranny, how much gas can you buy for $1000? How long does it take to "get back" your investment? I've driven 2000 miles in 4 years in my truck. 200 gallons over 4 years is real hard to justify a 700r4 just to save money.

A Prius that gets 50mpg? I've don't know anybody who gets over 35mpg in their Prius'. Or are two Prius' referred to as Prii?

[chadvicki]

Originally Posted by mrein3

Contrary to what many above have posted, you can tow all day long with a 700r4, just don't use overdrive. 3rd gear in a 700 is the same 3rd and final gear in a th350. If you are running a tranny cooler and watching your temp you can tow just as much as a 700r4 in 3rd as you can in a th350.

Overdrive on a 700r4 is .7 to 1. In other words you have a 30% drop off in your rpms when on the freeway in OD. (Not exactly but close enough for state work).

ALL factory rear end gears are 10% different then the next size. To go from a 4.11 to a 3.73 rear you will have 10% less rpms at the same speed as the guy with the 4.11. 4.56 to 4.11 is 10%. 3.08 to 3.43, 10%. 3.43 to 3.73, 10%.

Change two gear sizes is 20% (not exactly but again close enough for state work). Three sizes is close enough to 30% to just call it 30%.

SOOOOOoooo...
If you have 4.11s now and switch to 3.08, you achieve effectively the same change in highway rpms as getting a 700r4.

Switching from 28 inch tall tires to 33 inch tall tires will decrease your highway rpms approximately 20%.

What do you want to do with the truck? All this stuff costs money. If you drop $1000 on a new tranny, how much gas can you buy for $1000? How long does it take to "get back" your investment? I've driven 2000 miles in 4 years in my truck. 200 gallons over 4 years is real hard to justify a 700r4 just to save money.

A Prius that gets 50mpg? I've don't know anybody who gets over 35mpg in their Prius'. Or are two Prius' referred to as Prii?

Thanks everyone for your comments and help.

mrein3

I use this truck as a daily driver. I haul nothing and never tow. I drive about 100 miles a day sometimes less. All freeway. Right now I am getting about 11mpg and I have a 20 gallon tank. Is the 700r4 on Ebay? It looks like it is.

I just cant do it myself is the prob. I would like to get 15mpg if possible. I have new tires and wheels now. 265/75-16 radials. If I change the gearing to a 3.73 do I need to get a bigger housing?

Maybe it just needs to be left alone. I just hate getting only 150 miles to a tank of gas. (approx 150 miles)

Chad

[piecesparts]

If you have a 4.11 gear ratio, then you should be able to step down to a 3.73 or 3.42 without changing axle housings, you would have to look at the carrier assembly as to what will bolt to it. Talk to Randy's Ring and Pinion and see what they say.

My calculations agree with your combination and ratio---Do you know what rear end you have 10 bolt or 12 bolt?

[Boog]

Well since the engine and trans are both new you won't be wanting to replace either of them. The majority of your driving is freeway and you most likely won't be drag racing this heavy truck. A gear change can help you to an extent but that is a heavy, long wheelbase truck so don't expect great returns from the gear change. A couple of things you need to do is make sure the engine is tuned up good. New plugs, wires, cap, etc. Plenty of timing and advance working. Carb jetted right for the best fuel mixture, free flowing exhaust and check that fan clutch. A faulty fan clutch that engages too often can suck the fuel mileage right out of it. I have read that electric cooling fans are worth about 1 mpg. Proper inflated tires and a good front end alignment help too. Tailgate up or down? Myth busters found it made almost no difference in mpg, same with or without bed covers.

[chadvicki]

I have the 14 bolt.

I was thinking of getting it tuned up since i really haven't done that yet. Does it make a difference as to the brand of plugs, size and brand of wires, etc.....????

My exhaust is dual Flowmaster with crossover. However the size seems to be small. It looks like 1 3/4" coming out to the dual chrome tips pointed down which are small in diameter. Would a 2 1/2" or larger all the way back do anything for it?

Chad

[al's71gmc]

The bottom line IMO is your're looking for mpg's in a heavy 3/4 ton truck...You're not going to find them. 11 or 12 is about it. Mines a dd, but I only go about 25 miles a day. Tire size, trans, rear gears.... you'll pickup a few mpg's, but You're not likely to exceed 15. And for the cost? If I had to drive 100 a day, I'd be looking for a car. Good luck!
Almost forgot- Slowing from 75 to 60 increased my mileage from 8 to 11. That's not hard if you're driving 10 miles, but I'd have a real hard time going 60mph for 100 miles.