C-20 Mileage.

[Dtech]

Hey guys. I'm still a newbie to this forum. I have a 69 LongHorn and just recently put in a fresh 350 with standard flat top pistons, 64cc chamber corvette heads and an RV cam with .465 int/exh lift. Edlebrock Performer Intake.

But whats getting me it the MPG, currently it sits around 5-7. I got a Holley 4160, 600 cfm man choke, vac secondaries (I think). I have a #64 main jets, 31 acc nozzle and the idle air, acc pump and bowls are properly adjusted. The timing is stock HEI set at 8 degrees, cam spec sheet is 7. Also the trucks got a T400 and 4.10 or 4.11 gears (I'll confirm 2mrw). The truck starts fine, idles fine and performs fine (but less than what I was expecting)

I would love to see 10 or 12 MPG out of this. So my questions are. What are other ppl getting in there C20s, should I switch to an Edlebrock with inherently smaller primaries, is this normal, or am I doing something wrong.

Maybe thats why is has 70 gallons of fuel tanks on it.

[Longhorn Man]

10 - 12 is average for these trucks. If you live in a hilly area, then 12 may be a pipe dream.
I would swap out the carb for the edelbrock and see if that helps.

[1970cstblazer]

My '72 Cheyenne 20 Super is good for 13 MPG highway at 55 mph. Mine has a 402, original Q-Jet, MSD ignition, 2.5" duals with Blowmasters, and those wonderful 4.10 gears. The power is very good, but it is pretty thirsty.

My '70 K/5 Blazer with a totally original 350/4 speed, 3.73 gears/ 31" tires would get 16 MPG all day long.

[Griffin]

I think the TH400 and the 4.10 gears are hurting you. You may need the gears if you're doing any hauling, but they're killing your mileage if you do a lot of highway driving. Also the TH400's are a very heavy transmission and produce a lot of heat. It would take a lot of work for a mild small block to spin a th400 and 4.10 gears in a 3/4 ton truck. The best soultion would be to swap for a 700r4 transmission to take advantage of the overdrive. This will only help if you do most of your driving over 50 mph. Other suggestions would be a slightly higher than stock stall speed converter in the th400, an additional trans fluid cooler (less heat= better effficency b/c less slippage), a slightly taller tire would effectively "raise" your gear ratio (4.10 to a 4.05, etc...) or look around for a used complete TPI set up and make to switch to fuel injection. There's also dozens of little things to check like, tire pressure, whether you have a stiff wheel bearing increasing your rolling resistance, etc...but most of these won't result in any noticible savings.

[MrC1]

Sounds to me like something isn't right. Power valve blown? How do the plugs look? Do you ride the brakes? (haha)

My C20 has a 700R4, so it's not a fair comparison. That said, I've never calculated MPG.

[Sinister]

Just for comparison, my C-20 gets 10 mpg. 350/TH400/4.10gears.

[Skirkpat]

4.10's and California stop and go traffic = Horrible gas mileage, I would find a better geared rear end, won't have as much in the seat acceleration, but will pay for itelf many times in fuel savings

as far as overdrives go, it would take alot of fuel savings to pay for a 700 r4 swap in my opinion, and I would look at a used gearvendor OD setup before going that route, if I were to have a Grand laying around, JMO

[Dtech]

Well I figured the gear weren't helping at all so I tried to look up a OD unit. And WOW 2800 from Gear Vendor, I'll just go out and buy a 700R4. The gears are definitely on the list to go. 3.73 posi unit is what I need. Its go a 12 bolt in it. Here are some pics.

[Longhorn Man]

that's not a 12 bolt, that is an Eaton/corperate HO52. The gear you have, is the tallest you will find.

[Sinister]

Hey LHM, did the Eatons ever come with 3.73's?

[kookykrispy]

Nice lookin longhorn you've got there. I think your truck should be getting at least 10mpg.

I get about 10-11 mpg with a bone stock 350 engine with a q-jet... that's with a 3 on the tree and 4.56 rear gears.


In your holley, do you know what size power valve you are running? It might be worth looking into. If the power valve is opening to soon, then maybe its giving the engine more gas than it needs.

Here's a good quick article on selecting the correct power valve size for your engine:

http://www.holley.com/data/TechServi...wer_valves.pdf


If you finally decide to give up on the holley and go to a different carb, I'd go for a quality rebuilt quadrajet. (or get a used one and rebuild it yourself). It is the original equipment on these trucks. If the q-jet is properly set up and tuned, it will perform great! The little secondaries on the q-jet make for economical cruising, while the big secondaries can open wide when you need the extra airflow. The secondaries are vacuum operated, so it only opens as much as the engine needs. I love my QJ.

A overdrive would be nice if you are not towing with the truck, and would definitely help your fuel economy regardless of what carb you use. The 700r4 is an option, but also don't overlook the 200r when thinking about a 4 speed auto for your truck. The nice thing about the 200r has a little higher gear ratios than the 700r. The ratios of a 200r: 2.74 1.57 1.00 with a 0.67 OD. whereas a 700r4 has 3.06 1.62 1.00 with a 0.70 OD.

The gear ratio split on a 200r is not as wide as the 700r4, so your engine will not have to lug down as low when it shifts. The 1st and 2nd gear ratios of the 200r are higher, making it a better match with the low gears of our trucks. The 3.06 1st gear of the 700r4 is so deep it is almost unuseable with low rear axle gears... Compare this to the 2.48 1st gear of a th400. Another thing to note: the 200r is shorter dimensionally, and has a different spline output than the th400, requiring some modification to the driveshaft. The 200r can be 'beefed' up during a rebuild just like the 700r to be a very reliable automatic overdrive transmission.

[Dtech]

Coooool an unique diff, anything special about these diffs.

Here are the numbers off the carb.

Holley, List 1850-3, Type 4160, CFM: 600, Stock jets 66 (rejetted to 64), Power valve 6.5. I rebuilt it before I put it in with a run-of-the-mill rebuild kit.

I have the old q jet, But am weary of them cause they blow. Every once in a while i run across an old guy that swears by them and wants to rebuild it for me. I always seem to loose their #s.

How would you tell if the power valve is shot?

Oh an here's another oddity about the truck. Its got this Tach box but has no tac in the cluster (pic) below.

[Longhorn321]

Originally Posted by Dtech

Coooool an unique diff, anything special about these diffs.

.

Not sure what you mean by "special" but the diff is called a "no-spin". It has some clutches that lock when there is a difference in rear wheel speeds I think. Mine has a sticker on the inside of the glovebox door and the booklet that came in the owners portfoilo that describes how it works.

[wesmigletz]

Dtech, I would recommend taking your Q-Jet to Carlos at Burbank Carburetor and Fuel Injection. He is on Magnolia, just a few blocks West of Victory and East of Buena Vista. He does excelent work, and will fix the throttle shaft bushings (if needed) and seal your well plugs). If you stop by, just tell him his neighbor Wes referred you.

BTW, did you set your mixture screws with a vacuum gauge? Tune for highest vacuum. Work in 1/8 turn increments. If your idle mixture screws are less than 1 to 1 1/4 turns out once tuned for max vacuum, your carburetor will be going rich at cruise. You can clean this up by inserting a strand of wire (start with a strand of 16-18 gauge wire) in your idle feed restrictors. Once you get the idle feed squared away, your idle will clean up, plugs will stay cleaner, especially in traffic, and your MPG will pick up. There is good information on tuning your idle feed restrictors in the forums at www.innovatemotorsports.com

Also, what is your total timing? You should probably be running around 38* total advance (set your initial, so that you have 38* total, and if need be, have your distributor curved), with your vacuum advance disconnected. A proper timing curve will help your mileage.

Wes

Originally Posted by Dtech

Here are the numbers off the carb.
I have the old q jet, But am weary of them cause they blow. Every once in a while i run across an old guy that swears by them and wants to rebuild it for me. I always seem to loose their #s.

.

[Longhorn Man]

4.10 is the highest gear ever offered in the HO rears, there was a 3.90 that was in the aftermarket, but its been discontinued. Might have been other aftermarket ones, but its all moot now anyways since it's all outdated and unavalible.

[Longhorn Man]

that was for an aftermarket tach that was either removed to keep, or died and disposed of.
The eaton diff is a very strong piece, but it is just outdated. They are just obsolete.

[Dtech]

I just checked and it has 4:10 gear ratio.

[Longhorn Man]

not all eatons were no spin units. The no spin is actually uncommon.

[Longhorn321]

Originally Posted by Longhorn Man

not all eatons were no spin units. The no spin is actually uncommon.

Very interesting. I prolly should have noted that my knowledge is limited to one truck with this diff.

[special-K]

In stop and go you`d want 4.10s over a taller gear.it`s at high speeds that low gearing hurts you and the opposite for high gearing.Everyone has opinions on carbs.Mine is that a Quadrajet is your best bet in all cases except downsizing to smaller cfm of when ultimate perormance is the goal.I like Holleys,but never a 600.And,that`s is the agreed upon concensus in my circle of motorhead friends.If you had rebuilt the Q-jet we wouldn`t be having this conversation.
What`s special about these rears is there hasn`t been a better one made since it`s last year of use in`72.The 14-bolt,it`s replacement,is also unbeatable.But,a bulky hog by comparison.They got it right on the HOs with ease of service and no excessive weight while retaining more than enough strength to hold up to anything for 40 years and counting.Ring & pinions are hard to find.But,you`ll never need one if you properly maintain the rear(or not,but that`s a gamble).Basically,keep up on the lube.
4.10 is an ideal ratio for a truck with o/d.No need to drive a dog when you can pull stumps and cruise at interstate speeds with 4.10s.3.73s are cool,if you get a later rear.

No matter what,driving fast costs more than driving slow.Replacing transmissions and rears can take alot of gas money.Slowing down is my best economizer with a non-o/d carburated truck.

[Mayo]

I got a 68 C-20 with a 292 and a Dana 60. I am not sure what my gearing is, but I would guess around 3:73 or 4:10. The previous owner kept a gas log and would get between 9-12 mpg, so I would guess it still gets around the same.

[special-K]

Originally Posted by Mayo

I got a 68 C-20 with a 292 and a Dana 60. I am not sure what my gearing is, but I would guess around 3:73 or 4:10. The previous owner kept a gas log and would get between 9-12 mpg, so I would guess it still gets around the same.

Your`s could be a 3.54 ratio with the Dana60.Back then it jumped from 4.10 to 3.54.That`s what my white`72 GMC has and I get 12.5 with 12.50/33s.

[Mayo]

Originally Posted by special-K

Your`s could be a 3.54 ratio with the Dana60.Back then it jumped from 4.10 to 3.54.That`s what my white`72 GMC has and I get 12.5 with 12.50/33s.

One of my dad's friends told me it had a 3.54 in it but my dad thought it was a 4.56. My brother and I have always assumed it was 3.73 or 4.10 for some reason. Is there any way to tell what gears are in it without tearing it apart?

[Longhorn Man]

acording to the books, the 3.54 wasonly for big block 3/4 ton trucks, and if memory serves, only if with an autoamtic.
With that being said, there's been 2 or so surface on here with inline 6 engines.
I'm thinking that an inline with a 3 or 4 speed in 68 would be a 4.56 gearset... a real screamer on the freeway. The person who built mayo's truck used it as a push car for his dirt track race car, and the 4.56 would have been ideal

[sleepertruck72]

I previously had a 400sb in my C20 with a TH350 w/ 4.10s I usually got around 5-8mpg depending on how fast I drove on the freeway. usually around 70mph.
I swapped to a crate 350 and installed a 700r4, same 4.10s. I now get just about 11-12 if I drive at 65-70mph. At 75-80 I get ten mpg at best I would say. 700r4 or 200r4 is definitely worth the money if you do alot of highway driving. As for rear end gears, I like the 4.10 vs. 3.73 personally.