All about gas mileage now.

[RaysK20]

I have a 73 K20 with a semi tired goodwrench 350. It currently has a edlbrock 600 carb and intake, HEI. dual 2 inch exhaust and stock manifolds. Its going threw a th350/205 trans and case, gears are 4:10 and tires are 235/85/16.
A to B mileage on the highway at around 65 mph I get right around 11 mpg. My wife is freaking out because it seems to take a 20 dollar bill in gas to run to the store. My plan of putting in a 700R4 is in the works and I am gathering up the needed parts for a complete swap. But I also want to redo a motor and replace it at the same time so the truck is not down forever.

I have a few engines to choose from, I have a 283 and a 327 sitting in the shop as well as the 350 currently in the truck. I plan to use the above carb and intake etc. on whatever combo I come up with. I dont hammer the truck in the mud or tow anything much more then a hay trailer once in awhile. The truck is more just your daily driver type. I am really giving the 283 a hard look as it seems like less cubes to feed may help. But I have searched and come up with different opinions such as more cubes will work less hence better economy. I know I wont see 20 mpg but would like to twist at least 16 out of the old blue ox.

[Slowleadmccey]

I've been in the same boat with the truck I just got and in my researching (asking around to my fellow gear heads) I've learned that the 350 is fine as far as a motor goes. The 700r4 is also a good choice, although you shouldn't tow anything heavy with them. A higher gear ratio in the rear end, and road tires will help too. Also, how you drive it makes a big difference.

[Corts60]

I would definitely look at changing gear ratios if your are not towing or mudding. That is where the gas is going. I was able to get 16 MPG out of my 325hp 396 that had a 4-speed and 2.41's in the rear...granted it was a Nova but they don't weigh a whole lot less than these trucks.

or maybe an overdrive unit?

[83burban]

Should consider going to a nice rebuilt quadrajet - tuned well they can produce great mileage - I got 22.5 avg from California to New York and back with my set up in an 83 suburban.

Granted I was drafting, but only for 2/3 of the time, and I would have probably pulled 17 to 19 if I wasn't.

[RaysK20]

I was thinking about one of them Jet rebuilt quads as well, if its in the budget after the rebuild I will probably go that route. I thought the 4:10 gears would be about right with the 700's over drive. with my current tire size it should work out to 2100 rpm or so at 65. I tore into the 283 block today and discovered it had a crack between the freeze plugs, so I guess thats that. I got my heads back that I am going to use. they are 462 casting old school fuelly heads. Thay now have new 1.94/ 1.6 valves guides and hard seats. I will add the springs whenever I settle on a cam. Summit racing has a few house grinds that may work out. so I guess its between the 350 and 327 at this point. I already have a set of forged .30 pistons for a 350 but the compression would be near 9.5 so that may be to high for a heavy truck. Im all ears guys!

[donut]

What is your mileage at 60mph? Mine seems to improve some when I can keep the cruising rpm below 3k. (also a K-20 with 4.10's, tbh-350)

[RaysK20]

On the highway my mileage stays about the same from 55 on up to about 70. If I try and go faster then 70 the truck starts to run hot so I dont know. 65 is its comfort zone on the vac gauge so I use that as my baseline for running up the road.

[Clyde65]

I would consider changing rear end gears to a 3.73 or maybe even a 3.42

[tucsonjwt]

For general reference, I have owned several old squares and many new GM trucks and SUVs (all 2 WD). Here has been my experience, and I drive very carefully.

73 C20 454/400 (unknown rear ratio)- 9-10 mpg, city or hwy, 7-8 pulling heavy trailer on highway.

76 C20 350/350 - (unknown rear ratio) 9 city, 12 hwy

83 C20 454/4spd/4:10 + gear vendor - 9-11 city, 13-14 hwy

All of the above had stock Qjets, then Carter, then Edlebrock - made no difference on mileage. If it weren't for the gear vendor lowering the final drive ratio by 20%, I would probably be around 7-8 mpg city on my current 83.

In talking with other square owners, 10 mpg city is average for an old truck (no matter 350/454) and worse for a 4:10 or 4:56. Highway can be 14/15 with a good 350 and some kind of overdrive transmission with low number rear. Rear gears are available down into the 2s - sometimes used as city delivery vehicles. That would make a difference, but if you like get up and go performance, forget that option. I think 4 WD takes 2 mpg away from you on an old truck (not on a new one, necessarily.)

Now for the new trucks, all 1/2 ton, 2WD, again - driven like grandma on her way to church:

Tahoes/Yukons - 4.8/3:42 - 18-19 city, 24-25 highway
Sierras/Silverados - 4.8/5.3, 3:42 - 18 city, 22-24 highway
3:73 - 17 city, 21-22 highway
Silverado - 4.3/3:42 - 19-20 city, 24-25 highway

So, carefully chosen and driven new trucks/suvs can about double your gas mileage over an old truck. Drive like a teenager and you will get bad gas mileage in a Prius.

It sounds like money is the concern you have. If so, I would carefully add up what it costs you in gas now, and what it will cost you to make improvements
(transmission, rear end, carb, etc.). The quickest way to save money on gas is don't drive - consolidate trips, eliminate unnecessary trips, etc. If you can make the improvements discussed here cheaply, then go for it - it will provide you with valuable real life experience. I would start with a low number rear axle ratio - in the 2s - and see if you can tolerate it, then a 6 cylinder engine. Pretty soon, you also can drive like grandma.

Although it is very unpopular on this old truck website, sometimes it is better to buy a new or newer small vehicle for fuel economy and drive the truck only when you really need a hauler. This only makes sense if you drive a lot of miles every year.
Good Luck.

[zac]

Assuming your K20 has a 14ff, 3.21's are the best you can do, but good luck finding that axle. I am finally getting the 3.42 swap done on my '79 K20 (it was a pain finding that rear axle, even). But with a th400 and 203 I'm praying for 10 mpg with a 350. It just weighs too much to get "good" mileage.

A half ton can get good mileage. And I don't believe a 205 costs you anything in mileage. It is a 2wd truck most of the time. I have had two K10 Suburbans--a '70 and a '72-- with 350/465/205 combos and 3.08's, and they can get 17-18 easy on the highway. Withj Q-jets.

[RaysK20]

All really valid points so thanks for the info. I would drive a smaller truck if it suited my needs, but being 6'4 they get kinda cramped, and also are not practical for the work I do. {concrete}. I did have a nice 6 speed cummins dodge that gave me 20 mpg all day long but the current economy made me downsize nearly to riding a bicycle. So yes money is a concern as far as commuting at least 100 miles per day. I would slap a 250 strait six in without a second thought if i thought it would put me above the V8 mpg. mark. I could care less about high horses or a paticular sound. So what of it, you think a 6 banger with a 700r4 could be the way to go? Just so you know machine shop service of any type comes to me free as our family owns one. So building a new engine is alot cheaper for me then one would think. Any six cylinder guys care to chime in?

[Trick76]

6.2l diesel all day baby. Cheap as long as you dont want to go fast

[Buick455]

I have not driven my 1987 on the road in some time but I believe I got at least 14 MPG. This is a 3/4 ton 4wd with a 350 CI engine and turbo 350 trans and EFI. Other than weight your pushing allot of air with a truck. The EFI should be better on mileage. I am currently putting my 87 running gear in an 1986 1/2 ton as the body on the 87 has allot of rust. So I will basically have 3/4 ton 86/87 when I am done. I am staying with EFI as it runs and drives very good with EFI.

[GmtGmt]

Originally Posted by Trick76

6.2l diesel all day baby. Cheap as long as you dont want to go fast

x2