Gas mileage help please, (my first post)

[307K10]

Hey everybody!
Love all the info and pics on the site! I've been a member for a few months and haven't posted anything. Just been looking and reading a lot of great stuff. I could really use some help. I have my Grandfather's '72 K10. It's a LWB with 3 Spd manual and 307 V8. The engine was overhauled about ten years ago and is stock besides being bored .030 over. I started tearing it apart in May this year to restore it and will post some pics when I get home from work. Woops. Anyway, sorry to make this so long, my problem is the truck only gets 7 mpg. I would drive the thing everyday all day long if I could. I love that old truck and its full of memories and I will never part with it. I'm wondering if there is anyone who has some advice on whether I should swap trannies or just change the diff gearing to help improve the gas mileage. It is 4wd so I know there's more to it than would be for a 2wd. Also if anyone out there has a K 10 and is getting more than 10 or 12 mpg please let me know how. My wife is very supportive and loves my old truck too, but she kinda cringes when I talk about wanting it to be a "close to daily driver." Thanks for any help! Thanks again for all the other great stuff on here. I've used a lot of other ideas already.

[Ol Blue K20]

7mpg thats not good, my 72 k20 with a 402 gets about 15mpg. My 70 El Camino has a 307 and gets between 11 and 12mpg guess which gets driven the most. I've heard that 307's do bad cause underpowered but don't quote me on that just what I'v been told

[307K10]

Wow! Almost 15mpg out of a big block? Now I really feel bad for my little small block. What kinda gears does yours have? Is yours a 3 or 4 spd? When I was little, my dad had a 72 Nova with a 307 and he said it got pretty good mileage. I know these old V8's weren't the most efficient even when new, but surely mine could do better. Thanks for the info.

[72c10projectc]

Get a 4.3l v6 vortec with a 4l60e
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[307K10]

I had thought early on about swapping to a newer engine/trans but I finally settled on trying to keep it mostly original the way my Grandfather ordered it new. I have changed a few things, but trying to keep the old V8. Thanks for the suggestion.

[Warrens69GMC]

What do you mean by "overhauled"?

Are you talking a re-ring or new pistons/new bearings or ???? If you have it .030 over and used the stock heads, you could have low-low compression like ~7.5:1

Can you get the casting # of the heads? If its 185,882 etc, etc.. then you probably have very low compression. To wake it up, you really need (416) 305HO heads and a cam like the Edelbrock Performer. Those two componets really wake up a 307.

[307K10]

All the internals are new. Pistons, rings bearings etc. Cam and all is new too. The heads are stock they were checked when the block was honed and everything. Crank is original, just cleaned up by the machine shop. I'll have to get back to ya on the head castings. I never thought about it changing the compression ratio. It's never been a really quick truck but it does seem to run good. I've checked timing, dwell and all that, upgraded the coil to a little "hotter one but seems that other than running better, nothing has helped the gas mileage. Thanks.

[Ol Blue K20]

My truck is 410's and 3spd auto

[69GWC]

What kind of carb and is it in good shape? Might be time for a new one.
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[307K10]

Originally Posted by 69GWC

What kind of carb and is it in good shape? Might be time for a new one.
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The carb is a replacement stock style 2 barrel Rochester. Back when I had the engine rebuilt I replaced the original with a reman. I was young and dumb cause I didn't keep the factory one. Plus it took three remans to get this one that actually seems ok. I only assume the carb is good. It doesn't sputter or backfire or seem to "load up" like the previous two did. But you may be on to something. I've actually used some of the info on this site about how to adjust the carb and thats how I got it to be as good as it is.

[Mustard72]

Is that 7mpg around town or on the hiway? That makes a big difference. Did you mention what gearing it has in the rearend? How hard do you drive it? I find my right foot does more to hurt gas mileage than any tuning could possibly overcome...

Welcome aboard, by the way.

[307K10]

Originally Posted by Mustard72

Is that 7mpg around town or on the hiway? That makes a big difference. Did you mention what gearing it has in the rearend? How hard do you drive it? I find my right foot does more to hurt gas mileage than any tuning could possibly overcome...

Welcome aboard, by the way.

HA! Yeah, in the cars I have, i tend to drive them a little hard. But with the Ol' Chevy, I go easy on her. The 7mpg is highway. It seems like you can putt around town all day and not use any gas. I don't know exactly what gearing I have. Other than pulling the plugs in both diffs to inspect the fluid, I ain't had 'em apart. With the 3 spd and 32x10.50 tires on my 15 rims, she runs right at 3000 rpm at 60 mph. So I normally drive around 45 - 50 mph. The rpms there are around 2300-2500. I shift around 2000 rpms, seems to make for the smoothest switch between gears in the old 3 spd.

Mister B, I do run synthetic and keep her tuned up. Plugs and wires are all still new. I've took out everything that ain't bolted down to reduce weight. I probably over inflate my tires hoping that it helps. Makes the rough ride a little rougher. But I love it. I've tried about every filter and fuel tip I can find and just figured that now my only option is mechanically altering the truck to get any increase in mileage.

The truck feels like its geared really low. When I around 14, Papaw let me drive it and I actually took off in high gear in a rough tobacco field. The truck did struggle for a moment but didn't stall. After I got up to about 10 mph, I went to shift to second and realixed I was in third. Ahh, to be young and dumb again. Now I'm mostly dumb.

[Mister-B]

Try switching to full synthetic oil. If you're running premium unleaded, don't. It's too rich and not necessary for these engines. Check your air filter. Consider a K&N. at the very least, you won't need another filter ever again, since you can wash it out. Check your plugs. Are you carrying around a heavy bed mat or other unnecessary junk that's weighing it down? Check your tire pressure. These are all simple and cheap things that add up. I'd do that before changing transmissions and overhauling an engine for gods sake
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[70c10stepside]

Thats not good at all. My built 355, turbo 400 and 4.11 gear gets 12-14 lol Carb got a bad tune?

[307K10]

Originally Posted by 70c10stepside

Thats not good at all. My built 355, turbo 400 and 4.11 gear gets 12-14 lol Carb got a bad tune?

Wow! See I knew these V8s can do it! I'm starting to think I may be a little out of my knowledge range when it comes to tuning the old girl. I've been a shade tree mechanic all my life. Ha! i quess you can't tune one by ear like I do my guitar. Lol. I'm just thankful to hear so many of you guys are getting above 10 mpg. I still have plenty of time to get the mileage issue worked out. I have enough body work to keep me busy for a while. I'll start a build thread soon and post pics for you all to see. Thanks for all the replies and help. I love this site and will be on here as much as I can. Well I'm fixing to leave work so I won't get to reply for an hour or two if anyone responds. Thanks again, you all are awesome. (You have to be.. you have old Chevy trucks!)

[scorchingeng]

I used to have a K10... man it's heavy. Hard to get that mileage up with that big heavy NP205 in it.

I have read a lot of advice from experts that say the same thing. You need more torque off the line and taller gears. Your 307 is burning a lot of fuel to get that beast moving. Your 3 speed is rapping out your RPM way high. That 3000 rpm would destroy your mpg. The goal would be to get it to around 2000 rpm at 65 mph. You need some taller gears and the horsepower to support it.

In the end, I did spend some money. I upgraded to a sbc 350 for $2000 brand new. I bought a 700R4 auto and installed that for 4 speeds. (They, as in advance adapters they, make an adapter kit to mate it between 700R4 and the NP205.)

I got about 10 in the city, and 14 on the highway. STILL not great. It's just a beast truck.

Then, I swapped out my 4.10 gears and put in 3.73s to lower the RPMs some more. I got 11 in the city and about 15 on the highway... lot of work for that tiny improvement.

Now I have a 2 wheel drive. I guess my friends were right, I kept trying to change my climbing beast with poor gas mileage to an El Camino.

Here are some other suggestions:

Electric fan setup, or at least make sure your clutch fan is working.
Do you have full time 4wd? Install manual hubs.
Upgrade (with some work) to a lighter NP208... probably not worth it.
They, I forget who they is this time so google it, make an "overdrive" that is inline with your dive shaft.
Grease your wheel bearings with synthetic grease.
Make sure your alignment is good.
Try a vacuum secondary 4-barrel 600cfm carb - they get good gas mileage with small primaries and they are cheap.

And after all that money is spent, you can buy a LOT of gas. LOL.
Also, you won't be pulling as many people out of the mud.

Lived it.

[mrein3]

I used to have a 4x4 with a 3-speed manual transmission. I still have the truck but I have since replaced that transmission with a th350 auto.

Back then my engine was a tired, cam flattening 350.

Prior to the transmission swap I averaged 9.4 miles per gallon. I pull a boat with my truck so many of those miles were pulling a 2000 pound then a 3200 pound boat/motor/trailer.

BEFORE you get all crazy replacing things I would take my smart phone, my friend's smart phone, my GPS, my friend's GPS, girlfriend's, mom's, dad's, or anybody's phone or GPS and make sure your speedometer is correct. If your speedo is correct your odometer is correct. If you odometer is NOT correct there is NO WAY you can determine what your fuel mileage is. No way. Its not possible.

Once you get your odometer squared away fill the truck to the top. While you are standing there filling it write down what the odometer reads. Go drive your truck for a few weeks/days/whatever then go back to the gas station. Fill it back to the top. Again while you are standing there write down your odometer again. Subtract your odometer reading from the first fill up from the current fill up. That is miles driven. Divide by the amount of gallons the 2nd time. That is how much you used. There is no other way to accurately figure out your mpg.

Now that number is your fuel mileage or mpg's. Anything else is a guess. (Sorry for the rant. I see lots of folks post about how their 572 big block is getting 29.3 mpg then in the next post say how their odometer is off by 20% ).

I used to get an average of 9.4mpg with the 350 and 3-speed manual. This is with 3.73 gears and 235/75R15 tires.

Since the 3-speed manual to th350 swap I average 9.1 mpg. These numbers are low because I do a lot of stupid things with my truck like pull a boat, plow the driveway, take it ice fishing, and other hydrocarbon burning activities that this truck was built to do. Oh and I took out the tired old 350 and replaced it with a 383 I built.

Long story short, make sure your speedo is correct, figure out what gears you are running, and post the numbers. Without those numbers there is no way of knowing what is what.

Based on what my truck used to do (350, 3.37s, 3-speed manual) I think you should be able to get 13-14 mpg on the hightway out of that 307 with that manual tranny, depending on your axel's gear ratio and not pulling anything.

[dhaymaker]

Originally Posted by mrein3

...I would take my smart phone, my friend's smart phone, my GPS, my friend's GPS, girlfriend's, mom's, dad's, or anybody's phone or GPS and make sure your speedometer is correct. If your speedo is correct your odometer is correct. If you odometer is NOT correct there is NO WAY you can determine what your fuel mileage is. No way. Its not possible... )...

Another way to just verify your speedometer/odometer is correct is to find a stretch of road where mile markers are in place. Look at your odometer when you hit a mile marker and also look at the odometer at the NEXT mile marker. If you tick off a mile between markers, your speedo/odometer is accurate. This may not be as accurate as the GPS method, but it does present a simple, quick check!

[ges55]

X2 on the GPS

[steelhorse]

On an old Dodge I had I switched from full time to manual hubs and made a huge improvement in fuel mileage.

[blasher]

Like a previous poster mentioned, definitely start with a GPS to confirm the accuracy of your speedo/odometer. With the size of your tires, it's very possible that you're not getting accurate data for your calculations, unless you, or the previous owner, changed the speedo gearing or added a Stewart Warner type inline adjuster. When I bought my truck it was off by 25% (the diff gears where changed and bigger tires added) The speedo said 48 when I was traveling 60.

Just for reference I have a ‘69 K10 with a 305 V8 with a Quadrajet, 3 speed manual, NP 205, 3.08's and 31x10.50 tires and I get 16.5 mpg highway. It's a great setup for cruising but wouldn't do well hauling or towing because it's geared too high.

[69gmcc10]

In high school the 305, Qjet, 4spd, 4x4, 3.73 gears, 235/75x15 tires I had got 14 mpg. Same truck and my dad put a 350 in it and it gets 16 mpg now. I am not saying you need a 350, i am just giving you numbers of what mileage I had.

Do you have points or an hei, that will be a big help, especially if it is tuned correctly! that would be the first thing I would do. You can either buy a new one or rebuild a used one, do some research on how to dial in your timing as well as you can.

I would then rebuild a Q-jet after talking and ordering a complete kit from cliff Ruggles (cliffshighperformance.com) or another reputable Q-jet expert with proper needles and jets. Rebuild it yourself, dont go re-man. Rebuilt and tuned correctly they can get better mileage than the 2 barrel. At least talk to someone about re jetting the 2 barrel.

Taller gears help and so do OD transmissions, but regardless if you have them or not you need that engine running as well as possible.