1985 gmc swb 305/700R MPG

[Firebirdjones]

Another way to look at it. We spend over $100 a month to have internet service at the house. It's more than $1200 a year just to sit here on this computer and type, lol.

I know people that just have to have their smart phones and pay well over $100 a month for that service (something I don't care to do)

That's more than I'm spending in gas driving a classic.

[68c10owner]

Thanks for the input guys. If I could afford a newer truck i would but I cant so Im stuck with what Ive got. So since Im stuck with it I want to make it better. Unfortunately I live check to check so Im not thinking so much about how much it will cost me per month or year. Or even by the day for that matter. I put in $20-25 at a time and since gas prices are fairly low here right now it will last 3 days. Maybe more but since my gas gauge doesnt work I dont want to push it although I have before and that is why I now carry a gas can.

I know my gearing is far from ideal. I just got this truck in September so its going to take me a bit to get it sorted out. Had it a little over a month and lost the transmission so thats new. I know its needs a new engine but dont really have the funds for that yet so that means the gearing wont get changed for a while now.

I didnt mean to turn this into such a big discussion but I do appreciate the input. I was mostly curious what others are getting with similar combos to see if it was normal or way off. Sounds like Im about right for what I have.


firebird, the year cut off is 1975 and older for smog. you still can get a ticket if you are inspected and the smog is removed but there are no biannual smogs required. Some counties up here dont require them either depending on where it is. Once I get it smogged I plan to register it in another county that doesnt require biannual inspections and I do happen to have a address to use in that county. Once that is taken care of I can install a free flowing exhaust, remove the computer controlled carb and run something more efficient. Im not really going for max power with this since its my daily driver and only vehicle but I will be going for strong low-mid range torque.

[68c10owner]

Small update--

Filled the truck up today and did the math and it was 14.77mpg. What I've been doing is driving to work at my regular speed, 66-67mph and then driving home slightly slower. I'll have to GPS it and see hat it actually is but 55mph on sped is 66-67mph and I dropped it to 50 mph on the speedo. So guess it isn't too bad over all. I still think it can be better but probably not by much. I might start collecting parts for a gear swap and maybe that will increase it some and give me a little better pep too.

[knockretard]

Where are you getting your odometer readings from? I know you said your speedo is off, if your getting your mileage from the speedo your results would be off by the same % as the speed readings.

[68c10owner]

Originally Posted by knockretard

Where are you getting your odometer readings from? I know you said your speedo is off, if your getting your mileage from the speedo your results would be off by the same % as the speed readings.

I am getting them from the odometer. I know that will be off as well but I write down the mileage when I fill up and then again the next time I fill up. Then I subtract the mileage and then divide that by how many gallons it took to fill up. The odometer reading will not be correct but its accurate enough to figure the mileage the way that I am. Plus I know how many miles a day I drive and the math seems pretty close. Based on how much it takes to fill up my truck each time its fairly accurate. Its certainly not getting better or worse mpg because the speedo isnt correct. It gets what it gets. The odometer is still reading miles.

[knockretard]

it's reading miles but it reads them slower in direct proportion to the speed your traveling.

For example:

if the odometer thinks you are traveling at 55mph for an hour it will read 55 miles.
Now if you are really traveling at approx. 65-67mph that's a difference of 10-12 miles in just an hour of drive time.

If it's only off 10-15% you could actually be getting 2-3mpg better mileage than you think.

[68c10owner]

Originally Posted by knockretard

it's reading miles but it reads them slower in direct proportion to the speed your traveling.

For example:

if the odometer thinks you are traveling at 55mph for an hour it will read 55 miles.
Now if you are really traveling at approx. 65-67mph that's a difference of 10-12 miles in just an hour of drive time.

If it's only off 10-15% you could actually be getting 2-3mpg better mileage than you think.

The speedo is mechanical and driven by a speedo gear and the output shaft. It doesn't "think" anything. All it can do is read out what the gear reads. Like I said before I know how many miles it is to work one way so even though the speedo is reading slow the odometer is fairly close. I get what you are saying but I've run this truck out of gas enough times to know it's not getting better than what I've figured.

[knockretard]

The odometer is also mechanical, driven by the speedo. If the speedo is off the odometer is off.

[Firebirdjones]

Throw a GPS in it and drive for a few days. Use the mileage from the GPS. That's what I do when I want to compute exact MPG figures.

I find that even though I recalibrate every speedo I have with various tire sizes and rear gear changes to get the MPH spot on, the mileage ticker still (in most cases) doesn't click off exactly what it should be. Sometimes they are still a 1/2 tenth off per mile. So on a tank of gas and a 200 mile run the mileage is off by 10 miles.

10 mile discrepancy can make a very big difference when computing your MPG figures.

[MTCK]

Everybody preaching correlation between the speedo and odo are correct.

I had an 83 K10 305 700r4 with stock 27-28" tires and 2.73 gears. I got 11-13 mpg with a fairly heavy foot. Overdrive was useless with those gears. One thing I haven't heard you mention tuning up is the ignition. Does the truck have ESC? Good plugs and wires? Timing set properly?

You aren't too far off from optimal. With $20-30 a month in gas savings it will take a long time to make any drastic changes pay. Make sure she is running as good as possible, tires aired up, keep your foot out of it and watch the boneyard for a 3.42 or 3.73 complete rear end. Good luck.

[68c10owner]

Guys, I dont doubt that the odometer isnt reading 100% correct. Lets see if we cant just figure this out a bit. The mileage between my last 2 fill ups was 123.2 miles according to my odometer. My last fill was on 2/9/16 and recent was 2/12/16. On 2/9 I filled up before heading home and yesterday before leaving for work so we are talking about 2.5 trips to work and back. My work is 27.1 miles one way so that would be 135.5 miles just going to work and back the 2.5 times. Not to mention probably 4-7 miles of around town driving. Lets just call it 5 miles. So a total of 140.5 miles is what I come up with. It took 8.337 gallons to fill up one 16 gallon tank as I only use one at this time. 140.5 divided by 8.337 is 16.85 mpg. So if that correct then knock was correct about getting the 2-3 mpg better. Thats actually pretty impressive if this is correct.

I mentioned running this thing out of gas a few times. I was also going a shorter distance and wasnt always filling it up because gas was much more expencive then so clearly my math in my head was off. I would put $20 in every couple days and at $3.00 a gallon it didnt go far. Today gas is $1.79 and only cost me $15 to fill it up which seems I didnt need to since its a 16 gallon tank and only took 8 gallons to fill. I might try to stretch it 3 days between fill ups until I can get the fuel gauge working.

[68c10owner]

Originally Posted by MTCK

One thing I haven't heard you mention tuning up is the ignition. Does the truck have ESC? Good plugs and wires? Timing set properly?

When I got the truck in September I changed the plugs, wires, cap and rotor. Seemed to make a difference. We did bump the initial up to 8* from zero but cant go any more than that until I smog it as I think the initial is supposed to be close to zero. It does ping right off idle but if I put my foot in it a little more it stops. I also have the EGR removed and Ive heard this could cause some pinging issues. Im putting a new EGR valve and EGR solenoid on this weekend.

ESC? Would that be ECU? It is a California model so it does have a ECU and a computer controlled quadrajet 4bbl.

[MTCK]

ESC = electronic spark control. Computer also controls ignition timing with a knock sensor instead of standard mechanical / vacuum advance. If you are supposed to set timing to 0 btdc with the module unplugged (assuming you have ESC) that is what I would do.

Also if you want to calculate your actual MPG, use your GPS and your spedo, and find an actual ratio instead of guessing how many additional miles beyond your 2.5 trips you went. Or as another member suggested, use your gps odometer for an entire tank of fuel cycle and calculate the ratio that way.

[68c10owner]

Originally Posted by MTCK

ESC = electronic spark control. Computer also controls ignition timing with a knock sensor instead of standard mechanical / vacuum advance. If you are supposed to set timing to 0 btdc with the module unplugged (assuming you have ESC) that is what I would do.

Also if you want to calculate your actual MPG, use your GPS and your spedo, and find an actual ratio instead of guessing how many additional miles beyond your 2.5 trips you went. Or as another member suggested, use your gps odometer for an entire tank of fuel cycle and calculate the ratio that way.

Im pretty sure it has the ESC. We did unplug the module when setting the timing. We were trying to get it to run better and it did run better with 8* but it pings even on 91 so I will be resetting it a little. My smog tag on my radiator support is painted over so I dont know exacty what it needs to be but thought I read it was zero and could go about 4* without the smog guy saying it wont pass because of timing. It does run very good even though its tired. Gears would be a nice improvement for performance. Im not sure what ratio to go with. This really isnt a "performance" truck but I would like more performance from it. Im thinking 3.42 or 3.73 so whichever gear I come across first is probably what I`ll get.

[beamn7]

The CA models had a version of ESC in the fact that the dist is controlled by the ECU but did NOT have a ESC module or knock sensor. Relies solely on the advance curves built into the ECU and inputs from the TPS and BARO sensor. I always thought this was quite odd but is fact. 85-86 CA models only... Leave it up to CA...

[slotard]

My speedometer gears are way off so I can't say for sure, but in my experience most vehicles leave the factory with a fairly optimistic speedometer and a fairly accurate odometer. If you're going 60 it might say 63, but the odometer should still record 60 miles in 1 hour at that speed. I can't say if these trucks are that way, just that it's fairly common.

[68c10owner]

Originally Posted by slotard

My speedometer gears are way off so I can't say for sure, but in my experience most vehicles leave the factory with a fairly optimistic speedometer and a fairly accurate odometer. If you're going 60 it might say 63, but the odometer should still record 60 miles in 1 hour at that speed. I can't say if these trucks are that way, just that it's fairly common.

All modern speedos are slow on purpose. If it reads 60mph you are actually running about 57mph. With mechanical speedos it might be more but mine is over 10 mph off. There was a tire change that caused it and possibly a gear change. The glove box shows 3.08's but the guy I bought it from says the rearend was replaced. It appears to be original to the truck based on the condition of the rest of the under carriage but who knows. I am looking into changing gears so I'm not going to spend a lot of effort on making it read correctly just yet. My commute is 60 miles round trip and I am able to make it at least 3 days on a full tank. Since my fuel gauge doesn't work either I don't want to risk more than 3 days.

[Tom]

Originally Posted by 68c10owner

I would love a LS engine in here not only for the mpg they bring

What MPG? My 98 with a vortec 350 knocks down 22mpg in an extended cab, I'd love to see a 5.3 beat that.

[68c10owner]

Originally Posted by Tom

What MPG? My 98 with a vortec 350 knocks down 22mpg in an extended cab, I'd love to see a 5.3 beat that.

They certainly offer better mileage than my smogged out carbed 305. My vortec 350 never got better than 15mpg. I've read others post about their LS engines bringing in 20+mpg. Very few vortec owners have claimed that milage as you have.

[slotard]

Originally Posted by 68c10owner

All modern speedos are slow on purpose. If it reads 60mph you are actually running about 57mph. With mechanical speedos it might be more but mine is over 10 mph off. There was a tire change that caused it and possibly a gear change. The glove box shows 3.08's but the guy I bought it from says the rearend was replaced. It appears to be original to the truck based on the condition of the rest of the under carriage but who knows. I am looking into changing gears so I'm not going to spend a lot of effort on making it read correctly just yet. My commute is 60 miles round trip and I am able to make it at least 3 days on a full tank. Since my fuel gauge doesn't work either I don't want to risk more than 3 days.

My tranny isn't original, so it shows around 20% over actual speed... shows 30 when GPS says 25, shows 60 when GPS says 50, etc. I imagine it's fairly common given the age of our trucks.

[68c10owner]

Originally Posted by slotard

My tranny isn't original, so it shows around 20% over actual speed... shows 30 when GPS says 25, shows 60 when GPS says 50, etc. I imagine it's fairly common given the age of our trucks.

I'm sure it is and I know mine is off as 55mph is actually 66-67mph per my gps. I'm not worried so much about the accuracy of my speedo as I am in how far I can go on a full 16 gallon tank. So far I've gone at least 180 miles and then put $20 bucks in which didn't fill it so not sure how much was left. Gas gauge reads empty so no way to track it u til I fix that.

[Jake Wade]

Originally Posted by Tom

What MPG? My 98 with a vortec 350 knocks down 22mpg in an extended cab, I'd love to see a 5.3 beat that.

If you say yours gets that, then I have to believe you. I have never personally heard or experienced that kind of MPG from that particular truck, 19-20 tops with 16-17 being the norm.

[85/Silverado]

I WISH I was getting 10mpg...

[Firebirdjones]

Originally Posted by Tom

What MPG? My 98 with a vortec 350 knocks down 22mpg in an extended cab, I'd love to see a 5.3 beat that.

Easily doable Tom, I tune the 5.3's and achieve that quite often. These factory truck tunes are pretty horrible.

My 6.0 LQ4 in my 72 4x4 blazer with a 4L60 and 3.73's, 33" tires, knocks down 22 mpg highway with my SD tune if I drive it conservative ~70ish mph, and that thing is shaped like a brick and scales 5,000 lbs. Don't need to use the 5.3's to achieve 20+ mpg.

[Bigstevex4]

Ah fuel mileage the stuff legends are made of. having owned 84k20 350 auto at 65/14.5 -89 k 1/2 ton 350 Manuel no cruse. 65/15.8MPG - 99 1/2 ton 5.3 auto cruse at 65/17.2MPG- 2002 HD 6.0 auto no cruse at 65/14.5 2002 HD 8.1Allison cruse at 65/ 11.8mpg have a monster tired 90 jimmy 350 Manuel 10mpg maybe.83 c10 fresh built 350 273gears Manuel 13.5MPG happy as hell. most those 4x4s had 373s 84 343s. these are the best I ever got checking on the 400 mi trek from Denver to Moms in NE Wyoming. So my observation is there are a lot of MPG claims I just smile and keep my mouth shut.