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Old 08-08-2019, 07:10 PM   #1
LostMy65
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Calculating Mpg.

My speedometer doesn't work. And even if it did, it probably wouldn't be accurate due to drive train changes I've made.
I want to check my mpg.
Tell me if there's something wrong with my proposed method for checking my mpg.
I'm going to fill my tank, drive xx miles at 55 mph on a straight freeway, and then fill up again and note the gallons to fill. And then divide miles driven by the gallons used.
Now, my plan to get my actual miles driven will be to use google maps to plan a starting point to destination using google maps for calculating the distance from point a to b.
Anything wrong with my idea?
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Old 08-08-2019, 07:14 PM   #2
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Re: Calculating Mpg.

i wouldnt think so...
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Old 08-08-2019, 07:48 PM   #3
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Re: Calculating Mpg.

Sounds good to me. The longer the trip the better.

SAE actually had a standard way of filling for their fuel economy rallies, too. Something like fill it with a fully squeezed handle until it clicks off. Then squeeze the handle halfway until it clicks off. Then squeeze it a quarter of the way until it clicks off. Then you have a good chance of a more consistent "full" tank.
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Old 08-08-2019, 08:41 PM   #4
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Re: Calculating Mpg.

Quote:
Originally Posted by mattfranklin View Post
Sounds good to me. The longer the trip the better.

SAE actually had a standard way of filling for their fuel economy rallies, too. Something like fill it with a fully squeezed handle until it clicks off. Then squeeze the handle halfway until it clicks off. Then squeeze it a quarter of the way until it clicks off. Then you have a good chance of a more consistent "full" tank.
Yeah, I forgot. I plan on using a stick to verify fill height is the same. My truck is a 65, so there isn't anything in the neck.
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Old 08-08-2019, 08:44 PM   #5
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Re: Calculating Mpg.

Thanks guys,
And, no. I'm not planning on burning $50 dollars of gas just to see what my mpg is.
I have a destination.
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Old 08-08-2019, 08:48 PM   #6
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Re: Calculating Mpg.

My truck has a rather low mile 283. I don't know how it was built. So I'm also hoping to find out what rpm gets me the best mpg.
I'm thinking to find that out I would need to find the rpm that gets me the highest vacuum, correct?
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Old 08-08-2019, 09:02 PM   #7
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Re: Calculating Mpg.

Do you have a smartphone? Download GPS Speedometer. It will track miles traveled accurately.
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Old 08-08-2019, 09:14 PM   #8
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Re: Calculating Mpg.

Quote:
Originally Posted by LostMy65 View Post
My truck has a rather low mile 283. I don't know how it was built. So I'm also hoping to find out what rpm gets me the best mpg.
I'm thinking to find that out I would need to find the rpm that gets me the highest vacuum, correct?
I remember back in the 70's after the first oil embargo, you could purchase "Mileage Gauges" that were just vacuum gauges actually. I'm not sure but I think a couple of vehicles actually came with one stock (correct me if I am wrong). So maybe higher vacuum equates to better mileage to some extent. I always heard from the old guys that for an old car/truck with no overdrive, ~45 MPH was the sweet spot. "Modern" cars with OD, ~55 MPH but I guess gearing plays a big part too.
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Old 08-08-2019, 09:24 PM   #9
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Re: Calculating Mpg.

Quote:
Originally Posted by truckster View Post
Do you have a smartphone? Download GPS Speedometer. It will track miles traveled accurately.
Yes I do.
I've used a similar app in the past.
I'll do that too.
Thanks
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Old 08-09-2019, 10:18 AM   #10
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Re: Calculating Mpg.

Quote:
Originally Posted by WorkinLonghorn View Post
I remember back in the 70's after the first oil embargo, you could purchase "Mileage Gauges" that were just vacuum gauges actually. I'm not sure but I think a couple of vehicles actually came with one stock (correct me if I am wrong). So maybe higher vacuum equates to better mileage to some extent. I always heard from the old guys that for an old car/truck with no overdrive, ~45 MPH was the sweet spot. "Modern" cars with OD, ~55 MPH but I guess gearing plays a big part too.
I have one of those fuel efficieny gauge in my garage.

Higher vacuum = less air being pulled in = less fuel

the 45 / 55 mph thing has a lot to do with aerodynamics. I've heard 48 MPH is where that starts. Our trucks aren't aerodynamic, so it has a larger effect than it would on more modern aerodynamic vehicles.

Gearing changes the cruise RPM. Lower RPM is typically better as that is less times fuel is pulled into the cylinders, but it also depends how optimized the engine is at that RPM.

Brake Specific Fuel Consumption is used to measure how much power you make for the fuel used. It can be used to determine where your engine will cruise most efficiently.
https://en.wikipedia.org/wiki/Brake-...el_consumption

Modern cars (just about anything in the last 15+ years) have EFI, computer controlled spark, and variable valve timing. This allows them to optimize the engine over a wide range. Without VVT, the RPM range is more limited. You can still optimize the ignition and fuel at your cruise RPM.
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Old 08-09-2019, 12:29 PM   #11
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Re: Calculating Mpg.

Quote:
Originally Posted by LostMy65 View Post
My truck has a rather low mile 283. I don't know how it was built. So I'm also hoping to find out what rpm gets me the best mpg.
I'm thinking to find that out I would need to find the rpm that gets me the highest vacuum, correct?
Actually in theory for fuel economy you want high gear which gives you low vacuum. There are odd situations depending on how quickly the carb goes into enrichment that might hurt fuel economy, but with proper fuel injection or properly tuned carb you want a higher gear and lower rpm. Maybe 1800 rpm cruising? Something worth experimenting with.
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Old 08-09-2019, 01:21 PM   #12
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Re: Calculating Mpg.

Quote:
Originally Posted by mattfranklin View Post
Actually in theory for fuel economy you want high gear which gives you low vacuum. There are odd situations depending on how quickly the carb goes into enrichment that might hurt fuel economy, but with proper fuel injection or properly tuned carb you want a higher gear and lower rpm. Maybe 1800 rpm cruising? Something worth experimenting with.
I'm going to go for a trip today and do some experimenting. I'm anxious to see what it gets.
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Old 08-09-2019, 07:23 PM   #13
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Re: Calculating Mpg.

Maybe it was best not knowing.
Drove 80 miles and it took 8 gallons to fill back up = 10 mpg
Although, when I got onto I-5 southbound, I got stuck in a traffic jam. Stop and go and 20mph. I got off I-5 at Salem and came back up Hwy 213. Filled back up in Mulino. So, it's tough to say.
So much for my experiment.
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Old 08-09-2019, 09:45 PM   #14
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Re: Calculating Mpg.

Now you have a starting point. Was this a level trip or is there a bunch of up and down? Extra gas burned gaining altitude never seems to be fully recovered on the downhill side.

For those that don't want to read through 27 pages, what is your current setup?
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Old 08-10-2019, 12:15 AM   #15
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Re: Calculating Mpg.

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Now you have a starting point. Was this a level trip or is there a bunch of up and down? Extra gas burned gaining altitude never seems to be fully recovered on the downhill side.

For those that don't want to read through 27 pages, what is your current setup?
My truck has a rather low mile 283. I don't know how it was built.
Th350. 3.73 rear gear. 31 inch tall tires. Fitech EFI. HEI.
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Old 08-10-2019, 01:45 AM   #16
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Re: Calculating Mpg.

I was expecting you'd get 12 to 15 depending on road conditions, traffic, etc. For a little perspective, i drive a 2018 Silverado WT extendeded cab with a v/6, a 6 speed A/T and active cylinder management . (Fancy term for going into V/4 mode when coasting or under light throttle cruising). I use it on a weekly service route and routinely haul a few hundred pounds of stuff. The route is part hwy and part city and the mileage, as read by the computer, has been stuck at 19.9 for a long time. Using E85 however, will cause it to fall to the low 17's. I don't have a lead foot or drive aggressively either. My point is that if a modern full size rig can only knock down 20MPG combined, it's kind of a reality check about the potential for a 50+ year old truck. I would gues that getting your hwy cruise RPM's down might get you to the 12 to 15 I was expecting you'd get.
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Old 08-10-2019, 10:59 AM   #17
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Re: Calculating Mpg.

My 75 is similar. 350/TH350, 3.42 rear gear and 28" tires. Eddy 1405 carb, performer intake, headers, otherwise stock. An rpm calculator puts engine rpm for both mine and yours at about 2700 @ 65mph. Mine gets about 12 mpg @70~75 and 14 @ 60~65. Nowhere around here to safely drive at 55 for long distances.
I burn the cheapest gas I can find, usually E10 but sometimes 100% gas if I am filling portable cans for mowers at the time.

Might be time to check timing and tune the EFI.
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Old 08-11-2019, 11:28 PM   #18
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Re: Calculating Mpg.

Quote:
Originally Posted by LostMy65 View Post
My speedometer doesn't work. And even if it did, it probably wouldn't be accurate due to drive train changes I've made.
I want to check my mpg.
Tell me if there's something wrong with my proposed method for checking my mpg.
I'm going to fill my tank, drive xx miles at 55 mph on a straight freeway, and then fill up again and note the gallons to fill. And then divide miles driven by the gallons used.
Now, my plan to get my actual miles driven will be to use google maps to plan a starting point to destination using google maps for calculating the distance from point a to b.
Anything wrong with my idea?
I used a free version of an app called "Speedometer" and liked it enough to buy it. Even free it gives accurate trip distance and speed.
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Old 08-25-2019, 02:19 PM   #19
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Re: Calculating Mpg.

I googled C10 mpg

http://www.fuelly.com/car/chevrolet/c10_pickup

http://www.fuelly.com/car/chevrolet/c10

.
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Last edited by LostMy65; 08-25-2019 at 02:40 PM.
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Old 08-25-2019, 04:08 PM   #20
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Re: Calculating Mpg.

interesting...
that 1986 number is about dead on with my 305 C10 when I bought it new
and that 1981 number is what my 350 C30 got before the LQ9
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Old 08-25-2019, 09:00 PM   #21
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Re: Calculating Mpg.

I tend to appreciate those numbers over what you see sometimes in the forums. Some may have very well gotten 20 mpg with their 283 back in the day. But I would think with our trucks, 12 to 16 highway would be reasonable.
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