Calculating fuel efficiency on a 1970s car

i got a 1976 gmc motorhome with a bone stock oldsmobile 455 v8 engine and i'm doing a full interior replacement and i figure while i'm at it work on the engine and transmission to improve fuel economy but all the info i have found online requires a fuel injection system and im wanting to keep the original 4 barrel carburetor so any help is appreciated

You need to know how fast fuel is being burned and how fast the vehicle is moving.

To get the fuel flow you could put a flow meter in the fuel line. Say you get 20 MPG at 30 MPH. That's 1.5 gallon per hour = 192 fl oz per hour = 3.2 fl oz per minute. If you want data every 10 seconds or so, you will need a flow meter that can measure in increments of about half an ounce (about 15 ml).

You also need a way to measure rate of travel. Older systems put a magnet or two (or 8) on the driveshaft and used a reed switch (or, morte recently, a Hall Effect switch) to sense the rate at which the driveshaft turns. Knowing the differential gear ratio and the circumference of the tire you can get distance per magnet passing.

Best bet is to look at getting the engine in best possible tune or swap it out for something more modern or a Diesel .
Keeping your motor home parts light is a good move too , as is keeping it aerodynamic .

Fuel injection may not be a hard conversion I’d look into it , measuring what’s going on with an Arduino will not be a great help …

It might help the OP see how bad the fuel economy is and how very little he can do about it either through modication of the engine itself or driving habits.

Plus sounds like a fun project as outlined by @johnwasser… if we started doing only practical and useful and won’t buy the commercial version projects this whole hobby would collapse. :expressionless:


fill up to full tank, drive 50 or 100 km, fill up again and notice how much gasoline you added ....

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Perfect. If you want, you can make a calculator using an Arduino and an LCD touchscreen.


Diesel isn't a option as they are too big and won't fit tho I don't have to worry about aerodynamics too much as it's a 1976 gmc motorhome (most aerodynamic motorhome ever built) plus it's super light for its size (as heavy as the car it's based on) also fuel injection isn't a option on this engine without dumping $100k into it

This one's old enough to use a cable from the end of the transmission to the speedometer

I was thinking more knight rider led bar thing

Also I should mention that this motorhome has nothing related to a truck of it's day and for practical purposes it's a oldsmobile toranado front clip

@johnwasser description is the only way to get the numbers in a carbureted vehicle. However a flow meter may be problematic. I would guess the rate of fuel when under light load would be in the range of 50 liter/hour. The issue with a fuel flow meter is compatibility of the flow meter materials with gasoline.

Fuel injection can be done for small money, £100k is not a realistic sum - have a look at Megasquirt, Omex and others , there are also mapped ignition systems possible with Arduino etc.
Modifications such as this might make a real improvement - you can even do throttle body injection , keeping the car body .
Also maybe an lpg conversion ??

Deisel conversion?

I mean $100k(not £) for a entire engine rebuild plus custom heads, new crankshaft, new connecting rods, new timing system and more plus I can't afford half the parts needed to do a throttle body type one and throttle body injection isn't worth it compared to the carburetor and I would actually go diesel I'd I could afford it

Could you just purchase a newer engine? If I recall the 70's emission approaches tanked the mileage.

I plan to swap the 70s emissions equipment with newer parts to get some fuel economy back but also I plan to modify a newer transmission to fit on the engine and front suspension (it's fwd 3spd auto)

Just to keep things realistic, the fuel mileage of something that size with a mid-70's 455 v8 would probably set a world record if it achieved 15mpg on a level road. Monitoring the weight of the fuel tank would probably be good enough for calculating average mileage over short distances.

Older "fuel economy" gauges were often simply a vacuum gauge monitoring manifold vacuum, a heavy load or accelerating rapidly drops manifold vacuum and reduces efficiency.

You would probably be better off monitoring the ignition timing, manifold vacuum, and exhaust gases (oxygen sensor) to detect when the carburetor or ignition were getting out of tune.

You might also want to do some research to see if the carburetor needs any modifications in order to run the alcohol/gasoline mixture that is commonly sold these days (depending on which country you are located in).

Trying not to be negative but it seems the increase in fuel economy while maintaining was achieved by fuel injection.

Is there any way you could get an emissions waiver for your vehicle (age etc).

Some vehicles have a fuel return line, if so you will need two flow sensors and do some extra math

The return fuel line would be for fuel injected vehicles where the fuel pump is in the fuel tank and the fuel pressure regulator is on the fuel rail.

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So pretty much what I'm hearing is don't even try unless I'm swaping the entire engine or useing a shitty electric carburetor