Cough, Splutter - How Much MPG?! 😱

[GCorvette]

Off to France and the first long journey has already revealed that perhaps something needs a tweak.

5.7 190hp 1973 standard carb, improved air filter but just 12.38mpg cruising at 60mph on motorway the whole way!

Will need plenty of these over the next week.

What does everyone else get out of theirs?

 

[Mad4slalom]

Off to France and the first long journey has already revealed that perhaps something needs a tweak.

5.7 190hp 1973 standard carb, improved air filter but just 12.38mpg cruising at 60mph on motorway the whole way!

Will need plenty of these over the next week.

What does everyone else get out of theirs? View attachment 17794

Never really tested it but always guessed 10-12 about right. Only way is to absolutely brim it , zero the mileometer, drive say 20 miles then brim it again and work it out from there.

 

[johng]

Measured mine over the last 700 miles and it's averaged 15.7mpg

 

[James Vette]

When these cars were made, people enjoyed stopping at petrol stations and didn't even look at how much it cost.

 

[Mad4slalom]

Never really tested it but always guessed 10-12 about right. Only way is to absolutely brim it , zero the mileometer, drive say 20 miles then brim it again and work it out from there.

Is there a filter on that pic ? Cos it dont half look gorgeous

 

[CaptainK]

The only time I measured my MPG (1968 327ci/300hp) was on the long journey home from picking it up. It was about 18MPG I think, just cruising at 70mph all the way back (duallys etc, all other speed limits on other roads). That was with a Carter carb on it that I later found out was overfuelling. I've since put on a similar rated new Edelbrock carb, so I suspect the MPG should be better now. I've not measured it though, as I don't care about the MPG. I just drive it and enjoy the experience.

Out of interest, when cruising at the 60mph you state, what RPM are you doing in your top gear? From memory mine is about 2700/2800 rpm in fourth (manual gearbox) at 70mph. Interested to know what other people get.

 

[Forrest Gump]

Out of interest, when cruising at the 60mph you state, what RPM are you doing in your top gear? From memory mine is about 2700/2800 rpm in fourth (manual gearbox) at 70mph. Interested to know what other people get.

At 70mph in 4th gear my car is at 2650rpm. That is with a 3.08 ratio differential and a Borg&Warner 4 speed manual gearbox.

Some time ago I did a speed to rpm table:

 

[Nassau65]

When I was using C3’s every day, years. ( 74-82) around town I thought I was getting around 15mpg, on a bit of a run M25/M1 etc about 18ish. It was the same for 350ci equipped Caprice’s, firebirds, Camaro’s etc. they all seemed to do about the same.
pretty frightening mpg figures for today.

 

[Nassau65]

Of course when these “old bangers ” were built, fuel economy was not really a consideration. It did what it did. Period.

 

[Nassau65]

In one of my mustang books, it goes on about the new 1971 style being longer, wider and heavier. It also says it’s more thirsty, quote “ but at 12 cents a gallon, who cares” that was the attitude in those days.

 

[James Vette]

When I was using C3’s every day, years. ( 74-82) around town I thought I was getting around 15mpg, on a bit of a run M25/M1 etc about 18ish. It was the same for 350ci equipped Caprice’s, firebirds, Camaro’s etc. they all seemed to do about the same.
pretty frightening mpg figures for today.

You sure have had a good life with nice cars.

 

[James Vette]

I would ls-swap but you lose everything about the classic V8. Maybe there is a way to rebuild the classic V8 but with better fuel economy instead of putting a newer V8 in it.

 

[Nassau65]

You sure have had a good life with nice cars.

They were just old “yanks “ in those days James, nothing really special once a few years old. A lot of flash for a little cash the dealers ( inc Daddy) would say. And if you were real cheap you went for a six pot. I used a 76 Firebird 6 pot for a few months, great car, and very economical over a V8. Just no oomph from the lights or that V8 burble.

 

[CaptainK]

At 70mph in 4th gear my car is at 2650rpm. That is with a 3.08 ratio differential and a Borg&Warner 4 speed manual gearbox.

Excellent thanks. I still need to check my diff, but there was a debate on another thread that I might have a later 3.08 diff, instead of the 3.70 mine should have come with. Mine is also around that RPM at 70mph. I say "about" because both the RPM and Speedo ready solidly without wobbling, but the problem is that my speedo appears to be out a bit, so determining what 70mph actually is is quite hard. Normally I use a GPS speedo as backup, and I've also just learnt my car to know about 63mph on the speedo is about actual 70mph.

Anyway, long story - that kind of backs up the theory I have a 3.08 diff. I still need to rotate the wheels and count the driveshaft movements when I get time.

 

[maxuk15]

MPG figures are in from the Le Mans 2022 trip and the average mpg was 21 !!!!
Speed around 65-70 mph.............
Not bad for an 81 V8
Very happy but still cost £424 (ouch)
But well worth it

 

[Nassau65]

That’s not bad. The later C3’s steadily increased in mpg over previous years. By 1981 the second oil crises had come and gone, but manufacturers did take note and gradually increased mpg on many vehicles. The old ways of just pouring the petrol into the carburettor was slowing dying out.

 

[maxuk15]

It also has a different performance cam in !!!!
Mild ish ..........
Edelbrock carb no emissions stuff so good work there !!!!!!!!!!

 

[James Vette]

Cheaper than a typical holiday though.

 

[GCorvette]

Thanks for the feedback.

Now I am back from an absolutely fabulous trip, the numbers are in:
903 miles covered.
£660.92 of fuel consumed.
£12.21 Average MPG

Shocking? Yes, Clearly something needs a tweak (although RPM sits around 2700 when cruising 60-70mph), but I had a brilliant time, so no complaints!

 

[Roscobbc]

Thanks for the feedback.

Now I am back from an absolutely fabulous trip, the numbers are in:
903 miles covered.
£660.92 of fuel consumed.
£12.21 Average MPG

Shocking? Yes, Clearly something needs a tweak (although RPM sits around 2700 when cruising 60-70mph), but I had a brilliant time, so no complaints!

There is certainly someting amiss there Graham - even my 489 with a comparitively huge 950 cfm double pumper does 15/16 at similar speeds. If you are using the correct diameter tyres (27.4 ish tall) 70 mph should mean a 2550/2600 if equipped with a 3.08 rear end. Is it an auto box?. That should have at least another 5 mpg in it at cruise speeds. Has it got a/c? are you using it?