Too many variables in your experiences to draw any solid conclusions.
My experience is a bit different: I rebuilt the engine in a '70 B.
All new innards but the crank and rods. It got around 20 mpg during the
breakin period (mixed driving) but I couldn't stand the sound of the
engine howling at 4,000 + RPM at road speeds. So I came by a top-fill
LH overdrive tranny and installed it. Soon after that we took a trip
from TX to CA and OR. Drove the coast highways in CA & OR, camped in
Nat'l and state parks. With two adults and our camping gear aboard, the
B got 25 - 27 mpg for the trip.
I have a '69 B now that has a fresh engine built to '67 specs and the
same type OD as the '70 had. It got about the same mileage as the '70
on a trip from Central TX to NoCal and back. Same deal ; two adults and
camping gear. So at least the two cars compare well.
I really have a tuff time believing 30 mpg from a B without OD at 70+
mph. Shoot, it's a long stroke engine, with lots of internal friction.
Those piston speeds are waaay up there at 4K.
Cheers,
CR
Aaron Whiteman wrote:
> On Jan 15, 2008, at 2:32 PM, Charles & Peggy Robinson wrote:
>
>> Tell John Twist. He thinks overdrives are strictly "feel good"
>> add-ons.
>
> That corresponds to my anecdotal experience.
>
> Dads 1978 MGB got 28-31 MPG @ 70 mph on extended trips. It had no
> overdrive and ran all day at 4000 rpm (or more, I don't remember the
> exact figure).
>
> My 1975 MGB gets 26-30 MPG on the same trips. It has overdrive and
> runs at say 3500 rpm.
>
> The '78 had a 25D distributor and a downdraft weber and was recently
> rebuilt (less than 20K miles). The '75 has a 45D with pertronix and
> dual SUs, but is pretty tired. I need to get over and buy dad's car,
> if only for the engine.
>
> My experience says the OD doesn't matter one bit regarding mileage,
> but when my OD broke and would not engage, it was disquieting for me
> to listen to the engine at the higher revs--even though I knew it was
> just fine.
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