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MG MGF Technical - 40mph in 4th or 60mph in 5th

My daily commute to work has been altered slightly. Basically its an 18 mile journey done mainly on a dual carriageway.

I would usually travel at 60mph in 5th for the majority of the trip but due to roadworks the speed limit has been reduced to 40.

Now I'm averaging 40mph in 4th gear for the journey.

Should this have much effect on economy. The reason I ask is I've recently switched to Optimax but I seem to be using more fuel. Not a great deal but maybe 30 - 40 miles less per tank.

Any thoughts?
Bob Millar

Bob,

Just an idea, but doesn't constant slowing down, and speeding up, familiar to habits surrounding roadworks contribute more to fuel economy??

paul weatherill

But I'm not slowing down and speeding up. There are no roadworks on the road I'm on (A77) but they are building alongside it and have reduced the speed on the A77 for the safety of the workforce. Anywhere I was doing 60mph in 5th I'm now doing 40mph in 4th
Bob Millar

Bob, IMHO you will be travelling the same distance in a lower gear. Therefore more revolutions of the engine.
This should prob hold true until higher speeds when drag becomes a larger compomemt.
Ddraig

I should probably take a look at RPM at 40 in 4th versus 60 in 5th then?
Bob Millar

You should see a slight improvment in ecomomy, because the wind drag is less, however you should see a slight decresse in eco because the engine turns more per mile in a lower gear. 5th is fine at 40, just dont expect much acceleration (but then at a constant 40, why do you need acceleration?)
Will Munns

The engine/transmission has to produce the same torque to drive the car at 40 regardless of which gear you're in. There may be more power lost at higher revs, but also the engine may be in a more efficient part of its torque curve.
60MPH in fifth is pretty efficient use of the car, but I wouldn't have thought that 40MPH in 4th would be measurably worse. What do you mean by averaging 40MPH - surely 40MPH is your maximum ?
Steve

> What do you mean by averaging 40MPH

18 mile journey. 16 miles of this are at a constant 40mph. Then about two miles of town driving.

To be honest I'm just a bit dubious about Optimax's claims of inreased economy
Bob Millar

>>16 miles of this are at a constant 40mph.<<
I think you might be seeing a petrol engine's poor efficiency at light throttle.
I'd guess the MGF needs about 15BHP to maintain 60MPH, whereas 40MPH probably requires about 5BHP. A diesel would still give good efficiency at such light throttle, but not a petrol engine.
Optimax might help at keeping the engine clean, but probably no better than Texaco or Esso.
Steve

I had a rented Fiat Stilo the other week which (when it wasn't breaking down) had an instantaneous fuel consumption display on the trip computer. No idea how accurate it was, but I found fuel consumption to be almost directly proportional to the amount you had the throttle open. Upshot was that doing, say, 40mph in 4th was actually more economical than in 5th because you had to have the throttle open more in 5th.

FWIW, totally unscientific!

Tim.
Tim Jenner

> I had a rented Fiat Stilo the other week

I drove one a while back. Such a dissapointment and I've always been a Fiat fan having owned an Abarth Uno, a Turbo Uno and a Punto Sporting.

The Stilo was really a dissapointment. One of the worst driving positions ever and totally gutless. I like the styling and I was planning on buying an Abarth Stilo if I liked it but no chance in hell.
Bob Millar

>> dubious about Optimax's claims of inreased economy

Haven't we already decided that the F won't benefit from the above claims coz there is no knock sensor on the K series, or am I barking up the wrong tree here?
Phil

I thought the knock sensor advances the timing to eliminate detonation and wasnt an "economy" issue
Bob Millar

Better fuel => advance timing => more power => better economy.

The Stilo was the worst car I have ever driven (and I've driven some terrible cars...). I ached all over after 2 hours in it (I've done 9 in the F with no trouble), it was unrefined, gutless, it wandered on the motorway, the radio lost reception if you so much as moved, the remote boot release didn't work, the oil pressure light came on 5 times in 600 miles (3 days), it was less economical on a run than my VVC and all the electrical equipment was unreliable. Even the wash/wipe didn't always work. Needless to say, not impressed!

Tim.
Tim Jenner

Am I missing something here!!!

why don't you put the F into 5th gear at 40 then your fuel consumption will be even better that at 60.

T
thicko

Car labours a bit at 40mph in 5th and isnt very responsive.

And if your witty enough to write snide comments how about being brave enough to put up your real name!
Bob Millar

Wow!!!!!
<Am I missing something here!!!

why don't you put the F into 5th gear at 40 then your fuel consumption will be even better that at 60.>

If that is a snide comment from Thicko, read some of my others eslewhere. Get a life you sad b*****d
Dick Pullar

This thread was discussed between 18/11/2003 and 19/11/2003

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