SOT: MPG and differential

Bad choice of words on my behalf!

The car was in gear, so the engine was rotating. The only thing which wasn't happening was a spark as the ignition was turned off. The fuel pump was driven off the crankshaft (or perhaps the prop shaft; I can't remember which). As far as the carburettor was concerned, the cylinder was sucking fuel from it, so as there was a demand the float valve was not cutting off the petrol. There was no spark so no ignition, and the petrol sprayed in through the inlet valve would have met a cylinder which was cooling, albeit slowly. Eventually, the spark gap would have cooled enough to allow petrol to condense on it. In a way, it's not that different from flooding by too much use of a manual choke when the engine's cold.

I don't quite understand. Are you saying I should have done everything which I did, but /also/ kept the accelerator pedal on the floor? It's the way I got the car restarted - kept the accelerator on the floor while turning the engine continuously with the starter motor. Eventually the petrol in the flooded cylinder was driven out sufficiently to allow a spark to form.

Actyally off te CAM shaft if it was original. I drove one of these once fixed up by a mechanic friend. He dumped weld on the camsaft and filed it down to mosty drive the fuel pump. Nice cars.

I can't

Yup.

Yup.

I find there are two sorts of those signs - the ones that flash and say you are going too fast, and the ones that show your speed.

The former go off when I am doing 25 in a 30.

The latter are within 1MPH checked against GPS calibrated speeds.

Andy

My wife's car can show both instantaneous petrol consumption, and average over a period.

A couple of years ago I found out that the most economical speed is too slow for my patience.

I turned onto a major dual carriageway in a flat part of the country, and found just in front of me a tractor. In a contraflow section.

We drove for about 15 miles at a steady 30MPH, and the car recorded 80MPG.

Andy

On 15/07/2025 21:51, Vir Campestris wrote: w for my patience.

Possibly ok before 1988, but after that it would have been illegal according to the RTA.

As long as the engine had enough torque to keep in top gear at 30mph, that would probably have been the sweet spot.

I haven't seen the former for a very long time now.

The council probably made it a 20mph zone.

Agreed.

Must be a tiddly car. My jag is best at around 50-55mph (can get over

At 30mpgh its dropped two or three gears and isn't very efficient at all

How else are they supposed to get to the fields? He didnt say motorway.

Oddly not., You have a balance between an engine that is not particularly efficient at low power outputs, because there the friction in the engine is taking all the power and not the road wheels, and the speed at which the engine/gearbox reaches higher efficiency, but the aerodynamic drag starts to dominate. A light boxy car with a teeny engine is probably best at less than

What would be illegal? To take a tractor on a dual carriageway? I thought the restriction applied only to Motorways.

My car can do that on the level (Nissan Micra).

Absolutely right! I have no idea why I read "dual carriageway" as "motorway"!

Well, the OP did state "A couple of years ago I found out that the most economical speed is too slow for my patience." I assumed that he had tried speeds above 30mph and found a higher fuel consumption.

Anyway, after (trying to!) read this, it's a lot more complicated than I originally thought, and your comment about engine friction vs aerodynamic drag makes sense:

Several sites I looked at a while ago suggested that for most cars slower uses less fuel. One here, for example, admittedly from the US:

nib

Well not exactly, but my Jag really doesn't do well at anything much below 50, and is not markedly less efficient at 60mph which is probably its best speed.

I'm not sure how accurate a reading you get if you fill to pump shut off. That can vary because of multiple factors. I'm also not sure of how much that variation affects the end result. It's probably only 1 mpg on a full tank refill. I always fill until I can see the fuel in the filler spout to do my calculation. I was surprised to achieve 59 mpg on local driving around town & villages in my diesel 1.6 Peugeot 2007 & 62 mpg on motorway trips.

There is a good reason why you should not do that on modern petrol cars.

It's a diesel.

I have never had a car new enough to have an EVAP system. The filler cap has always been good enough to stop any escape.

Or perhaps you simply didn't know it was there?

Possible but unlikely. This Peugeot is the first car I've owned where I can't do my own servicing.

I think it was last year. Might have been the year before. Certainly not before.

It does. That was 1500 RPM indicated.

(Oddly my wife's car and mine have pretty much identical gear ratios - hers undoubtedly for economy, and mine for top speed)

Andy

You're near me aren't you? Somewhere near Cambridge? There are a few around still.

They flash 30!

Andy