SOT: MPG and differential

Air conditioning compressor? Electric fan constantly on trying to keep the engine cool?

The air conditioning was checked last week and is booked in for a new condenser on Tuesday. I assume the compressor is okay but I will certainly ask. The fan is an interesting thought and would explain the lack of any correlation with steering. Is there an easy way to check?

I should think 63.5 mpg is fairly normal for a modern car driven in almost optimum conditions. I don't have to try very hard to beat 60 with our hybrid on a long non-motorway journey.

nib

When the noise occurs stop, open the bonnet, you can usually see the fan spinning.

Dave

Did the noise also occur with the car statioary but with the engine running? If so, you can eliminate anything transmission-related.

As a matter of interst, how did you measure MPG?

I have a 2008 Peugeot 308 1.6 HDi (diesel). I tend to accelerate moderately briskly but never tyre-shreddingly! I rarely exceed 70 on a motorway, though I tend always to drive at or slightly below the speed limit if it's safe to do so (as opposed to choosing to drive much slower than the limit). I also anticipate in good time so I come off the power when I see that I will need to slow down, rather than remaining under power and then braking harder later.

My car has averaged about 55 mpg over its whole 200,000 mile life so far. I tend to get full-tank to full-tank averages (fuel bought to fill to pump-shutoff divided by distance since last filling) of about 45-55 mpg. There will be some variation in when a given pump shuts off. When I was doing a lot of longer journeys on motorways and fast single/dual-carriageways (ie not with much around-town driving) I was averaging around 57 mpg. The best ever tank-to-tank figure was about 63.5.

If you, in a petrol car, can achieve 64, then I'm insanely jealous ;-)

It's harder to measure as it's a plug-in hybrid, but my usual way for single journey MPG is to use the driving modes to force it not to consume any significant battery and then rely on the trip computer. That sort of MPG is for quite gentle driving on A roads, maximising use of regen braking and all the usual tricks gentle acceleration, avoiding braking at all if possible, ... The last trip where I achieved >60 was Milton Keynes to NT Anglesey Abbey and back, for example.

As an overall average including local trips and/or using sport mode to good effect it's less of course, and you have to factor out the electricity consumption which involves making assumptions.

The best long-term I've ever got out of a pure petrol car was 54 mpg over 25,000 miles, and that was a Smart!

nib

Was that the car's reported reading or yours based on odometer miles travelled divided by gallons (litres) from the petrol pump?

Well, yes, but in this instance running as a sophisticated petrol car!

nib

A diversion from that. I believe (anyway it's my guess) that many modern cars measure the fuel consumption for the trip computer by summing the durations of the injector timings, thus making it very cheap (just software) to add this function. My old MG Maestro, with carburettors, had a large fuel flow meter in the fuel line to do this back in 1984.

That makes it very sensitive to the fuel type. I discovered this when I accidentally put some petrol in my Fiat Panda diesel. I'd just topped it up before a long trip so luckily only about 25% petrol and I didn't notice for a while, it ran all right, it was just a pig to start. Then I noticed that I was doing more than 70 mpg. I suppose the petrol makes the fuel less viscous, so it injects for less time to get the same power, so it looks like better mpg. Then I looked at the fuel receipt. Just topped up again ASAP and it gradually came back to normal. Didn't seem to do it any long-term harm, it was still going last I heard (gave it to a family member when it got old).

nib

Well not for a modern 30-40bhp engine. Remember that hybrids scale back on engine power so that the petrol engine can just manage to cruise the vehicle. Overtaking and accelerating is dine with the electric bit.

Ergo a 30-40bhp petrol - a modern morris minor - could do every bit as well on a long journey.

And most non turbo diesels under 2 litres can do that as well. Diesels are very economical under lighht load.

In both cases both of you are operating what you have at near its best efficiency.

And that nets about 60mpg for a typical reasonably aerodynamic small saloon.

Both of you are operating small engines at near peak efficiency at 60mph and each of you has a hidden acceleration aid - a battery or a turbo.

The hybrid has the slight advantage that not all energy is lost in braking. Some goes to recharge the battery.

You are a very lucky boy.

On my Ford Focus you can display the near real time figure for miles/gal on the dash. One thing I noticed is with a very light foot and going downhill the display tops out at 99.9 miles/gal. I have no idea if this is just the software limiting the displayed data of if the actual calculation has some limitation when little fuel is being used.

As OP, I think NY was referring to my car achieving 63.5 mpg. My car I can assure you is a 2008 Micra and categorically not a hybrid of any form.

I didn't do that as I was on a motorway. However, I went into neutral (probably not best practice on a motorway) and this did not affect the situation.

The drive computer. No aircon distraction.

The first of these. As a piece of history, I believe the Nissan Micra was the only car ever to exceed its declared mpg figure in Honest John's website. I think it got 101%.

The only reason I asked was that my Honda Jazz reads 9% high for mpg. That is due to an artificially high odometer reading which translates as a higher speed shown on the speedometer.

In that case, the car is only doing around 45 mph when the speedo shows

1. It's probably just Honda being ultracautious in that they could never be held responsible for a faulty speedo contributing to a speeding offence taking place.

It is in Construction & Rules - there is no positive tolerance, so, unless the speedo hardware is unusually accurate, it will always read low.

Chris

Read high, surely? Otherwise customers driving at the speed limit would risk prosecution? Mine reads about 10% high (compared to the satnav, that I assume must be accurate).