SOT: MPG and differential

Sorry, I missed that aspect.

That was my thinking too.

My mistake. :-(

Chris

Typically that will be pulses off the wheels or prop shaft so is subject to variation in tyre circumference. Its probably closer than the speedo though

That's the way I checked my actual speed, although you tend to err on the side of caution if you're not exact as the motorway "mileage" markers are 500 metres apart. So if you measure the time between four markers, you've done only 1500 metres, whereas a mile is 1609 metres. Your speed will be 7.25% "high". If you're trying to go by a steady speed indicated on the speedo, it's even more confusing as you'll have to add the 7.25% error to the 10% or so error the speedo is already showing!

The little marker posts are 100 metres apart, so you time 16 intervals, which is a mile to within about 0.5%.

nib

Yes I was.

I still don't understand how PHEVs can be more efficient that pure petrols because the energy to recharge the battery for the bursts of acceleration in between the low-power cruising still has to come from somewhere - the petrol.

On my car, the equivalent real-time consumption is a reported 999.9 mpg.

It is interesting to see the difference between

- lifting off the power when the car remains in gear (reported 999.9 mpg because the inertia of the car is keeping the engine turning so no fuel at all in needed)

- lifting off the power and pressing the clutch (reported 300 mpg approx because the wheels are not turning the engine and so a small amount of fuel is needed to keep it idling)

It's wheel rotations, but if you install the factory size of tyres it knows what the circumference is. Obviously temperature and pressure are going to vary it slightly but only 1-2%, not 10%.

I would guess that if you drive 100 miles according to the map the odometer is going to go up 100 miles, not 91 miles. If you drive it at a constant indicated

Theo

I use a GPS speedometer app (there are many of them) to check my speedo. Mine reads about 5 mph high at most speeds - I've checked 30, 40, 50,

They get it from wall charging the battery. (PHEV = plugin hybrid) The EV part is ~90% efficient, the petrol part is ~40% efficient, so you get some mix of efficiency depending on your usage.

Non-plugin hybrids have a more efficient drivetrain than a traditional petrol because they run the engine at its most efficient part of the cycle and recover energy from regenerative braking. They also allow the engine to be smaller because acceleration partially comes from the battery, rather than fitting a thirsty big engine just for peak acceleration.

Theo

There are several ways that hybrids might benefit:

1. Regenerative braking, energy that would be wasted in the brakes can be recovered for re-use

1. Engine run only in most efficient way, the engine is never allowed to idle or to run at light loads where it might not be so efficient; the electric drive is used in these cases, or the engine is run but at higher output recharging as well as driving

2. Engine optimised for steady high load, the engine doesn't need to be flexible at all loads because starting and low speeds are handled by the electric drive, so the engine can be optimised for constant speed and load, typically with an Atkinson cycle

1. As the electric drive adds to the petrol engine for peak acceleration, the petrol engine can be much smaller than would be used with a normal drive, and can be better optimised to normal driving rather than peaks

Cheers, nib

Although in my example it was for a PHEV used in normal hybrid mode, ie no added power from external charging.

nib

Modern petrol engines have some optimisations. Mine is a 1.5l 150PS turbo automatic which often runs in 2-cylinder mode. Going downhill it disengages the clutch when in "eco mode". It also makes aggressive use of stop-start operation. It often claims about 50mpg on longer journeys and about 45mpg for short trips. It has plenty of acceleration at all reasonable speeds. John

on my EV, indicated 30mph shows as 29mph on speed signs. No need for any extra work on my side.

I have not found these to be very consistent. There was a period of a few months when I drove past one three or four times a week, it often showed the warning when I was 3 or 4 MPH below the limit. At least once it stayed dark when I went past it at around an indicated 35.

In the mid-70s I coasted downhill in top gear (for engine braking) for over mile in my Viva HB with the engine switched off. I wondered how much petrol I would save. As it happened, the answer was a minus figure

- I had forgotten the HB had a mechanical petrol pump and when in gear it was continuously pumping petrol into the cylinders! But not only was it doing that, the cylinders were so badly flooded it took ages to get the plugs dry and starting to fire again. I must have used a lot of extra petrol getting the engine to restart. :-(

You live and learn...

Good luck trying to keep the count right and, at 60mph, keeping your eyes off the road for most of a minute looking for the next post.

I use similar speeds (32, 43, 54, and 65) although with the Jazz it's the automatic limiter which is set to those speeds.

Well yes, not so easy with just the driver, this sort of thing is much better done by a passenger. When I used to car-share to work on the M1 we checked each car we used this way, its quite easy, and we used a stop-watch to check the speedo as well. We'd also try reading the numbers from the posts at entry and exit to get a longer sample. There were no 500m signs then.

nib