Deciphering the curious case of a stalled car with a half-full fuel tank

Fuel gauge.

Fuel gauge.

Photo credit: Shutterstock

Gavin,

Thank you for your enlightening articles on motoring. I have a ‘previously enjoyed’ 10-year-old Toyota Hilux from UK (an Invincible). Recently, the car came to a stop yet the fuel gauge indicated that the tank was an eighth full. Since then, I have had to track the mileage every time I fill the tank to avoid repeat embarrassment. A friend who has the same model had a similar experience. What might explain the fuel gauge malfunction and how can this problem be addressed? 

Asante, Ndwiga.

Almost all fuel gauges are electric and only read when the ignition is on, but you cannot put an electronic gadget inside a tank of explosive fuel, so the electrics bit is mounted outside the tank and gets its information through mechanical linkage to a “float arm” inside the tank (a bit like the ball cock in a toilet cistern). 

As its name suggests, this arm floats on the surface of the fuel inside, and goes up and down as fuel is filled and used. This moves the linkage in the external electric gadgets (there are several different designs), and clever electronic tech “reads” that movement and translates it to the needle on your instrument panel.

So, there are two main reasons why you might get a false reading. Either the electric gadget is faulty (unlikely) or the float is jamming (quite likely), so it reports the decline in fuel level to a certain point and then stops moving although the fuel level continues to fall. From your description, that is the first thing I would check. It is a reasonably quick fix.

Meanwhile, I would advise all motorists to always zero their trip buttons on the odometer when they fill up their fuel tanks, and always do the simple arithmetic of how much fuel has been added and what mileage has been covered to know the regular kilometres per litre figure. 

That not only gives you a good idea of how much fuel you probably have left if the fuel gauge fails, but also gives you a regular report on your fuel consumption rate. If the car suddenly gets much more thirsty than usual, there might be a leak somewhere or your engine might have a problem. The simple ‘zero trip’ habit will give you that useful information with no trouble and at no cost.

As a post-script, you may have noticed that the fuel gauge needle changes a bit when you drive up or down a very steep hill, and even sometimes when you corner or traverse a slope. That means the float is mounted somewhere other than the dead centre of the fuel tank and so moves as the fuel sloshes from back to front or side to side. Some tanks have baffles to reduce this. The most accurate reading will be given when the car is stationary on a flat surface.

Does my speedo know I have fitted bigger tyres?


I have been your silent fan since the time of Camshaft (about 40 years ago!). I was fascinated by you recent reply to a reader called Carol regarding tyre size. Can I be flagged down by police for exceeding speed limit due to bigger tyre size? Please help me understand the relationship between my speedometer reading and tyre size. 

Thanks, Mamadi.

Big tyre

Your speedometer is calibrated for the circumference of the tyre that was originally fitted to the car.

Photo credit: Pool

If you fit a tyre with a bigger circumference, you will travel a greater distance than your odometer records, and will be travelling at a higher speed than your speedometer indicates. 

So yes, in theory you could be prosecuted for speeding even if your speedo needle was not over the limit. In practice, that should never happen, because the difference between your real speed and the indicated speed should be within the “degree of error” that normal policing would ignore, but as we all know, our traffic policing is not normal.

Unless you have also changed the wheel size, and modified the wheel arches of the body work and adjusted the suspension ride height (and perhaps even changed the final drive gear ratios in your differential) in order to fit extremely outsize tyres, it is unlikely that your “bigger” tyre (probably just a higher profile?) has a circumference more than 5 per cent bigger than its predecessor, and certainly not more than 10 percent.

And whatever the degree of actual difference is will be precisely the degree of under-read on your distance and speed dials…and the alteration to your car’s overall gearing.

If the difference is 5 percent, when you are travelling at 100kph, the speedo will read 95kph. If the odometer reads 100kms travelled, you will actually have driven 105kms. All your gear ratios will be 5 percent higher, possibly most noticeable in first gear and fractionally larger gaps between gears, and longer legs in top.

There are many different designs that operate speedometers, using various combinations of gears, cables, electrics, magnets, springs and other obscure techywhatnots. But all of them (except those that use radio signals from GPS satellites) are based on counting the number of rotations of the wheels.

From that, the distance travelled (displayed on the odometer) can be easily calculated if you know the circumference of the tyre. And speed is also derived from the number of rotations in a given time. Modern speedos do that calculation about once per second.

Oil filters are important, go for genuine replacements from the original manufacturer

Gavin,

I am a scanner of the dailies, I don’t usually read a whole motoring section to the word but I find your articles smooth and concise – it leaves some desire to keep reading. You have tackled engine oil, what about filters? I know the prices range from Sh300 at the juakali stand to over Sh1,500 in a dealer’s shop. What should one check? M.

Oil filters are not sexy parts of a car, but they are very important to protection of the whole engine, so start by comparing the difference between Sh300 and Sh1,500 with the tens of thousands of shillings it costs to repair a damaged engine, not forgetting the costs of its progressive deterioration on performance and fuel economy.

Also recognise that oil filters don’t just sieve big visible lumps of dirt out of the oil as it circulates. They should, and need to, trap almost invisible particles of dirt measuring only microns, while at the same time allowing clean oil to flow through easily.

That requires quite sophisticated filtering material, precisely fitted and housed. There’s an equal chance that cheap filters compromise on the cost, hence quality, of those factors, and that the most expensive filters include a substantial profit margin.

As there is lots of cheap junk available, and even reputable brands are often counterfeited, my own policy is to try to always renew filters at every 5,000km service with genuine replacements from the original manufacturer. The price might not be the best bargain, but at least they have a vested interest in looking after your engine for the sake of their own brand reputation.

Which do I go for, a Subaru SF or Mitsubishi Lancer?

Mitsubishi Lancer.

I am an avid reader of your column and offer my regards for the informative reviews. My question is, between a used Subaru SF and Mitsubishi Lancer, with fuel consumption and maintenance as the priority, which one would you advise me to acquire. 

Regards, Rosh


Both of these models have had numerous variants throughout their production history, especially different engine designs/sizes and power outputs ranging from just over 100 bhp to nearly 300 bhp, thus extremely diverse fuel consumption profiles.

The fuel economy difference between manual and automatic (CVT) transmissions can also be significant, so, on that topic, the answer depends on which variants you are looking at.

On maintenance, the general principle is that makes and models and variants that are most common (numerous over many years) on our roads will generally have more available spares from more outlets – both franchised and independent, up-market and jua kali – and that workshops of all styles and scales will be more familiar with them than makes/models/variants that are newer, scarcer or more niche.

Subaru Forester
Photo credit: Pool

So look around. Broadly, the Subaru SF and the Mitsubishi Lancer are not consistently so different in economy or maintenance for those to be a clear determinant of choice. Their image (especially visually) is definitely different, as distinct as town-and-country in an emotive sense, and that is up to your personal preference, lifestyle and location.

If your third priority after fuel economy and maintenance was all-wheel drive or luggage space, I could give you a more definitive answer. But you surely already know that. Personally, I would have no dilemma. I live among 10,000 trees I have planted on the side of a mountain. A young urbanite would likely make the opposite choice. So, what is your music – folksy or jazzy?

Check your brake discs…


Hi, 

I have a 2013 Prado V6 which vibrates/shakes when I brake. What could be the cause and what do I need to do to stop the problem. Your kind assistance will be highly appreciated. 

Regards, Morrison W

If the shaking only occurs when you apply the brakes (at any speed) the chances are that one of your brake discs is bent or its surface is damaged, so it grips and slips intermittently. Replacement is the probable remedy.

There may also be associated problems, such as faulty control arm bushes, or other related defects that allow the front suspension to jiggle back and forth under braking loads.

 When brakes have a problem, never put off getting them fixed. Brakes are very near the top of the safety-critical list.

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