Tyre Pressures and Fuel RON

Where Does It State Tyre Pressures?

It’s as simple as looking inside your fuel filler flap. Either side of a crap representation of a smart it tells you the recommended pressures for the front and rear tyres based on the alloys that came with the car.

Adjusting The Tyre Pressures

The first thing you must be aware of is that the tyre pressures must be read and adjusted while the tyres are cold. Hot tyres heat the air inside them. Air expands as it’s heated so can give false readings.

Filling

Nice and simple, remove the dust cap from the valve, fit the pump nozzle to the valve and pump. The pump you have will usually be either a standard foot pump or a 12v compressor pump that attaches to the cigarette lighter socket.

The pump will have a gauge to tell you the current pressure of the tyre. Fill the tyre to the correct level and stop the pump, allow it to rest. After about 5 seconds check the tyre pressure on the gauge. Repeat if necessary.

Depending on how smooth and quickly you can remove the filling nozzle you may want to over fill your tyres by 1 or 2 PSI to make up for the air lost as you remove the nozzle. The tyres must be filled when cold.

Venting

If you over fill the tyre or find it has too high a pressure, you will need to drain the excess air out. Unscrew the dust cap from the valve and you will see a pin in the centre. Pushing this pin in will release air from the tyre, using a thumbnail is the easiest way. Although, expanding air cools down so be careful you don’t freeze your thumb.

Check the pressure using a gauge or a pump with a built in gauge until the pressure is correct. If you let out too much air, simply top up using the info above.

Over Filling

This is a neat little trick that increases MPG but decreases grip. The extra pressure increases the hardness of the tyre and reduces the contact patch on the road. This causes less rolling resistance but also less grip. I have increased the tyre pressure to 36psi in my tyres that require 30psi.

450 – 600cc Smart

This is from John’s (heinkeljb) 1999, 600cc.

Note that there is one tyre size option. 95 RON petrol (or higher) is recommended.

Crossblade

Strangely, also gives you pressures for tyre sizes that the Crossblade never came with.

450 -700cc Smart

This is my from my 2003, 700cc.

Note that there is a choice of three front and rear tyre options. 95 RON petrol (or higher) is recommended.

450- Brabus

This picture was found online by Peter Esser.

451 Smart

This is from a first edition generation two smart.

Note the new smart shows two options depending on the size of tyres you have fitted. 95 RON petrol (or higher) is recommended.

452 Roadster

This is from OnSmart Gavin‘s Roadster.

Note that there are three tyre size options. Also note that even though they had to print these specially for the Roadster due to different tyre sizes, they didn’t change the picture. 98 RON petrol (or higher) is recommended.

453 Fortwo & Forfour

It’s no longer on the inside of the fuel flap. It’s on a sticker on the back edge of the door shut.

Forfour

This is from OnSmart Gavin’s Forfour.

Note that there are different options depending on the number of people in the car. 95 RON petrol (or higher) is recommended on the standard models. On the Forfour Brabus it is recommended you use 98 RON or higher.
Thanks to MA56FMO from Forfour owner forum for that info.

Hashtag Models

Open the left hand front door and look for a sticker in the back of the door shut near the door striker.

PSI is no lo longer shown so time to get your brain unpacked and working for once. 100kPa = 1bar = 14.5psi
So, for example, 250kPa = 2.5bar = 36psi

Aftermarket Alloys Or Wider Tyres

A common question when fitting after market alloys is “what pressure should I put in them”. There are 3 answers that you can consider.

Firstly try 30PSI (approximately 2Bar) all round.
Secondly try the original pressure settings as stated on the fuel filler cap.
Thirdly you can do a little bit of calculation and work exactly what it should be.

More often than not, the actual circumference of the tyre will remain fairly similar to the standard tyres. The thing that changes is the width of the tyre. The pressure that pushes against the road is rated in (for example) pounds per square inch. So, in a tyre with 30PSI means the there is 30 pounds of pressure for every inch of the tyre. When you fit a wider tyre, you want to keep the total pressure the same pushing against the road. Because the circumference is similar, you don’t need to work out the contact patch size or anything complicated like that, you just need the widths.

Go to the Percentage Calculator and in the middle section, put the original tyre width in the 1st box and the new tyre width in the 2nd box. Click calculate.
In this example we started with 185 tyres and changed to 205 tyres. That gives a total of 90%

Take that percentage and put the number in the 1st box and then the existing tyre pressure in the next box. In this case we started with 90% and the original tyre pressure was 30psi. Click calculate.

That will give you your new ideal tyre pressure. In this case, 27psi.

What Fuel Should I Use?

Although it says Unleaded fuel 95 or 98 RON, you can use higher quality fuels like Shell’s V-Power or BP’s Ultimate which are rated at 99 RON. You are unlikely to ever come across lower than 95 RON in most of Europe, American should be wary as a lot of their fuel can weigh in at 88 RON and sometimes lower. Although the big hemi engines can run on that, the smart engine is a little more refined. I would suggest seeking out better grade fuel or using octane booster.

New Fuel Info

Ben Dalrymple contacted me about Total’s Excellium petrol. He says that he regularly experiences an extra 40 miles per tank over other fuels.

Excellium is a 97RON unleaded petrol containing detergents and other magic ingredients. Designed purely for an increased MPG and has too low a RON for some smarts listed above. Well worth a try if you have a Total garage in your area, prices are ‘competative’.

Super Fuels, Additives And Formulas

Personally I have always filled with Shell’s Optimax until it was discontinued, I then went to the replacement product, Shell’s V-Power. Although it seems slightly more expensive I notice an increased MPG making it cheaper overall.

During the year, Shell closed my local station for a refurbishment and I went over to BP’s Ultimate unleaded and recorded the worse MPG I have ever had.

My driving had stayed the same, I had not noticed any performance improvement over Optimax making Ultimate very expensive in comparison. There are many rumours flying about that BP is now no longer selling the original formula Ultimate petrol, apparently it is now rebranded Optimax that Shell discontinued.

Many of the super fuels out now contain detergents that keep the inside of the engine clean, couple that with the higher RON value means it has a cleaner burn. Smart recommend you don’t use any additives, so don’t use them in any of the fluids. If the additives were that beneficial, companies would add them already.