If you have a classic Mini, you probably did not buy it for its gas mileage or at best you simply take it for granted that it is small car and thus must be getting great mileage.
In today’s’ world of sky high gas prices with no end in sight it is not uncommon for owners to be saying to themselves that it is time to drive the Mini daily to save a little money on fuel cost. But what is reality about our little cars? Most of us that have had them for any period of time have spent most of our effort in trying to make them faster, increase horsepower, just to drive as fast as we can for the sheer joy of it. When was the last time you checked your gas mileage?
Checking the gas mileage in many modern cars is a simply matter of pressing a button the one dash and reading the number. The problem with these later cars is that it is difficult to change it much with adjustment or upgrades. We have the opposite problem with our early cars, it is easy to adjust (or miss-adjust) things that can make some very big changes in your mileage but finding a way to measure it in real time while driving is not easy or cheap.
So what are the components that we have control of that will impact MPG? Strange as it might sound, it really is the exact same things that we have dealt with all these years as we worry about horsepower; Ignition and fuel. Before we get into some details we should not overlook one of the most obvious- if your tires are not inflated high enough, they will produce more drag with resulting lower gas mileage. Check your tire pressures and keep them up!
The fuel economy of any Mini can be impacted by a few basics. What is the condition of your air filter? What kind of air filter do you have? The stock air filter is paper and even when new is restrictive. Most stock air filter should be changed at least every 6 months. By the time you can see dirt on them; you have already waited too long. The stock air filter on the Mini, whether it is a twin or single carb set-up, has the paper filter in a plastic or metal housing. An option to upgrade the filter for not only longer life but better gas mileage is to replace the stock paper with a high flow K&N type filter. The K&N filter not only has inherent better air flow but can also be cleaned and reused. The stock filter as used on a single 1 ½” and some 1 ¾” carbs is GFE1008, the upgraded K&N version of this filter is E-2601. Later model fuel injected Minis use the stock filter GFE1143, the K&N upgrade is E-9172.
The most popular way to upgrade the air filter is to completely replace the plastic housing and paper filter with a complete assembly from K&N. Various options can be found on the Mini Mania Website.
Spark plugs should be the next area of attention. While it is very unlikely that a fresh set of plugs will help your gas mileage, it is important to change them simply so you can some method to help determine if the carb is set correctly. A quick inspection of the spark plug can provide at least provide some insight if the car is running rich (poor gas mileage) or lean (could burn a piston). There is not much science in this method as the subjective judgment of color is not all clear. And of course it very much depends on how the car was driven just before the plugs are inspected. In general it can be said if the porcelain around the tip of the plugs are black and sooty (not oily) then the carb is running rich. If the plug is all white- then the carb is lean. It is the ability to evaluate shads of black, white and grey is where the subjective process comes in. More details can be found here.
So now what do you do if you find black or white spark plugs. The Minis’ of all years until they finally upgraded to fuel injection had one sort of SU carb or another. As such, all of these cars have a choice of carb needles and adjustments. While the most common adjustment is the idle screw, the mixture adjustment is next most common. Regretfully, it is also the most commonly miss-adjusted. Reading the color of the spark plugs can help get you in the ball park, but if you really want to get the best mileage, a little more sophistication is in order. There are a number of choices, everything from a complete dyno tune-up to the simple use of an exhaust gas analyzer. The options for the do-it-yourselfer are more limited. One option to consider is the use of a “Colortune”. The Colortune is a unique spark plug with a glass top that shows up the fuel/air ratio in a colored form. More details can be found here.
There have been a large number of articles written on the care and adjustment of the SU carb and many of them can be seen on the Mini Mania Website.
Something as simple as changing your oil could also result in a small improvement in MPG. The Mini is very unique in that the same oil is used for the transmission that is used for the engine. The standard practice is too use a very light weight oil, 5W30, to reduce drag and improve mileage. And this works in a Mini but remember that the oil will need to be changed very often as it get very beat up in the tranny as is circulates to the engine.
If you are comfortable that you have all the easy things under control and you still want to do more, here is a short list of things that will take some real time and effort but are sure to pay back in improved ‘Miles-Per-Gallon’. The transmission final drive ratio in most Minis was set for the highway speeds of the 60’s. With today’s demands of high speed driving, dropping the final drive from a common 3.44:1 to say a 3.1:1 or even a 2.95:1 will yield a noticeable improvement and even a longer engine life.
When building a Mini engine if gas mileage is to be considered be sure to aim for a compression ratio of no more than 9:1. Better mileage and cheaper gas will result. Using a single 1 ½” SU carb on a 1275cc motor will for sure yield better MPG than a 1 ¾”. If you are really serious you will also run 145 tires in place of the more common upgrade to 165 (The wider the tire the more rolling resistance).