everything you do to a car usually has effects some maybe you wanted and thought about and others maybe you didn't. By it's definition the more efficient you make your engine, transmission or car the more fuel you can save but if you use this to accelerate more quickly or go faster, then you use more fuel (more efficiently !). You make the ultimate straight line speed car but it doesn't handle. Everything has it's pluses and minuses your first job is to work out what you want from each mod and do the pluses as you see them outweigh the minuses. Trust your own feelings don't blindly believe people who tell you what they did, no 2 cars of the same type, model or year have been the same and to do the same mods to both would produce 2 different cars. Get peoples input and try things you think might work maybe even ones you don't think will work if the outcome is worth the effort, remember it may not do what you expected but might have some other effect which could be a plus.
 
 
Air filters
    - run the biggest you can fit, if you run ram pipes inside make sure you have a distance of at least 1.5 of the carb diameter of clear air before the top of the filter. If you can't achieve this get shorter ram pipes, don't run with out air filters, cheap way to increase air flow yes, but also quick way to destroy engine.
 
 
Brakes
    - if your increasing the power/performance of your car you probably need to increase the effectiveness of your brakes. Look at other cars in the range you might be able to swap bits with ease.
 

Cooling fans
    - loose it! This eats power and does nothing useful most of the time. Get a cheap (£5+) second-hand electric one from the scrapyard, either use a temperature sensitive switch or a switch on the dash. I usually just switch mine on near the end of a journey or coming traffic queues. Figures vary on how much a fan blade cost you in bhp, but it all adds up, it also means your engine warms up quicker.
 
 
Dynamo cut-out
    - while it's charging a dynamo will sap 1-2 bhp,  basically you want to stop this when you need the that power to accelerate.
    Easy option is to miss out the f (field/thin) wire between the dynamo and control box and replace it with a switch on the dash. You then just have to remember to switch it off say during a race and turn it back on during the slowing down lap.
    More complicated is a pressure (vacuum) switch so that when the vacuum in the inlet manifold drops to between 0-5 in hg it cuts the power to the field wire. This means under max load/full acceleration the dynamo will stop charging and as you lift off the throttle it will start charging again.
 
 
Drag - frontal area
    - reducing this will increase your top speed and acceleration over 50 mph. Things like exterior mirrors (check age of car as these can be a legal requirement), bumpers, door handles and even aerials. Stand in front or behind the car and look at what sticks out!
    Another way of reducing the frontal area is to lower the car (this probably doesn't work if it's a 4x4 off-roader), if your car is 50" high and you lower it an inch that's 1/50 or 2% reduction in frontal area not much you may think but it will make a difference.
 
 
Drag - aerodynamic
    - like frontal area, reduce it and you can increase your top speed and acceleration over 50 mph. Unlike frontal area you can't just say take that off it will be quicker. Do you drive with the window open, is the top down. Wind noise is probably your engines power being converted into sound! You could even argue for the highest polish finish, flush fitting glass etc.

    as an example when ford added a second door mirror as standard to the xr3 they had to add extra spoilers to the front to get the drag figures back down and the top speed back up.  
 
Drag - general
    - an overtightened or under lubricated wheel bearing, a dragging brake shoe all sap power. If something produces more noise or heat than usual it's your power being being wasted. The classic being under inflated tyres, every time the bulge in your tyre moves a bit it produces more noise and heat and waste your power
 
 
forced induction
    - no not super chargers or turbos, think about the air travelling around your car, why not make a sealed air box and use the cool external air to pressurize your carb.
The obvious place to feed your carb air from would be the front by the side of the radiator opening. The less obvious is just in front of your windscreen (why are heater/air ducts put here) this is usually a high pressure area, on a rainy day (with someone else driving) watch what happens to water on a bonnet. Usually at 40+ mph the water will come reasonably straight down the bonnet then as it approaches the windscreen it will go sideways to the outside of the car, this is where the high pressure area starts (any cooling slots in your bonnet here could be having the opposite effect.
 
 
Electronic distributors
    - there are many of these on the market and people more than often go for them. Don't forget the modern version of your engine probably had an off the shelf electronic dizzy which was designed to go on for years with little to no maintenance and will come straight out of the scrapyard block and run.
 
 
Fuel filters
    - they don't give you anything but could save you a fortune. I always fit one between tank and pump this has two advantages. It's not pressurized and the pump doesn't get clogged up.
 
 
Locked diffs
    - unless your an ex-kart champion i'd leave well alone, cheaper than lsd yes but one or both of your tyres must be slipping whenever you turn, can cause understeer.
 
 
Lowering
    - can be a great way of improving handling, reducing frontal area and drag but remember speed bumps, ramps and all the other things that hit your sump, exhaust or various other bits. Also remember to check clearances on the car do your drive shaft clear everything are you actually sat on your bump stops instead of your springs (i was). Lowering the front more than the back can increase high speed stability but may cause more drag?
    Remember after altering your suspension your castor, camber and toe-in have probably changed for the worst or best? Check it!
 
 
Oil
    oil coolers - worth having in most cases. Usual problem is where to fit them when i ran out of the obvious i removed the heater matrix and fitted it in there, this does make for a very warm car in the summer, so not for everyone. If you can afford it always fit it with a thermostat, oil that's too cold is too thick and doesn't work properly.
    Oil filters - the biggest you can fit is usually best this allows best flow/pressure drop and some say helps cool the oil more, debatable.
    Oil pressure - how much do you need? On an 'a' series 60 psi is plenty anymore is a waste of bhp. Two choices here adjust the pressure relief valve or as some mini racers do modify the pump to reduce it's capacity thus a little more power to the wheels.

 
tyres
    - or maybe wheels and tyres

    wheels - don't have wheels that look great but are the wrong width for your tyres!
 
     Tyres general - try to have the same tyres across each axle if you can't get the same make/type check the size, rolling diameter, compounds,  and wear rating if any of these are different buy a pair otherwise you'll have different grip on each side!  
 
    Tyre pressures - i don't run the recommended pressures, vary them a bit at a time check how they wear, how they handle at different pressures. Don't be stupid about it just because race tyres run very low pressures road tyres won't they'll tear off of the rim!
Having said that if your sensible about it you might be surprised at the pressures you end up with. Don't forget unless your doing the indy 500 you'll want to keep the 2 front tyres at the same pressures and the same with the back ones. Finally just keep checking them they'll change with temp, weather conditions and you might just spot a slow puncture?

    tyre sizes - generally go for the widest, lowest profile if you want good acceleration and handling. You may find you want wider tyres on the rear - be aware if you have to drive home with a spare that a different size the car will try going sideways when you brake or accelerate hard so take it steady.

    tyre compounds - it you want to go around corners fast you want the softest compound you can get. If you want to sprint or hill climb that's great but if then want to drive it round the track a few time you might have no rubber or grip left after a couple of laps. Look around see what others use ask around the tyre people.

    second hand tyres - i've been using them for years. You have to trust the people who sell them, there are many garages that sell them and it's useful if your trying a tyre that your not sure is going to work on your car.

    try it i ended up with narrower, softer tyres on the front than the back it gives me the feel i want but that might not be for you.

    remember any change in tyre size or profile will make a difference to your speedo, brake effort, acceleration and top speed.

    someone in motor racing once said the most important thing on the car was the tyres it doesn't matter how much power you have or how good your suspension is put on a poor tyre and you have a poor car.
 
 
Water injection (but not really)
    - have you ever noticed how your car may seem to have more grunt in the wet? That's because it does. A cheap use of this fact can be as simple as using an electric screen wash pump connected to a paint spray nozzle at the air box intake. Try it be surprised!
 

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