I'll break it into two parts and hope I don't confound things.
First, in reference to gearboxes with your truck and planes. Most prop planes have a single/single-set gear reduction. From memory, but I would need to look it up to be certain (and coincidently I will be looking those sections up hopefuly soon when I get three prop blades back from Texas anydaynowortwoweeks... 
), a Packard built Merlin mustang has a 1:2 fixed ration with the prop, meaning for every two full circular roations of the prop, then engine goes through one cycle/rotation.... prop strikes are very bad because at best you maybe bend a prop and sheared some of the reduction gearing before bending some rods or worse, and on the opposite side of the spectrum you can overspeed in a steep dive - unpleasantries avoided by your land-loving truck thanks to neutral and the ability to shift up to 5/6 gear while pushing mach 75+ on the highway.
Manifold pressure... uh-boy, lets give it a try.... In a nutshell, it's measuring atmospheric/air vaccum "pressure" the engine is intaking/consuming/sucking-into-the-carb. Mind you, with engine off on the ground you'll get a reading of the neutral/natural-atmospheric/barometric pressure (~28.6" or whatever the weatherman says it is at the moment). While at idle, you're starving an engine of something to "idle" it back (usualy air with the throttle back), so normal/average reading at idle would be ~10-15". Throttle is one of three (the other two being prop pitch control and your fuel mixture control) controls and governs how much air/atmosphere you're letting into the engine - MP tells you how much air your throttle setting is letting into the engine which can vary with a number of circumstances, so it is not accurate to describe it as a "throttle position indicator".... Confusing enough yet???.... here's a great in debth explanation: http://www.askacfi.com/421/what-is-manifold-pressure.htm
Take the good information provided by Vinkman and it starts to make sence why you wont get much more or less than mid 30"s and how the german 0.9/1.0/1.1 ata converts.... now, I msut admit, clueless am I about how precisely the british boost system works but I'm assuming it has something to do with their metric "bars" for pressure measurements.
Thanks, actually that does make sense. Vinkman's post and your post answered another question regarding the meaning of ata settings vs pressures in US rides. I assumed as you said that british readings were in bars also. I didn't however know what ata was...which appears to be a relative measurement to atmospheric pressures if I understand correctly rather than a more direct indication given in US and british planes.
The manifold pressure indication was confusing to me because I never realized that in aircraft you're able to control more things vs just throttle. If I got this right then in a modern vehicle I control throttle...which is fuel and air. The modern vehicle controls the ratios and all I see is engine rpm increasing or decreasing. With an aircraft I also control those things but I have instrumentation telling me how much air I'm giving the engine....manifold pressure...which changes according to alt. because at higher alt.s the air is less dense therefore for a given amount I'm letting in then pressure decreases for the same throttle settings. I had forgotten about mixture control...something I don't directly control in a modern vehicle or in AH. Does AH then vary mixture automatically? Do you have to mess with mixture much in a real plane or is that something you rarely adjust?
Thanks all
