When I made SPL boxes, I used to go the opposite, bigger box tuned higher (50-60Hz) for more efficiency with the medium power levels I was running. If your running stupid high power then smaller box is better to get the peak efficiency. the smaller box helps keep a load on the sub to handle the power. If your running stupid high power then your ports become less linear in how they operate due to velocity of the air and non linear compression of the air. Smaller boxes help to keep the pressure behind the sub to keep it from unloading as much when this happens (well that's the overly simplified reason of why smaller boxes work better)
My personal philosophy is bigger box tuned high. You get better efficiency with a larger box and the impedance curve is more predictable, but it also has LOTS more peaks.
Your tuning frequency of the box should be tuned to the peak of the vehicle. The way I find this is to run my computer subwoofer in the car which is a TC sounds sub in a small sealed 0.5 CU FT box. The box is small enough that the bass region is all below the resonant frequency of the box/sub. This gives a perfectly predicitable impedance and with active equalization it gives a completely flat response to 15 hz - 80 hz in open air measured in my back yard. It however is extremely inefficient so it mates well with the 1400 watt QSC pro audio amp it's powered with. The reason for all of this is to see what the car is adding to the sound. If you put a perfectly flat subwoofer with perfectly flat frequency response in the car, you can see what your car is adding to the bass. Also by looking at the impedance curves of the sub while in the car, you can judge what frequencies add or subtract to the loading of the sub in the car. Also since the sub is small, you can position is any which way to see what difference angle, position, etc have on the bass response. Usually, the peak frequency of the car coincides with an impedance oddity with the sub. This poorly describes how I do it, but It works well to find out exactly what your car does to the bass response. However, when you start getting to stupid pressure levels, things become MUCH more unpredictable due to cabin leaks, panel vibration leaks, and even atmospheric pressure differences in these cases, testing and measuring over and over again is the only way to get that perfect edge.
With ported boxes playing in their linear range, the impedance will dip to near DC resistance levels at the tuning frequency if the port is large enough to stay linear. On each side of the frequency band before and after the tuning frequency you'll have a massive peak in impedance around 10 to 40 to even 50 ohms depending on how your ported box is aligned. Avoid these areas because well, your amp wont put out as much power into 10 to 40 ohms... Also, after the peak above the tuning fequency, the impedance dips back down to more normal levels but not quite as low as the tuning frequency. This dip is much larger on larger boxes. After this dip the impedance slowly rises back up to 10 to whatever ohms do to regular coil inductance.
For maximum power delivery and efficiency of the box/speaker make your tuning frequency the frequency which you burp at. This does two thing, it reduces excursion of the woofer as the port loads the rear of the speaker 180 degrees out of phase and it also has the lowest useable impedance letting your amp put maximum power to the sub. This frequency is the most efficient frequency that your box/speaker will play at. It can get loud with other frequencies, but it is more a function of the cabin gain, not the box efficiency.
Also of minor importance, the port will reinforce frequencies that are an even harmonic of the tuning frequency more than the other frequencies. However, its not that much more reinforcement and can usually be ignored.
Best scenario would be to tune your box to the frequency your car excites at. However, all the calculations in the world wont get you peak efficiency because as soon as you load your box into your car and slam 3K to 4K or more watts to the sub, things change. The calculations will get you close, actual measurements and UNDERSTANDING of these measurements will get you there. I had one car where I would measure where I needed to be, built the box to do what I had previously measured, and then put it in the car and found out things shifted because of the displacement of the actual box and proximity of the ports to walls and the change in tuning frequency of the box and loading of the sub... I did this measure, build, then test sequence 3 times before I got it right, each change of the box changed parameters in the car which shifted the peak efficiency frequency each time.
With this system, I hit 153.3 to 153.4 at 44hz every time in a station wagon with one 9515D on one MMATS D200HC in an 8 cu ft box tuned to 44 hz for competition. That's pretty **** good for 1200 watts and one 9515 with big 3 done on stock 90 amp alternator electrical system. You can either slam several kilowatts of power into your subs on a marginally efficient system, or you can tune for absolute peak efficiency and get good numbers with less than 2000 watts.
