sTTu,
Don't panic mate! Sciatic pain is almost always nothing to do with your leg itself, but usually something pressing on the sciatic nerve anywhere between your brain and the uppermost point where you feel the pain. If standing brings relief, and sitting pressing the clutch pedal is worst, I would suspect lower back...
I have had 3 prolapsed ('slipped') discs in the past - none caused by the car, but couldn't drive any manual car during the periods of extreme pain. Be very very careful if yours is a prolapsed disc don't exercise AT ALL while it is painful, try not to sit still for any length of time, and if you must sit, try and recline as much as you can (lower back pain increases as the angle between thigh and torso decreases), else it will make it worse and delay your recovery. If you have private health cover, ask your doc. for a referral to a specialist for an MRI scan (if no private health you will probably be better and forgotten all about it by the time you get an appointment on NHS!!!)
My vertebrae is now permanently weakened, so I have to keep my back muscles in shape to prevent re-occurance. However, I find the Porsche sports seats the most comfortable BAR NONE - so don't change your car until you are sure that is the problem! My car has rock hard suspension (lowered chassis, GT3 wheels and tyres) and I have driven to and from Scotland 5 times since, and even Monaco and back, all non-stop (apart from fuel stops of course) and all without pain and without pain-killers.
Ironically the car I couldn't drive when I first 'slipped' two discs, was a MB 230TE automatic - so a 911 tip will not necessarily make any difference!
Ibuprofen (i.e. Neurofen) does help, but if it's a 'slipped' disc you may need something stronger on prescription and to rest it... I literally lay on the floor 24/7 for 6 weeks (apart from essential 'crawling' to the bathroom) before I recovered the first time around, but within a couple of months of that I manually lifted and stacked 750 bales of hay off of our field in a couple of days without a problem!
I've gone on a bit, but get it checked ASAP!!! If it is a prolapsed disc, the longer you leave it, the longer it will take to recover (unless you go for surgery of course).
PM me if you want more detail...
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