MissSpentYouth Posted August 16, 2007 Share Posted August 16, 2007 Hi Guys, I have a 1300 spitfire on a 72 plate with ford 5 speed convertion. It seems pretty high geared how can I tell if I have a 3.7 or 3.9 diff is it stamped on the casing ? My speedo well under reads so cant go by that, It feels like it is going slightly faster than double the rev counter in 5th say 65 at 3000 RPM cheers Ben Link to comment Share on other sites More sharing options...
Deleted User Posted August 16, 2007 Share Posted August 16, 2007 Jack up the rear end, lean in and turn the propshaft one whole turn, count how many times the wheels turns.There is your answer! Link to comment Share on other sites More sharing options...
rotoflex Posted August 17, 2007 Share Posted August 17, 2007 It's the only way to determine it as far as I've ever heard. Link to comment Share on other sites More sharing options...
the_nutter Posted August 17, 2007 Share Posted August 17, 2007 If you can read the numbers stamped on the diff they will give you the ratio:FR is 3.63:1FH is 3.89:1FC is 4.11:1Be aware, if replacing the diff, that there are different sized input and output flanges! Link to comment Share on other sites More sharing options...
Steve AKA vitessesteve Posted August 17, 2007 Share Posted August 17, 2007 I recommend that you count the turns of the output for each turn of the input. It is possible that the diff you are looking at has been rebuilt into a different casing. Link to comment Share on other sites More sharing options...
MissSpentYouth Posted August 20, 2007 Author Share Posted August 20, 2007 Just jacked her up and made a few marks with tip ex measured the circumference of the UJ flange with a tape measure a bit of elementary maths and hey presto a 3.896 diff thanks guys it was running a standard one after all boy a 3.7 diff must be way over geared you would need a V8 to make it sporty :-) Link to comment Share on other sites More sharing options...
MissSpentYouth Posted August 20, 2007 Author Share Posted August 20, 2007 Now that I know what diff I have can someone tell me what the MPH to RPM is in 4th i.e. 60MPH is 3500RPM. That way I can calibrate my speedo it is way off with the 5 speed conversion. I have standard 13" wheels with 155 / 80 tyres. cheers. Link to comment Share on other sites More sharing options...
Deleted User Posted August 20, 2007 Share Posted August 20, 2007 http://www.mintylamb.co.uk/gearspeed/:) Link to comment Share on other sites More sharing options...
Chasj Posted August 21, 2007 Share Posted August 21, 2007 Hi.Sorry not to be too technical,suggest you borrow a GPS system and check the true speed of the car,this together with correct rpm readings will give all information you need.Don't forget when working out information to take into account different sized tyres,and,when using GPS take readings on a straight level road for 100% accuracy.as a matter of interest my MkIV on 165/70 R 13 tyres give the book readings for the standard 155 R 13 tyres,the speedo only being 1 mph out at 30/50/70 mph..Thinking of speedometer accuracy I have driven Skodas for 9 years now,been very good cars indeed.On my first 2 the speedos were 1 mph under reading throughout the range,the same as a friend of mines VWs.no surprise I suppose as the same manufacturer using thesame components.However my new Skodas speedo is way out,reading 34mph at 30 mph and 70 mph at 66 mph,not an improvement,also a pain as the engine needs 98 octane fuel,doesn't half rev.though,good sports engine,maximum torque at 4,400 and maximum power at 6,000 rpm.Despite all its electronic gadgets,traction,diff lock stability etc.,prefer the RWD gadget free Spitfire even if a little pedestrian.Charles. Link to comment Share on other sites More sharing options...
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