Thank you very much for figuring that out for me. I would agree that i have a 4.1:1 diff like you said because the car did come with a rollbar and racing harness when i bought it, and if i remember correctly the person i bought it from said that the previous owner to him built it for racing. With that information and you saying 4,000 Rpm for about 50 Mph i would assume that my speedo is off, giving me the 4,000 rpm for 40mph that i previously said.

Edit: If this post makes no sense I will reword it for you.

OK I've finally got it. I get wheel/motor of 1.74 which is more or less 1/2 of 3.44 because only I wheel is turning with my method. Pushing the car like Cheleker has both wheels turning. So Evan if you get 2 that means you probably have a 4.1 :1 differential which is really for racing to maximize acceleration. There's a list somewhere of speeds for different diffs. I estimate, from a 3.44 diff that gives 16 mph/1000rpm, that a 4.1 gives 13mph/1000rpm or 52mph at 4000rpm. No Minis came standard with a 4.1 diff. If you have one your speedo is going to be very inaccurate. There is a number after the serial number on the speedometer that is the turns/mile specification. The most common diff is a 3.44 that matches with a 1280 turns/mile speedo.

Cheleker, my method seems to have a 1/6 factor due to the operation of the differential. If I multiply my 5.75 turns by 6 then divide by 10  you get 3.45 which is just about right. Maybe the the ratio of diff gears is 6:1.

Helpful to have two people. Car on the ground in 4th gear.

Mark the crank pulley and a corresponding point on the timing chain cover, or, if fitted, set up the pulley opposite the TDC mark on the cover.

Mark a front tire in 1/8ths with 12 o'clock marked bigger. Chalk works.

Person in front pulling the car forward and counting crank rotations. Go 10.

Person on the marked tire side counting rotations and partial rotations.

Divide 10 by tire rotations to get final drive ratio. E.G.: just short of 3 turns = 3.444 final drive. A 4.13 final drive will give just short of 2.5 turns and just over 3.5 turns will mean a 2.765 final drive. 3.1 and 3.2 are harder to tell, but it can be done.

Repeat to check numbers.

Will work for all Minis except the very few 998s that were built with other than 1:1 drop gears and cars modified with non-1:1 drop gears.

There are lots of variations to the above that can work.

This was my idea based on knowing (1) the 4th gear gear ratio is 1:1 and (2) the drop gear ratio (primary gear, idler gear and first motion shaft input gear) is also 1:1. Therefore, 1 turn of a front wheel the motor should turn the motor the same number as the diff ratio. I should have said monitor the turns of the crank pulley NOT the fan. Watching the fan would work if the fan pulley and the crank pulley are the same size. However, my theory doesn't work anyway. I tried on my Cooper S with a 3.44 diff. Its easier to monitor turns of the front wheel for each turn of the crank pulley and I did 10 turns of the crank pulley to make it more accurate (more turns would result in even higher accuracy). For 10 turns of the motor I got 5.75 turns of the wheel or 1.74 motor turns/wheel turn. I apologize for misleading you with my faulty thinking. This method could still be used to determine the diff ratio if other measurements were available for different diff ratios. It only takes a few minutes and I would recommend doing, say 100 turns, for better accuracy. Somebody more versed than me in Mini transmissions and gear ratios could probably also work out this out from basic principals. If you redo the measurements the same way as I did then we could tell whether you have a 3.44 diff. Any differences could be used to predict your diff ratio. 

As an aside,  what color were the plugs when you removed them?

What are you using for a speedo? And what are you using for a tach? A 4.35 CWP is 53 mph at 4000. It's rare to find a CWP any shorter than a 4.35. I'd think you are clearly doing something wrong. Steve (CTR)

I had an earlier post asking about my Minis top speed. Thanks to the people who helped answer my question we figured that I had a 1275 A+ engine, but the question as to why the car wouldent go over 75 Mph was still at hand (I would go about 40 Mph at 4,ooo RPM). I was told I could check my FD ratio by jacking up one front wheel, take out the spark plugs, and put it in 4th, spin the tire and count how many times the fan turned.

After doing so the fan spun two times for every one revolution of the wheel. Now I'm stuck wondering what exactly is my ratio, and if the speeds im producing are correct for said ratio?