How do you work out gearing?

[wanderinstar]

I intend having my speedo recalibrated, but in the meantime how can I work out my correct speed. I don't want someone to do this for me. but tell me how to do it.
I presume the speedo will be accurate if using 145/80x14 tyres. I have 185/70x14 on now and according to one website they will mean I am going 4.6% faster than speedo shows. meaning showing 30mph but actual speed is 31.4mph.
The crunch is I have also fitted a 3.73 diff in place of the 4.22.

I know there are some people out there who can work it out. But how do you do it? Could also do with working out correct mileage for working out fuel consumption.

[bmcecosse]

So - take your 31.4 mph X 4.22 and / 3.73 = 35.52.
ie for every 10 mph on your speedo you will actually be doing 11.84 mph.
So if your speedo at the moment is say 1504 tpm (which mine is) - then to correct the readings you will need a speedo with approx 1270 tpm. (1504/1.184).
Note - it seems when they 'recalibrate' your speedo (at vast expense) it's just a tweak of the spring to get the speed reading ok - but the mileage reading is NOT corrected and will read wrongly. The answer is to get a speedo as near the necessary tpm as possible.
It would be handy if someone would come up with a list of all know 'Minor type' Speedo tpm ??

[Alec]

Hello Ian,

if you look on the face of the speedometer, you will see a number, not sure for a Minor but usually in the order of four figures. This is the turns of the speedometer cable per mile.
Disconnect the cable, and put a little pointer on it (card or the like). Mark the tyre and the ground where it touches and then push or drive slowly counting the turns of the cable. Say twenty revolutions for arguments sake. Now measure how far you have travelled. From that you can calculate turns for a mile. Compare the actual turns to the figure on the dial and that is your error, from which you can work out a correction factor.

Alec

[wanderinstar]

Right, thank you Roy. Apparently the number 1504 is something to do with gears inside speedo. I cant remember the figures that John Ostick said but they go up or down in something like 17 or18s so the next figure down is 1486 and then 1468. That apparantly equates to one tooth on a gear inside. With mine he will calibrate the actual MPH and also the mileometer. It will cost me £90-- I hear you gasp. But a speeding fine is £60 plus 3 points.( I know I have 2) Plus I will then be able to work out my mpg accuratly.

[bmcecosse]

Well indeed in your line of work - you can't take any chances with your licence! You could however just mark the face of your speedo with the new correct speeds in the short term.