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Rpm Query

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#1 speedwell1


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Posted 09 April 2019 - 08:00 AM

Hi all, my rev counter is a pathological liar, pretty sure it's reading double so I've done some calculations to work out my rpm at certain speeds based on the rolling tyre circumference.
I've got 10" wheels and a 3:765 final drive with a 1:1 4th gear

My calcs are in 4th
2000rpm 30 mph
3000rpm 45mph
3500rpm 53.1mph
3800 rpm 57.6mph
4000 rpm 60.6mph
4500 rpm 68.26mph
5000 rpm 75.85mph
5500 rpm 83:43mph
7000 rpm 106 mph

Do these sound about right? I appreciate there may be a slight variation

#2 nicklouse


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Posted 09 April 2019 - 08:20 AM

put your info into here http://www.guess-wor.../Tech/ratio.htm

 but yes.

#3 DeadSquare


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Posted 09 April 2019 - 08:41 AM



Not far out.


According to the BMC workshop manual,   "Road speed in top at 1,000 rpm" ... 3.675 ratio is 14.7 m.p.h.


2,000 rpm is 29.2 mph and 7,00 rpm is 103 mph.

#4 zero_wlv


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Posted 10 April 2019 - 12:59 PM

Yes,  those calculated speeds are spot-on for a 3:765 final drive.

I've no idea why an original Mini tachometer would read double,   but if you've picked up an aftermarket tacho somewhere that's intended for a 2-cylinder engine,  this would read double.     Also,  I think some aftermarket tachos have a switch or jumper somewhere that you use to switch between 4 cylinder and 8 cylinder (and probably 2 cylinder).

#5 spiguy


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Posted 10 April 2019 - 06:13 PM

Is the rev counter from an MPi but fitted to an SPi / carb car?  If so then it will read double. The MPi has no dizzy and uses a wasted spark setup with two coils, which fire alternately and the rev counter is connected to one of them. Thus the rev counter only 'sees' half the sparks that it would on a traditional single coil and distributor so the rev counter is different to take this into account. 


There was a post back in 2017 which covered this and a claimed mod that can be done to the clocks (use at your own risk etc.).


Of course if this is not a case of MPi clocks in a non-MPi then probably of no interest! 



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