Rev Counter misreading

[Jonny XKE]

My car is a 4.2, is fitted with a megajolt electronic ignition, has a 3.54 diff and a 5 speed box (first four gear ratios are identical to the original four speed box it just has an overdirive fifth).

The rev counter seems to be over reading the rpm.

With the help of my passenger! please see below the rpm's in different gears/ speeds.

Gear Speed rpm
3 50 5000
4 20 1700
4 30 2500
4 40 3000
4 50 3700
4 60 4500
5 40 2300
5 50 2600
5 60 3300
5 70 3700
5 90 5200

Does it seem to be misreading to you guys? The engine sounds like its notrevving at the rpm indicated. The rev counter is original.

[Heuer]

Put your figures through this simple rpm to mph calculator: https://wahiduddin.net/calc/calc_speed_rpm.htm

You need the radius of your tyres, Longstone Tyres have the spec for all brand/sizes of tyres.

[Jonny XKE]

Thanks for that david .. the tyres are 185VR15 and not sure what the transmission details are

[Heuer]

Your diff is 3.54 and 4th gear 1:1 (direct drive). You don't need to bother with the other gears unless you want to know specifically.

[Jonny XKE]

The calculator says in 4th gear at 4500rpm the car should be doing 100mph, when its actually doing 60! I know my speedo is correct as I have measured against a GPS.


Engine Speed
4500
RPM
Transmission Gear Ratio
1:1
Differential Gear Ratio
3.54:1
Loaded Tire Radius
13.3inches

Vehicle Speed
100.6
MPH

[mgcjag]

Do you have a multimeter with an rpm function..if so just clip it onto one of your spark plug lead and run the engine and compare to your dash rpm...Steve

[Jonny XKE]

N I don't but its going in next month for some work so they could try that for me. Will have to get it sent off to b recalibrated I suppose which is a pain

[Heuer]

If your car has the camshaft mounted tach generator then it is the problem. The magnets weaken over time and result in low RPM reading. Repro generators have a poor reputation for failure so see if you can get hold of a good second hand one from M & C Wilkinson or Ken Jenkins. Fitting is easy. Unlikely to be a problem with the gauge.

SNG has them for special order: https://www.sngbarratt.com/uk/#!/Englis ... 55015a3cf6

[Jonny XKE]

I'm not sure but the car was fully restored 3 years ago so might it be something else such as the megajolt causing the issue?

[Heuer]

The tachometer is isolated from all other systems - two wires from generator go to tach, a completely closed system. The Megajolt should have no impact on it but you could connect a laptop to it and it will give you the rpm.

The s/h generators are rare and expensive new so unlikely it would have been replaced. I have often wondered about replacing the magnets or re-magnetising them but the housing is sealed. I had to replace mine.

[christopher storey]

David : this tacho is over-reading not under-reading so poor magnetic flux is unlikely to be the problem. I suspect that with this being a 1968 car it may have the electronic ignition driven tacho . These are notorious for a. misreading where electronic ignition has been fitted as her, and b. having the resistors and/or capacitors degrade over the years. If it is the electronic type, there is a potentiometer inside the case which can be used to try and correct the reading. However, it is unlikely to achieve accuracy throughout the rev range : replacement of the components is needed for this

[Heuer]

In which case it could indeed be the Megajolt. Maybe read the MJ support thread and see if anyone else has had such a problem.

[Jonny XKE]

I'll do that thank you all