Speed and engine speed...

[Allister]

Hello,

One of my customer owns a 1969 E-type 2+2 4.2L Jaguar with standard specifications, that are 3 S.U Carbs, 4-Manual Gearbox and a 3.07:1 differential ratio.

The speedometer has been recently overhauled and works perfectly. The speed indicated (in km/h) matches with the speed given by a Sat Nav App.

The REV counter has also been recently overhauled and works very good. The engine speed displayed matches with the engine speed measured with my stroboscopic lamp.

From there, who could explain to me why the rev counter displays 3000 rpm for a 200 km/h speed ?!?

IMG_0630.jpeg (113.25 KiB) Viewed 6496 times

Thanks

[cactusman]

Afraid the obvious answer is that one instrument is wrong. 200 kmh is about 125 mph so unless you were on an autobahn best say nothing!

[Allister]

Thank you Julian for your answer. The speed limit in France is 130 km/h. I suppose that my customer confounded km/h and mph...

Concerning my problem, could the tyres' size explain the situation ? They are 185 P15...

Anyone knows another way to check the engine speed than the stroboscopic lamp ?

Julian (too !)

[cactusman]

I'd start by using a gps and see if indeed the Speedo more or less keeps accurate up to say 130 kph....ignore the tacho. If it does then the tachometer is under or over reading. If the Speedo over reads then either the tyres are very wrong sized or much more likely your speedometer is calibrated for a different differential ratio of gearbox angle drive to the one you have. Pretty sure a six pot motor generates three spark events per revolution of the crank.If you are using a tachometer setting on your strobe or auto multimeter make sure it is set to six cylinder or you tachometer measure will be way out.

[Allister]

Thank you Julian.

I will make a new speed test soon and will let you know.

Kind regards,

Julian

[malcolm]

Yeah, I'd say speedo was wrongly calibrated. 3000 revs is a fraction over 70mph, with standard tyres on a 3.07 diff. or approx. 115kmh.
You presumably knew roughly how fast you were going when you took the pic, i.e nowhere near 125mph?

[Heuer]

Try using this nifty little RPM to MPH calculator which takes everything into account: https://wahiduddin.net/calc/calc_speed_rpm.htm

[malcolm]

David, what is the loaded tyre radius of a standard 185/15 tyre?

[mgcjag]

Hi Malcom..there isnt a standard 185/15 it depends on manufacturer.....Loaded tyre radius is from center of hub/wheel to ground...as the tyre is loaded with the weight of the vehicle this will be smaller than center of hub/wheel to top of tyre.......so the distance a tyre travels on the road is calculated by its loaded/rolling radius......Steve

[malcolm]

OK, thanks Steve. Tape measure out! As a follow-up question,am I right in saying that the transmission ratio in top gear is 1:1?
Trying to use David's calculator.

[mgcjag]

Hi Malcom...if you have a 4 speed yes..1:1 its in the service manual....Steve

[malcolm]

Yes, thanks Steve - I was only being a bit lazy; my manual is in the car which is currently enveloped in a big cover! Interesting using the calculator; just 0.1 of an inch difference makes a difference of 0.5 mph at 70 mph. So tyre pressures can be quite impactful (is that a word?) on true speed no matter how calibrated/accurate your speedo might be.

[mgcjag]

Hi Malcom...a mechanical speedo is never 100% acurate.....if you have one recalibrated you need to give a distance traveled by the rear wheel in relation to a specific number of turns of the cable at the speedo end....as you have found out tyre pressure can be impactfull ...pressure is also affected by temperature....Steve

[Series1 Stu]

......... and the rolling radius increases with road speed due to the effects of centripetal force. This is more prominent on the older type of high aspect ratio tyres where there's a lot more sidewall flex.

Regards

[Allister]

I tried to use the calculator but I failed !!

My RPM is 3000, isn't it ?
My Gear ratio (in 4th) is 1:1, isn't it ?
My Differential ratio is 3.07:1, isn't it ?
My Loaded Type radius is... ?

Help please !!

The speedometer can't be wrong. It's very easy to test it with any Sat Nav App...

Julian

[malcolm]

As above, you'd need to measure the loaded tyre size, but as a guide my tyre size, (radius), on standard 185/15 vredensteins, is 12.5 inches.(I took the covers off Steve!) Put that into the calculator along with the other figures you have listed and you'll see a speed of about 72 mph

It is certain that either your speedo is wrong (likely) or the rev counter is wrong; but if the latter, were you really doing 125mph??

They can't both be right!!

[mgcjag]

Hi Julian..yes...yes...yes(assuming you have a 3.07 diff)...measure from center of your spinner to ground..eg a Vrederstein 185r15 has a diameter of approx 25.24in..so on the car the radius would be around 12in i entered these figures and get 69.8mph.. Steve
Ps...what tyres do you have..you mention 185p15.. is that a typing error

[steve3.8]

Alister

The ratio between the rev counter and the speedometer in 4th gear is a fixed given , tyres play no part in that . If you are confident the overhauled speedo and rev counter are correct i can only suggest the gearbox angle drive has maybe been swapped for the wrong ratio ? as 200kph @3000rpm is well fantastic .

[cactusman]

Certainly is Steve The workshop manual has a comprehensive list of engine speeds vs road speed for various gears and differential ratios if i recall right....assuming of course your angle drive is correct!

[Allister]

Good evening guys,

The angle drive is an interesting trail to explore ! Usually, when an angle drive is used, it has a tendency to not make the cable turn and so the speed displayed is under the real one, even no speed displayed at all !

Is the angle drive the same for all the 4.2L manual gearbox E-Type ? The E-type that we are talking about was an American one (Automatic Gearbox, Zenith Stromberg carbs, 3.54:1 differential ratio) on which the gearbox, the carburettors, the differential, the speedometer, etc have been changed to turn it into an European version.

Julian