Mathematics and Griff Speedos

[dnb]

This is a strange co-incidence...

I'm designing a new dash for the Griff at the moment, so am doing some maths to work out some component values. (In case you want to know how I discovered this...)

It turns out that when you use standard sized tyres, the 4-pulse-per-rotation speed sensor on the Griff produces a frequency that is numerically equivielent to the vehicle speed (in MPH) less EXACTLY 10% EG at 30 mph, the reading from the sensor is 27Hz.

Is it a co-incicence that every Griff I've seen has a 10% speedo error?
Or is this one conspiricy too far?

[2Munkys]

Dunno what it is but when I go through the Quarter at an indicated 120 I get a ticket for 112.5. 245/45/16 boots on the back.

[antonyj]

Is the accuracy of the tacho more reliable?

If so..........Have a look at this

My speed is way under if I use the tacho,

[mikedavis1]

Don't know about the Maffs but I get up to a 10% error comparing the speedo to GPS: Speedo = 70, GPS = 66 : Speedo = 80, GPS = 74: speedo = 100, GPS = 91 etc etc

I kinda round things down a bit on the dial...

Is this normal?

M

[nicbowman]

It is some kind of legal requirement to not allow the speedo to underread, or you can get off a speeding ticket. So manufacturers deliberately set the speedo to overread by 10%. It's pretty normal.

Nic

[ruaricoles]

According to our GPS thingy our Griff speedo (1994 500 with 245/45/16 rears) is actually spot on. Although the speedo itself has been replaced twice for some reason by previous owners.

Perhaps the exception proves the rule....

[jeffsy]

dnb said:

It turns out that when you use standard sized tyres, the 4-pulse-per-rotation speed sensor on the Griff produces a frequency that is numerically equivielent to the vehicle speed (in MPH) less EXACTLY 10% EG at 30 mph, the reading from the sensor is 27Hz.

...yeh, wot he said really

[Ozstyle]

Hi,
I did some maths on speedo readings(using gear & diff ratios and tyre circumference), worked out speedo should read approx 28.6 mph for every 1000 rpm in 5th for rover gbox. For early T5 gbox 31.0 mph for 1000 rpm in 5th, late T5 box same as the rover - all using 235/50/16 rears. All corresponds to the table in the bible. I found this info usefull when trying to determine the accuracy of my speedo. But don't reckon my Griffith can hit 186 mph at 6000 rpm!!!

Ozstyle

[virgil]

All speedos can read up to 10% over (to compensate for wear and not put the driver in danger of speeding inadvertantly) but can not read over.

I do wonder if TVR used this margin to massage the performance figures a little. The HP reads higher, the speedo higher etc.

Mine too reads 10% high according to my GPS.

[dnb]

Virgil - exactly my point... 1Hz per mph is a pretty good engineering approximation