speedo problems

[PaulN]

Hi
I have a puzzle
My 1973 stag when doing 60mph is showing 4000 revs
Other stags I have driven show 60mph at 3000 revs

The gearbox has just gone through a major service with a professional who also owns and runs a stag ..his stag also is doing 60mph at 3,000 revs

This mechanic specialises on BW 35 and 65 gearboxes and has checked out

The differential ratio which is correct
The speedo cable which is fine
The colour of the cog on the back of the speedo which is correct

I believe the Stag was originaly fitted with 185/14
My car is fitted with 195/75 R14
Is this causing the difference shown on the rev counter ?

If not ..any other possible causes ? ..perhaps the speedo head its self is untrue ??

Any help or insight would be appreciated..
Regards
Paul

[barkerwilliams]

What speed does a satnav show when the car is driving at 60mph?

https://tiresize.com/tyre-size-calculator/

185/70/14 shows 834 revolutions per mile, 195/75/14 shows 791 revs per mile, or about 5% difference not the 33% you are reporting.

Difficult I know to corroborate but does the rev counter seem to be reporting the rpm correctly? There are cheap electronic rev counters on Ebay for £10 to £15 that couple to the ignition coil and you can jury-rig in a few minutes if you need to check revs.


Alan

[sujitroy]

f this is anything to go by, I have an auto bw35 stag with 185/70 x 14. my rpm at 70 mph is typically 4000 rpm

[Carboy0]

I would mistrust the rev counter, mine was reading 50% low until I had it rebuilt.

[Phil S]

download a speedo app on your phone and then you can check revs against speed against what your speedo reads
my zf auto box is 3000 rpm at 80mph

[Niallcat]

Hi Paul, the mechanicals must be correct, there is no scope for a variation other than minor such as tyre diameter so it must be calibration of instruments as the root cause of such a large variation, try a drive with a friends car to compare speedo or as suggested a back up to the tachometer.

[Carboy0]

As Phil S has suggested, if you have or can borrow a portable SatNav, you can double check your speed by GPS against your speedo.

[PaulN]

As always Im really impressed with the speed and feed back from SOC
Im a sleeping member really but now the car is back on the road Im going to attend a few shows and hopefuly meet some of you guys

I never thought of checking with a sat nav and thanks for the calculation I would have a clue how to do that ..im please its only 5% variation on the tyre size I was worried im burning more fuel because of more revs linked to the larger tyres

When im doing 60mph the rev is def showing 4,000 revs ..but obviously now Im thinking im doing 70 not 60 as the car is revving high it seems to me ..if its the speedo thats out then when im doing 70 im really doing 80 which is very worrying as Im liable for some speeding points

The ZF box is a nice piece of kit I was seriously thinking of it if the mechanic had condemned the gear box ..initially it went to him because it wouldnt change into 3rd unill it hit 3,000 revs but if the rev counter is out then it changing into 3rd at 2000 revs in reality or am i losing the plot ?
It was only when I spoke to him as he runs a stag that the 1000rpm diffidence came to light

My way forward think is sat nav to check speed to put my mind at rest so Im not going to get done for speeding if the speedo is out..I wonder if would get away with a prosecution it if I could prove after the event that the speedo is faulty ? ..just a thought

Buy a cheap rev counter to tie into the coil to check revs .. but technically if the sat nav proves the speed is correct then the rev counter has to be faulty there is no other conclusion
Im picking the car up on Sat Il let you know how this plays out

Cheers
P

[Carboy0]

My rev counter was rebuilt by these people Speedograph Richfield, it’s worth ringing them first. Then send your tacho, they assess and call or mail you. You approve their estimate, they let you know when it’s ready and you pay and it arrives in the post. About a two week turnaround. It seems a bit of an antiquated way to do it, when I asked they said that some people back out when they find out the costs. Mine was £85.00

http://www.speedograph-richfield.com/

[PaulN]

Great thanks Chris Il contact them ..
P

[Carboy0]

Have you done the SatNav test yet?

[PaulN]

Not yet
Im picking the car up on Saturday ..at 10am then a 2hr drive home so I can give it a good test then

[Hughesy]

This may help with RPM vs speed. I used it when I converted my Mk1 Tacho to digital.

[wilf]

Mine works out to 3000rpm at 60mph, so at 4000 it is doing 80, all checked against a GPS.

[PaulN]

Mine works out to 3000rpm at 60mph, so at 4000 it is doing 80, all checked against a GPS.

Hmm.thats what Im worried about..Ive already got 6 points cant afford any more ......Im off to Baldock at 8 to pick the Stag up so il up date the finding here Ive downloaded a speed app to double check actual speed
P