Speedometer accuracy

[stevie-a]

I have just done a 700-mile run in the car this weekend, all went well.
One thing was when I was sitting at 70mph on the motorway, I felt as if everyone was passing me

It was too many people for it to be everyone was speeding, so I reached my destination which was Gaydon in Warick for a big car event I have always attended.

on the way back I set my phone sat nav to read actual speed as well as seep limit.

Well I was shocked how far out my speedo was:

My car mph .............. Actual mph

60................................53

80................................71

70................................61

Are the all this far out

[Tuga2112]

i dont get overtaken a lot when my speedo reads 70. so i suspect not... but then again. up north maybe just everyone has slow speedos

i have checked that comparison with google maps before amd never noticed a drift of more than 6 to be honest. but i really nevr fully paid much attention to it.

[dapinky]

Are the all this far out

Mine isn't! - at 60mph (on the satnav) it is reading 60 on the speedo. Other speeds are a little out, but pretty much accurate all the way between 30 and 90 (which is where it needs to be accurate).

Of course, it wasn't always like that, and when I first got the car it was similar numbers to your results (except I never tried them as it was before Google Maps was around on a mobile phone). It is a simple (but awkward) thing to rectify...... and you only get one point where it can be 100% accurate, AND it remains so only until the tyres wear a bit, get replaced or change pressure!!!!

Basically, I set the tyres at the right pressure and drove at exactly 60mph - easy to do with satnav and a straight road, but was more difficult back in the day with a measured (exact) 1 mile distance and a stopwatch..... it took me a few passes to get it right, but now it is a 1 minute job of keeping a constant speed and then noting the exact position of the speedo needle.

If the needle shows 65mph at 60, then you need to remove the speedo from the cluster (after getting the cluster out of the car), dismantle it, and then manually move the magnetic disk until the needle indicates 65.......mark the position of the disc relevant to any point on the housing (sharpie pen)...... carefully pull the needle off the spindle..... align the markings on the disc/housing, and maintain that position whilst carefully replacing the needle whilst it points to 60mph marking..... reassemble the lot and test it again. It will certainly be a lot better, and at some point in the useable range it should be dead right - job done.

[FunElan]

My federal is the same - I reckon it reads 10% too high, which I've had happen on my BMWs before also. Just a matter of doing some quick math

[johnj]

Stevie-a my speedo figures are similar to yours. On the plus side speeding tickets shouldn't be a problem

[MPx]

Surprised you were consistently being overtaken - nearly all speedo's are hopelessly inaccurate. Pinky will know the detail better than me...but my understanding is that the law says that speedos must never read slow...but can be up to 10% fast. As a result all speedo's err on the safe side of reading fast even if not 10% - more typically 5%. Of course Lotus have always embraced reading fast because it makes their cars appear faster - so go for the full 10%. My Esprit clocked 150 once, but I know it can't get anywhere near that IRL.

[dapinky]

Mike is right with his reasons and numbers - but I think that he may be crediting Lotus with more 'thought process' than existed.....

Bear in mind that the speedo of the era was a basic mechanical item and it relies on a purely mechanical gear ratio between the gearbox and the speedo drive, then friction losses in the cable, and finally, magnetic 'pull' at the speedo itself - so it is unlikely to ever be completely correct as the main thing affecting it is the gear ratio at the 'box end of the system - and there are a finite number of options on the teeth which can be selected to fit in the gearbox system. I know that some cars would rely on a 'transfer box' fitted somewhere along the line to get the turns intot he right range, but not Lotus.

They would have got whatever ratio of teeth were on the shelf to give results 'close enough' with the other components - bearing in mind that the gearbox was from Isuzu, so should be okay to connect to whatever speedo unit those cars used - except they took the instruments from the Cavalier/Calibra/Astra.

I believe (although I have absolutely ZERO evidence to back it up, but it seems logical to me!!!!) that they would have taken the parts from Isuzu to turn the speedo, then have a look at the various gauge faces that Opel used, and chose the one which gave the closest results at 60mph, and within tolerance all along the range.

I'm sure there is more to it, but sometimes it just works, and sometimes things need to be made to work - bear in mind that the NA uses a different final drive ratio to the Turbo, and that the Fed/S2 cars use a different size ty(i)re - and the speedo "works" for all of them.

More modern cars with electronic speedos are more linear along their range, but generally still read 4 or 5 kph lower than an indicated speed on the satnav.

Far more interesting (to a sad git like me) is the gearing mechanism for the odometer.... again, it is a restricted range of gears that can be used during manufacture, AND it has to be far more accurate than the speedo (+ or - 2.2%). If you've ever looked at the back of the speedo you will see the open gearbox for the odometer - and it is a work of art for something so basic yet so complex!

[stevie-a]

Thanks for the answers,
I was just surprised that it was so far out (9mph on average)
Even my old Triumph TR7 was only on average 2mph out, and that was a British Layland and you can’t get much worse on reliability and quality of BL in the 70's.

[dapinky]

Even my old Triumph TR7 was only on average 2mph out, and that was a British Layland and you can’t get much worse on reliability and quality of BL in the 70's.

Ah, but the excellent engineers from Smiths Industries who made those dials probably thought that the BL tolerances were too loose, and they wanted to make it 0.5mph out!!!!!

[RonR]

Back in the day when Metal Monkey made replacement dial faces they offered an option to space out the speedo markings so instead of 170MPH top reading they used 160MPH.

170MPH.jpg

160MPH.jpg

It's around 6 percent difference, they said it improved the accuracy of the speedo (so as long as it reads correctly at one speed, it should be accurate at any speed). I chose this option but I've never bothered to check if it really is more accurate. Perhaps an old-timer on here has checked it?

If it's true that means Dave's method of setting the correct reading somewhere around the speed limit is the way to go.

[muley]

My S2 Speedo showed about 10 m p h. over.

[Denny]

My car mph .............. Actual mph

60................................53

80................................71

70................................61

Are the all this far out

Mine is very much the same. At least it's on the safe side

[Denny]

170MPH.jpg

160MPH.jpg

Did you wind your mileage back?

[stevie-a]

Well today I fitted this



Works great

[simonbuk]

Where did you get that from ?

[stevie-a]

I got it from E-Bay for £10.59.
Just watch most of them have green numbers, even though adverts show white, I purchased two and adds showed white but it was green, best thing to do is ask seller first.

in the contact seller I said this:

"You might think this is a silly question but I have purchased two of these from other sellers and when they came the display was green even though the advertised was white. Could you tell me is this defiantly white?"

most will say "sorry it is green"

But I got his is the one and as they said it was white

https://www.ebay.co.uk/itm/357166955229

[dapinky]

I've got one of these in the garage, which I bought for my trip(s) down to Gambia....... the Km readout on the dash is pretty poor, and when the Moroccan Old Bill like to hand out speeding tickets to foreigners who just want to pay and drive on, it makes life far easier when it is just in your field of view when driving.

I did notice that it wasn't great in very bright sunlight (which is a disadvantage when you are driving down Africa!) - but no issues at all in the UK. Mine came with a piece of plastic film which you can stick to the windscreen to improve the viewing contrast, so maybe that would have helped, but it got lost somewhere during the packing of the car, so I never got to try it.

I found it was pretty accurate, and ran about 2km lower than Google on the phone when driving at 60Km/h - so better than the dashboard on the Freelander - and much better than an Elan!!!

[stevie-a]

. Mine came with a piece of plastic film which you can stick to the windscreen to improve the viewing contrast, so maybe that would have helped, but it got lost somewhere during the packing of the car, so I never got to try it.

Yeah i have the piece of plastic film, but have not tried it yet,

I found it was pretty accurate, and ran about 2km lower than Google on the phone when driving at 60Km/h - so better than the dashboard on the Freelander - and much better than an Elan!!!

Again this runs about the same 2mph lower than Google, better that the M100 spedo that is out 9 mph out at 70mph.

[Tuga2112]

I got it from E-Bay for £10.59.
Just watch most of them have green numbers, even though adverts show white, I purchased two and adds showed white but it was green, best thing to do is ask seller first.

in the contact seller I said this:

"You might think this is a silly question but I have purchased two of these from other sellers and when they came the display was green even though the advertised was white. Could you tell me is this defiantly white?"

most will say "sorry it is green"

But I got his is the one and as they said it was white

https://www.ebay.co.uk/itm/357166955229

you guys with your nice, usefull and cheap gismos are an expensive bunch to follow !

i bought 1. apanrelty theres only 1 left now

[stevie-a]

you guys with your nice, usefull and cheap gismos are an expensive bunch to follow !

i bought 1. apanrelty theres only 1 left now