Speedo ratios.

[Wagger]

OK went for a good blast today. Really ran well and scary above 55 mph, bolts rattling somewhere. Then noticed on return that the tool roll had sprung undone. Bits all over the rear footwells. Jingling spanners.

My speedo is reading 55 at 3000 rpm, 36 at 2000, and 18ish at 1000. That is correct for a 3.63:1 diff but this is only rpm v gearbox output. Wife had the car with the satnav in. Foiled again. Must get a better phone.

There are numbers on both speedo and tacho, so there must be a 'Chart' somewhere giving the details of which to use.

Also, if somebody can tell me what the rolling radius of the rear tyres should be, then I can adjust pressures and get this accurate enough for the local speed camera.

Started to rain, so I'll get the torch and note the numbers ready for replies. Hope, hope.

[cliff.b]

mintylamb.co.uk

Should provide some answers about speeds in any gear for our cars using various parameters, like tyre size, diff etc 👍

 

[johny]

Or Richyrichracing.com which uses the same calculator but with different presentation...

[Wagger]

Thanks guys. I have something to work with now. I had seen that and rembered that t was approx 18mph per 1000rpm with at 3.63:1 diff.

I now need to decide what tyres and wheels to look for.

It is bearable up to 3000 rpm as I have a quiet silencer. It would be for too frantic with a rorty one.

[Colin Lindsay]

My speedo reads 50/60/50/60 so I just aim for somewhere in between. I changed the drive at the gearbox but have no idea how accurate it is at present; that's just reminded me to dig out the ancient sat-nav and check how far out we are.

[mark powell]

I bought one of these. Moves between my Midge and Pick-up, neither of which have accurate speedos.

https://www.ebay.co.uk/itm/304518526627?hash=item46e6b7f6a3:g:lQEAAOSwq1VinyQu

 

[Wagger]

I was pleased at how steady it was. Different models must have had different speedos to cope with variance in diff ratios.

My speedo is marked SN 6203/12 and the Tacho is RN 1319/63  & 3.55-1. Both are JAEGER.

The rolling radius at the rear is 10.5 inches so I'll go consult Archimedes and do some proper maths. Will use the sat-nave next outing for confirmation.

I reduced the float heights to 1/8 inch from 3/16ths before today's test and the flat spot and spitting has ceased altogether. I canted the engine anti-clock a while back and changed floats (not needles) and upset it at the time. The 2500 slaoons have the carbs horizontal and engine sloping back and canted to the right. On the Vitesse 2 litre, it was vertical. That does not suit the 2500 manifold angle. Carb bowls do swing a bit, but not enough.

This thing really does go now. Using top gear from 20 mph is just as exciting as going through the gears.

[Josef]

There will be another 4 (or maybe 3) digit number on the face of your speedo that will indicate the turns per mile it is (or, was originally!) calibrated to. Usually just below the trip meter readout on the rhs.

[Wagger]

My calculations give 51.6 mph for 3000 rpm with a rolling radius of 10.5 inches with a diff ratio of 3.63:1. I double checked the diff when changing the drive shaft u/j's last Autumn. It was tedious but I marked the flanges and rotated the prop 36 and a third revolutions. The diff flanges rotated 10 times. Did it 10times and got 2.75. Doing it 3.5 turns was just under one turn. It drove me nuts, but I wanted to be sure. My brother found it very frustrating as neither of us could count out loud.

I'll take the sat-nav next outing.

Will definitely look at finding the missing overdrive bits and fitting over next Winter. OR fitting a five speed Mazda box. That is expensive though.

Now, what size tyres can I fit guys on standard Vitesse steel rims? Some of you must know.

At least I know how not to trip our local Gatso!
 

 

 

[Wagger]

3 minutes ago, Josef said:

There will be another 4 (or maybe 3) digit number on the face of your speedo that will indicate the turns per mile it is (or, was originally!) calibrated to. Usually just below the trip meter readout on the rhs.

Your are right, the number is 1120. The computer lost it last time. Not sure if that is turns per mile though.

[Josef]

23 minutes ago, Wagger said:

Your are right, the number is 1120. The computer lost it last time. Not sure if that is turns per mile though.

Yeah 1120 sounds about right (120mph Spitfire Mk3/IV ones are 1248 or 1184 depending on the diff for example).

[Wagger]

2 hours ago, mark powell said:

I bought one of these. Moves between my Midge and Pick-up, neither of which have accurate speedos.

https://www.ebay.co.uk/itm/304518526627?hash=item46e6b7f6a3:g:lQEAAOSwq1VinyQu

 

I have aproblem with digital displays. Tend to round them down. Nuisance with time pieces too as 11:59 is nearly 12:00, Not twenty-one minutes to 12. Spent years reading voltmeters. I tend to round analogue displays up. Less chance of speeding. I see your point though. saves hogging the sat-nav.

[cliff.b]

If you have a smart phone there are plenty of free GPS based speedometer apps. I'm using digiHud at the moment which lets you save average speed, max speed, distance covered etc.

[Pete Lewis]

if the error is consistent you can remove the speedo mechanism  hold the alloy disc and carefully move the needle a few mph 

remember they need to read fast  not slower than   the old std  used to be 10% +4mph   so effectively  at 60 it could read 70 and be in tolerance 

we had a Smiths drive and magnetic reset  machine but i didnt think of keeping it  so it got skipped in 2001 

Pete

[Wagger]

I am really pleased that the Tacho and Speedo track so closely. Both double and treble within a percent or two. Sat nav is on charge awaiting a 'Charge' down the road tomorrow. I have worked out routes that avoid the cameras and the local cops seem to let me pull out. Must just wanna see the car maybe. It looks good from 25+ metres. Even better in the dark.

[Peter Truman]

It amazes me with modern cars that the drivers sit dead on 100kph not knowing that the speedo's read 5 tp 10% high, I frequently go to the daughters 30klm away down the Mornington Peninsula the run is virtually Freeway door to door, I set the cruise at 104kph to be within tolerance and very few cars pass me! 

Last night I had to make the run twice between 11PM and 4AM as the young grandson had a haemorage after having his Tonsils out last week, an Ambulance job and we had  to look after the other grandson & clean up the carpets! best time I did the run was 20min door to door.

Re speedo indicated speeds the Dolly Sprint at 95 is consistent with the modern at 100kph, probably a tyre size issue it runs 175/70/13, which are the handbook recommended size, they are Michelin as there freebees!

[Wagger]

At an indicated 80 my beemer is doing 77 according to the satnav. On the motorway in the UK, others still flying past me at 10+ mph.

[cliff.b]

1 hour ago, Wagger said:

At an indicated 80 my beemer is doing 77 according to the satnav. On the motorway in the UK, others still flying past me at 10 mph.

GPS suggests the speedos on both our modern cars read significantly high, one by almost 10%. 

Also, those roadside speed displays that seem to be everywhere now vary quite a bit compared to GPS. Some higher some lower, still, rather those than speed cameras which I presume have to be more accurate. 

Or do they? I have heard multiple times that you won't be prosecuted unless you exceed the relevant limit by 10% +2 mph (so 79 on the motorway). Is this because of a margin of error? I must admit, I have never personally known anyone that has been done for speeding on a motorway at less than 80mph.

 

[Wagger]

10% + 2mph is correct so that makes 79 mph for motorway. My exp cop school pal tells me that they'd rather we kept moving, so won't do you for 80. They will even let you tow a caravan at 60 on a two way two lane road, although they may just issue a warning when they catch you stationary.

I gave up towing as lorries would not keep below 56 on the 50 mph motorway roadworks sections. It was getting scary, especially when their limits were increased to 60. Towing in a narrow lane two is no fun. In fact the only fun in towing is reversing into a space. I learned at 16 to back a trailer full of hay bales into a barn with a foot to spare each side.

[cliff.b]

27 minutes ago, Wagger said:

10% + 2mph is correct so that makes 79 mph for motorway. My exp cop school pal tells me that they'd rather we kept moving, so won't do you for 80. They will even let you tow a caravan at 60 on a two way two lane road, although they may just issue a warning when they catch you stationary.

I gave up towing as lorries would not keep below 56 on the 50 mph motorway roadworks sections. It was getting scary, especially when their limits were increased to 60. Towing in a narrow lane two is no fun. In fact the only fun in towing is reversing into a space. I learned at 16 to back a trailer full of hay bales into a barn with a foot to spare each side.

Regarding the lorries & roadworks, I agree that when not slowed down by cars doing an indicated 49mph they travel faster but usually if I am following them my GPS shows they are actually doing exactly 50. So maybe they have more accurate Speedos or use GPS themselves. But consequently, if they are held up by a car driver who thinks he is doing 49 but actually about 45 they sometimes get frustrated.

Also, if I chose to travel at 79mph on the motorway the Speedos in my modern cars would be reading at least 85 🤔

[Pete Lewis]

a tachograph is more accurate than a speedo  

my days of running the factory calibration unit ended in 1993 when they closed manufacturing 

Pete

 

[cliff.b]

1 hour ago, cliff.b said:

Regarding the lorries & roadworks, I agree that when not slowed down by cars doing an indicated 49mph they travel faster but usually if I am following them my GPS shows they are actually doing exactly 50. So maybe they have more accurate Speedos or use GPS themselves. But consequently, if they are held up by a car driver who thinks he is doing 49 but actually about 45 they sometimes get frustrated.

Also, if I chose to travel at 79mph on the motorway the Speedos in my modern cars would be reading at least 85 🤔

This has reminded me that many years ago I had to drive a lot of miles between customers sites each day to fix things and as we were under contract to arrive within 4 hours, I was always "in a hurry".

Consequently I usually sat on motorways at 85, keeping an eagle eye out for the boys in blue, but I never got a pull and I'm wondering now if this is why 🤔. 

[Pete Lewis]

just he tolerance means we're not as quick as you think  Ha !

Pete

now you have to look out for the yellow   rectangular box on the side of the gantry   there's not a lot of blue's around these days 

[Wagger]

Actually it is quite good so far. 20 on the speedo gives 19 on the satnav, 30 gives 29 and 40 gives 38. So, I will be ok near home in the urban zones.

It was getting a bit hairy on my own so will take a pal next time to check at higher speeds. I need to watch the road.

[Wagger]

5 hours ago, cliff.b said:

Regarding the lorries & roadworks, I agree that when not slowed down by cars doing an indicated 49mph they travel faster but usually if I am following them my GPS shows they are actually doing exactly 50. So maybe they have more accurate Speedos or use GPS themselves. But consequently, if they are held up by a car driver who thinks he is doing 49 but actually about 45 they sometimes get frustrated.

Also, if I chose to travel at 79mph on the motorway the Speedos in my modern cars would be reading at least 85 🤔

I was using a satnav at the time. Set the car at 60 cruise and the satnav read 57. OK, sanavs work on linear distance ignoring gradients so will read slower anyway, even if the speedo is true. I have always found my BMW speedos to be quite close to reality and that is accross four models in twenty years. That's Germans for you.