speedo readings with a 3.9 diff??

[ericthered]

Hi all,
I just fitted a 3.9 diff to my traveller (its great!) but now I dont know how fast I am going.

My car is a 1963 with standard everything except for the 3.9 diff, the diff that came out was a 4.2 to 1.

Can anyone tell me what my speeds will read on the standard speedo please, I need to know what it will read when I'm doing 30mph, 50mph, and 60mph?

Thanks and best regards,
Eric.

[PSL184]

Far too many variables to give an accurate answer to your question. The ONLY way you will know is to use a Sat Nav to get an accurate reading.....

[bmcecosse]

The speedo would be reading ~ 5% fast even before you changed the final drive ratio. You will need a speedo with something like 1300 tpm. Certainly the 1248 tpm mileometer and speedo is DEAD accurate with my 3.7 final drive.

[mike.perry]

You are assuming that the standard 1408 TPM speedo is accurate to start with. There can be up to 10% error on standard tyres and diff so your indicated 60 mph reading could be as little as 54 mph. Sit a safe distance behind an HGV on a level stretch of motorway and check. The HGV will be doing 52-54 mph.
I have swapped my 4.22 diff for a 3.9 diff in my traveller and 175/80/13 tyres which are almost identical circumference to 155/80/14. I checked the 1408 speedo with a sat nav and found the speedo readings to be within 1 mph of the sat nav readings from 20 -70 mph. You are advised to have two people for this test

[MarkyB]

I found the same thing as Mike, the speedo was more accurate after changing to a 3.9 diff.

[liammonty]

Likewise- after changing to a 3.9:1 diff from the standard 4.22 in my '68 traveller, the speedo is now dead on according to GPS. All my Minors (5) have over read by at least 10%. I think they get worse as they get older due to weakening springs in the speedo mechanism. Lower profile than original (crossply) radial tyres also make the speedo over read a bit. What you should bear in mind is that even if the speedo is now really accurate, the odometer will probably be out, as the mechanism for that is direct, so will not have altered over the years unlike the speedo. So, your mileometer will most likely under read with the new diff.

[bmcecosse]

Yes - I'm sure that's true - the speedo spring does weaken with age leading to higher speed readings - so as the others have said - compare the mileage recording with a known distance to find out what's really going on....

[Neil MG]

Sit a safe distance behind an HGV on a level stretch of motorway and check. The HGV will be doing 52-54 mph.

The one in the inside lane is doing 52 and the one in the middle lane is doing 54!

I suspect GPS is a bit more accurate, but I take your point. If it seems like all the HGVs are doing 60+mph then your speedo is out!

[bmcecosse]

Some are regulated to 50 mph.......... I sat behind such a beast for many a mile on the A9 earlier this week.....

[mike.perry]

If it was on a single carriageway road then it should have been doing 40 mph, 50 mph on a dual carriageway.

[bmcecosse]

I guess I was 'lucky' then to be doing 50.........

[ericthered]

Thanks for the help and info chaps, I'll have to get my son to sit in with me and use his gps thing, I dont keep up with tech stuff, with a bit of luck it will now be spot on as a few of you have found .

Best regards,
Eric.

[charlie_morris_minor]

i can not find the post, but i am sure someone on here has stripped the paint off the face of their speedo and went for a drive without the glass in and got a friend to mark the 20 , 30 marks etc might be an option

[rocket3]

Hi, don`t trust the sat`n`nav readings as these things aren`t calibrated, being in the truck industry i often get drivers complaining that their sat`n`nav screens are reading up to 10kph slower/faster than their tacho`s, they then assume the vehicle is down on power etc,etc.. if you know someone who works at a, or have a tacho station near you he may oblige and run it up on the rollers for you as these will be calibrated aswell and do a speed for speed check. Rob.

[Alan Pearse]

Mike Perry, i stand to correct you. A vehicle of 7.5 tons and above traveling on a duel carraiage way with no central dividing barrier is restricted to 40MPH, even if the signage says 50 MPH. Only when there is a central dividing barrier and the limit of 50MPH is signed can it then apply to vehicles over 7.5 tons.

Road Traffic Act / Tachograph Regulations / Speed Limits of Vehicles / 7.5 tons and above.

Secondley B M if an HGV is resticted to a speed no higher than 50 MPH is must display advisory signage at the rear most point of the vehicle accordingly. Normaly would say something along the lines of (This Vehicle is Restricted to 50MPH MAX).

Eric, be advised that under the Road Traffic Act / Vehicle Consruction and use Regulations a vehicle speed recording device (speedometer) must be acurate to within + or - 1% at 30MPH but can be + or - 10% there after, however if your speedo fails to record an accurate speed to within 10% it is an endorsable penelty for displaying an incorrect speed.

[Neil MG]

Eric, be advised that under the Road Traffic Act / Vehicle Consruction and use Regulations a vehicle speed recording device (speedometer) must be acurate to within + or - 1% at 30MPH but can be + or - 10% there after, however if your speedo fails to record an accurate speed to within 10% it is an endorsable penelty for displaying an incorrect speed.

As I understand the Vehicle Construction and Use regs require a tolerance of +10% -0%. The speedometer must never under read.

I was also under the impression that the satnav system is very accurate so long as a steady speed is maintained. As the time and position on the device is by its nature accurate then there is no need for the unit to be calibrated. Inaccuracies during acceleration and braking, even slightly, are inevitable due to slight signal delays.

[chrisryder]

I was also under the impression that the satnav system is very accurate so long as a steady speed is maintained. As the time and position on the device is by its nature accurate then there is no need for the unit to be calibrated. Inaccuracies during acceleration and braking, even slightly, are inevitable due to slight signal delays.

i quite agree, speed being distance/time is going to be far better calculated by something that knows where you are and how long it took you to get there, rather than the rotation of a wheel, through gears and a cable that turns a magnet to make a disc work against a spring. far too many variables along that route!

[bmcecosse]

Yes - pretty sure GPS is deadly accurate if getting a good signal and holding a steady speed.

[PSL184]

The "myth" regarding sat nav inaccuracy stems from people who say "my sat nav is wrong because my speedo says so"
Sat navs, by nature, are deadly accurate and do not require any form of calibration.....

[gazzcross]

As above satnavs are really accurate but using a truck is a good guide aswell...most trucks are restricted to 54mph so if you find one thats overtaking its almost a guarantee that they're on the limiter and as has been said, the trucks use tachographs which have to be calibrated/checked every 2 years so its sure to say that they are pretty accurate