Underreading Speedometer.

[Vic Butler]

I have noticed that my 1977 MGB GT triggers speed signs in 30 & 40 zones even when the speedometer is reading the correct speed or just below. The other day I approached one of these signs which showed my speed as 31mph. I glanced at the speedometer which showed just below 30. I slowed down and when the sign showed 30 I was doing an indicated 25.
Tyres are 185/70 x 14 which have a slightly smaller circumference than the standard 165 x 14 tyres so I would have expected an optimistic speedometer. Over a distance of 6.1 miles ( home to a road junction where I turn off) the odometer reads the same as a 15 plate Sprinter a 10 plate Peugeot Partner van and my 1975 Land Rover so are these speed indicator signs set high? I can't believe my speedometer is reading that slow. Anyone else experienced this?
I'll be borrowing a Satnav to check the speedometer.
. Vic Butler

[Peter Cresswell]

Hi Vic
Firstly take a read of this other recent thread which is related:
http://mgbforum.mgb-register.org/viewto ... ?f=4&t=397
I am currently changing the gearbox and having the overdrive unit overhauled on my roadster (the reason is a long story!) and initially I was tempted by a good secondhand gearbox and overdrive unit which came from a late rubber bumper car. This of course is fitted with a blue label O/D rather than the black label unit in my 1969 car. The gearbox and overdrives are mostly between early 4 synchro cars and late cars, but a major difference is the drive ratio for the speedo cable which is in the O/D unit. So do you have the correct overdrive unit (a blue label one) for a late car. The difference is about the underreading error have? The label in question is the cover of the solenoid housing on the Overdrive.

I didn't buy the secondhand unit in question but have managed to get a good gearbox and I've taken my O/D unit to ODSpares in Rugby for reconditioning.

As far as the roadside speed indicators go, it wouldn't surprise me if there some 'over-reading' going on to help curb speeding drivers, and they don't need calibration like cameras and radar guns.

[David Wicks]

High Vic, I have the same problem in my 74 GT, in fact I was caught doing 36 in 30 Zone and had to go on a Speed Course. The indication in the car was 33! B****r
Anyway my solution was to "calibrate" the speedo with my TomTom. I found that at 30mph in 4th on the satnav, on a clear straight road, engine speed was 1500 rpm. I now rely on the tacho in built up areas rather than the Speedo.
For higher limits I reckon on a 10% over read and again the satnav corroborates this. Will I bother getting the speedo calibrated? Nah!

[Vic Butler]

Afternoon Peter.
I bought my1977 BGT in July 1979 and the overdrive, gearbox and engine are the original (just over 68,000 on the clock first time round and verifiable.)
David. I thought 33 in a 30 zone is within the 10% error rule.
I will check my speedometer against a Satnav and use the rev counter. I went through a police speed camera on the A303 last year at an indicated 70 and no speeding notice arrived.
. Thanks Vic Butler

[Vic Butler]

Hi David.
I misread your post. So you were doing an indicated 33 but were actually doing 36 not the other way round.
I keep the speedometer reading at whatever the prevailing limit is but until I have checked against a Satnav I'll keep slightly lower.
Peter. I had read that thread which prompted my writing this one. Once I have established how slow the speedometer may be I'll allow for it and use the rev counter in conjunction with the speedometer.
. Vic Butler

[Ian F]

Vic Butler wrote:
> I have noticed that my 1977 MGB GT triggers speed signs in 30 & 40
> zones even when the speedometer is reading the correct speed or just below.
> The other day I approached one of these signs which showed my speed as
> 31mph. I glanced at the speedometer which showed just below 30. I slowed
> down and when the sign showed 30 I was doing an indicated 25.
> Tyres are 185/70 x 14 which have a slightly smaller circumference than the
> standard 165 x 14 tyres so I would have expected an optimistic speedometer.
> Over a distance of 6.1 miles ( home to a road junction where I turn off)
> the odometer reads the same as a 15 plate Sprinter a 10 plate Peugeot
> Partner van and my 1975 Land Rover so are these speed indicator signs set
> high? I can't believe my speedometer is reading that slow. Anyone else
> experienced this?
> I'll be borrowing a Satnav to check the speedometer.
> . Vic Butler

Hi Vic,
I was prompted to write as I too have different size tyres on my BGT due to the difficulty of getting reasonably priced 165 x 14 tyres. I have Uniroyal 175/70 x 14 which have a slightly larger circumference than the standard tyre, and my speedometer under-reads. Accordingly I always have a GPS speedometer fitted to comply with the law and to avoid (as far as possible) any offences.
So, are you quite certain that your 185/70 tyres are actually smaller in circumference than the standard tyre? This may explain your observations on speed quite well?

You may have used one of these websites, but if not have a look:

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

Hope this might be a useful comment, please excuse me if not!

IanF.

[Vic Butler]

Thanks Ian.
I compared the 185/70 on a wheel against the standard 165 on a wheel and the latter was very slightly taller which led me to believe that there would be very little difference in the speedometer reading. I'll have to borrow a Satnav and check the GPS speed. I do have a different set of tyres now, still 185/70 but Dunlop now on LE alloys km but I don't believe there would be any difference to the previous ones which were Vredesteins (think that's how it's spelt)
. Vic Butler

[Vic Butler]

Morning Ian.
Checked the tyre calculation site and a 185/70 has a diameter of 24.2" and a circumference of just over 76" whereas a 165/100 has a diameter of 27" and a circumference of about 84". Therefore I would have expected the speedometer to overread.
. Vic Butler

[Vic Butler]

Measured the diameter of the wheel and tyre on my BGT and allowing for the slightly domed aspect of the tyre the figure is a fraction over 24" which is the figure on the calculator.
I don't have a 165 tyre to measure.
. Vic Butler

[Ian F]

Hi again Vic,
I believe the standard 165 tyre is 165/80, and not 165/100? I don't think that 165/100 ever existed as a radial tyre size. Someone will correct me no doubt if I am mistaken!
If 165/80 was the standard size for MGB speedo calibration, then your calculations will be out by quite a bit?

IanF