Underreading Speedometer.

[Vic Butler]

Afternoon Ian.
I had to use the size 165/100 because the calculator needed a figure in the box and as the standard 165 fitted was not a low profile I used 100 as the percentage like the second figure for a low profile tyre is. They weren't known as 165/100 as the aspect ratio (tyre wall height to tread width) was the same unlike a low profile tyre.
As the 185/70 has a smaller circumference than the standard 165, the former has more turns per mile which gives an overreading speedometer.
I'll see what a Satnav check throws up.
. Vic Butler

[Ian F]

Hi Vic,
I've attached an extract from the Longstone Tyres information section - worth a read if you can open it, or refer to the original on the Longstone website. Their information states that cross ply tyres were typically 100% aspect ratio, but early radial tyres should be assumed to be 80% aspect ratio in the absence of any other information - see quote below:

"Aspect ratios began to be reduced at the end of the 1960s to improve cornering. Previously no aspect ratio was given on radial tyres; and in these cases an aspect ratio of 80% should be assumed."

So I still think you should consider the original MGB Radials as having 165/80 x 14 tyres and a speedo calbrated accordingly,

IanF.

[Vic Butler]

Afternoon Ian.
Thanks for that so I will go on that data. That would explain why the speedometer is not grossly overreading as the difference between a 80 and 70 aspect is not as great as that between 100 & 70 which was what I was assuming.
Will have to download on the laptop.
. Vic

[Vic Butler]

Evening Ian.
I used the tyre calculator site you advised me of in an earlier post to compare a 165/80 & a 185/70. The former has a diameter of 24.4" and 852 turns per mile and the latter has a diameter of 24.2" & 859 turns per mile. It would take an extremely accurate speedometer to notice that small difference.
I'll borrow a Satnav sometime and compare the readings at 30, 40, 50, 60 & 70 according to the speedometer.
Your advice and help is greatly appreciated. Thanks.
. Vic

[Ian F]

Good luck with it Vic! I hope that some of the stuff points you in the right direction, but with so many variables to consider each case has to be sorted out on it's own set of data.

I have a smart phone (bought reluctantly a couple of years ago - but that's another story............) and I have downloaded a free app called DIGIHUD. It's a GPS speedometer and I really think it's a terrific thing to have in an older car with a speedo which may not be as accurate as you would like. The phone (a Motorola pay-as-you-go from Tesco) fastens on the the BGT windscreen with a suction cup, and plugs into the cigarette lighter socket. DIGIHUD has trip meters, averages all sort of parameters, and gives a nice bright speed readout.

GPS speedos will under-read slightly on long steep climbs or descents, and are unreliable in tunnels of course, but choose a flat bit of road and it will give you a good check on the B's unit. I just leave mine in place all the time if I'm on a long trip. Highly recommended!

IanF

[Geoff Norcliffe]

Hello Vic,
A little while ago I had an e-mail conversation with Avon Tyres technical Dept. about using their tyres on my BGT. Among many interesting points Avon told me that using a 70 profile tyre instead of the standard 80 profile would reduce the speedo reading by 4.5 mph.
Hope this helps, Geoff

[Vic Butler]

Morning Geoff.
Thanks for the information and that would explain why I had to reduce speed to an indicated 25 mph to get a reading of 30 on the speed indicator sign.
I thought that the slightly lower profile and smaller circumference would result in an overreading speedometer as the wheel is doing more turns per mile. It explains also why I tend to catch up other vehicles in speed restriction zones.
In future I will drive at an indicated 25 in a 30 zone and 5mph below the limit in others. I wonder then what my actual speed was when driving on a dual carriageways at an indicated 75 mph believing the speedometer overreads.
The MG has been tuned and is considerably quicker than a standard rubber bumper one so I will have to be extra careful. I'll borrow a Satnav and compare the speed with the speedometer reading.
. Vic

[Vic Butler]

Checked the speedometer against a Satnav today and it underreads between 2 & 3 mph so no problem.
. Vic Butler