Speedo Calibration?

[mike.perry]

Having fitted a 3.9 diff and 175 x 13 tyres to my '68 traveller I thought it would be useful to check the speedo accuracy. I measured the circumference of the tyres, 1.94m compared to 1.88m on the original 155 x 14 tyres and checked the speedo accuracy against the sat. nav. I was expecting the speedo to under read by about 10%. I was therefore surprised to find that the speedo was within 1 MPH of the sat. nav. throughout the range from 20 to 70 MPH.
I just wonder how far out the speedo was on the standard diff.
Next I will check the Series MM but only up to 60 MPH

[alanworland]

I have always found a 'progressive' error with my satnav ie it always seems to read less than the speedo says, whatever vehicle its in
I just thought it was something to do with naughtical miles?
Dont think I spelt that right but you know what I mean!

[bmcecosse]

Sat nav will be deadly accurate - your size 13 wheels have pretty much cancelled out the 3.9 diff, that and the original 'optimistic' speedo that all Minors have as standard! Would be interesting now to check the mileometer against the Sat nav for say 10 miles ?

[rayofleamington]

I have always found a 'progressive' error with my satnav ie it always seems to read less than the speedo says, whatever vehicle its in

99% of speedos will read between ~5 and ~10% high.
The 1% accounts for classic cars where it can read anything - including the windscreen wiper effect

The error is deliberate - all speedos will have some variation, and there is even variation between different brands of tyres and between a worn and new tyre. Making the speedo read high compensates for all of this.
If it read low in some circumstances there's nothing to prevent the car makers from being taken to court if you get done for a marginal speeding offence.
The amount of error varies between car makers - some typically increase the error so you think their cars are fast!

1.94m compared to 1.88m on the original 155 x 14 tyres

From those measurements I can guess that both tyres are 80 profile (80 profile is a default and therefore not usually stated on the tyre)

Depending on the age of the car, it may have left the dealer with crossplies - these are likely to have bigger circumference than the radials. Therefore any intentional speedo error was increased by fitting radials with a smaller diameter. As a compromise, 145's radials were fitted as original equipment (in the later years) although the rims width is correct for 135 tyres (145 has slightly bigger diameter than 135's, so gives less additional speedo error).

155's were not originally fitted to saloon rims as they are not in the working range for the narrow saloon rims. (155's are commonly fitted now as the traders sold them for van rims and most folks interpreted this as meaning they were ok for saloon rims - or sometimes because local garages couldn't get 145's)

[rayofleamington]

Sat nav will be deadly accurate - your size 13 wheels have pretty much cancelled out the 3.9 diff

missing half the point again ;-) The 13" (THIRTEEN INCH) wheels have tyres with a BIGGER diameter than the 155's on the 14" (FOURTEEN INCH) rims. therefore they haven't cancelled anything out.

[bmcecosse]

I saw that - but that's a size taken round the circumf, but may not give the true rolling radius. This is why I ask for the mileometer reading v sat nav over 10 miles so we can see what's really going on.

[rayofleamington]

but that's a size taken round the circumf, but may not give the true rolling radius.

a circumference reading will never be the true rolling radius (affected by width, profile and tyre pressure) - but certainly a circumference will tell you enough to compare one tyre size to another.

[bmcecosse]

Well - if it's not anythjing to do with the tyres -it must simply be that the original speedo was 'miles' fast! But the actual speed reading depends on a spring which can weaken with age - the mileage reading is a fixed gearing and will tell all !

[kennatt]

I have the same 175-13 on my split screen but no sat nav would be interesting to find out as mb says over 10 miles

[mike.perry]

Thanks for all the info. The tyre measurements were taken as a comparison. From the serial no. on the speedo the car was originally fitted with 5.20 x 14 crossplies. Has anyone got a crossply that they can measure the circumference of? When I have time I will do a 10 mile run to check the mileometer, however it is by no means certain that the error in the mileometer is the same as in the speedometer.

[bmcecosse]

Indeed not - because the speedo relies on a fine spring that weakens with age. The mileometer is a gear drive and will tell much more accurately how well matched the speedo is to reality.
For interest - what is the tpm setting on your speedo ?

[mike.perry]

1408 I think. Can't be bothered to go out into the garage again to check.

[bmcecosse]

Yep - that's much like my own - a 1098 with 4.22 diff. I run 155 x 14 tyres on standard wheels - and the mileometer is very accurate - it reads about 1% over (I do 'regularity runs' which depend on exact mileage and spot-on average speed!) but the speedometer is about 10% fast - judging from following trucks and signals on speed warning signs!

[IslipMinor]

These are the tyre charcteristic differences, using 135/80x14 as the reference. A 3.9 diff will give 8.3% more mph/1000 rpm and unless you went to a 145/80x13 tyre, all the other likely sizes are bigger than the original tyre (135x14). The overall outcome is that more or less whatever you do the odometer will underread, but until you get a 'gearing' of aroung 8% more, the speedo will just correct some of its built-in optimism!!

Tyre Sizes Sidewall Rad Diam Circ Rev/mi Speedo Diff
135/80-14 4.3" 11.3" 22.5" 70.7" 896/mi 60MPH 0.00%
145/80-14 4.6" 11.6" 23.1" 72.7" 872/mi 58MPH 2.80%
155/80-14 4.9" 11.9" 23.8" 74.7" 849/mi 57MPH 5.60%
155/80-13 4.9" 11.4" 22.8" 71.5" 886/mi 59MPH 1.20%
165/80-13 5.2" 11.7" 23.4" 73.5" 862/mi 58MPH 4.00%
175/80-13 5.5" 12.0" 24.0" 75.5" 840/mi 56MPH 6.80%

[bmcecosse]

Well - as I say my mileometer is VERY accurate - just barely about 1% high and that's with 155/14 tyres. But the speedo !!!

[IslipMinor]

Roy,

Makes sense - it's more likely that the 145 radial has the similar rolling radius to the 5.20 cross ply, more than the 135, and many contemporary road tests in the 50/60's found that the odometer was around 2/3% fast. So as your 155's are around 3% larger than the 145's, they effectively provide the near perfect odometer calibration correction factor! The speedo is another matter - might be worth repositioning the needle?

[mike.perry]

What a wealth of information this thread has thrown up. The only thing missing is the data for a 520.14 crossply. I think I will take a tape measure to the next meeting and jack someone's car up to take the measurements. I can't ever recall seeing 135 section tyres on a Minor, I thought they were just for Citroen 2CVs. Maybe some of you have longer memories than me, I didn't get my first Minor until 1969.

[mike.perry]

I checked the Series MM speedo calibration on the way to the NEC, about 2 mph fast at 30 mph and 4 mph fast at 60 mph. However the needle was wandering around that much it was difficult to get an accurate reading. I think I will just push the needle back 3 mph and check it again.

[Luxobarge]

What a wealth of information this thread has thrown up. The only thing missing is the data for a 520.14 crossply. I think I will take a tape measure to the next meeting and jack someone's car up to take the measurements.

Do you mean in order to measure the circumference? I would have thought it would be more accurate to measure the rolling radius, i.e. ground to wheel axle centre, then multiply by 2 and then Pi (roughly 3.14) to get the circumference. This is because with the car in the air, you will be measuring the unloaded circumference of the tyre, the actual rolling circumference is a bit lower as the tyre compresses at the bottom under the weight of the car and the rolling radius is smaller than the unloaded radius - does that make sense or is it as much gibberish as it sounds?

Should be done with the tyre pressures correctly set of course.

Also saves jacking the car up too.....

[bmcecosse]

Easier still - mark the ground and the tyre with chalk - roll the car forward one turn and mark the ground again with chalk - measure between.