Speedo Discrepancies

[F69]

My mate and I are both owners of 1972 TR6s (CP ) and suffer from inaccurate speedometer readings. Personally my speedo readings about right up to 30mph but the faster I go the greater the discrepancy, probably 8-10mph at 70mph ( although I can't be certain c'os the bloody speedo reads wrongly. I'm sure that the tyre size when the car was new was a 165 x 15 but its now fitted with 5.5J wires and 195x65x15 tyres. Is it true that increasing the size of the tyre will produce a false/inaccurate speedo reading. I intend to send the speedo and tacho to JDO Ltd for reconditioning in late autumn when I lay the car up.

Edited July 12, 2007 by Richard Goddard

[rpurchon]

the speedo on my car is the same.30=30 60= 65 90=100mph. i have had my speedo recalibrated by jdo.

before it was even worse i could blast down the motorway at 130mph? but so was everyone else.

you need larger dia tyres on the back.

i have checked my 6 and modern cars with 3 gps and the speedos always read higher than the gps

jdo did a good quick job with the speedo though.better to read ok at 30 than 90.richard

[stuart]

Richard try this site for tyre radius differences. http://www.toyo.co.uk/tyre_converter.php

Stuart.

[Richard Crawley]

It’s not the size of the tyre that affects the accuracy of the spedo but the rolling circumference’ although one usually influences the other. 195/60 tyres will affect the spedo accuracy; 185/70 is the nearest rolling radius to the original 165/82 (85) for the 5 ½ j rim. Most (including me) use 185/65 which are a common fitting on many modern saloons & so are an awful lot cheaper; typically £35 - £50 all up depending on make.

 

195/65 is a closer match on the radius but these are outside the recommended width for 5 1/2 j rims & won’t necessarily improve the handling. Many do fit them, however, & there is lots of info & arguments about tyre widths/wheels etc, on this forum so do a search & have a read up.

[preport]

Yes, I have that problem too. My speedo reads about 10% fast which is directly related to the fact that I have calculated that the circumference of the present tyres is 10% smaller than the orignial tyres. The odometer is 10% too. The advantage is that you have a bit of a safety margin when going through speed cameras. The disadvantage is that the actual fuel consumption is 10% worse than you think it is!

 

Nick

[Ted M]

Hi Richard,

You may find this web site http://www.carbibles.com/tyre_bible.html#sizes has a complete description for tyre sizes - I found the various numbers confusing, especially when looking for modern tyres for my 1973 TR. Triumph (Leyland) workshop manual quotes two tyre sizes 165 or 185 for the TR6 and only one set of road speed / engine RPM figures. (471 rpm/ 10mph in top, no o/d).

My local tyre dealer says that his computer conversion system shows the 205/70 is the correct direct replacement for the old 185 size. This size, I believe, will not fit the TR body or the 5 1/2J wheel rim. The second number in modern tyre descriptions is the profile ratio, width / height. You probably know that. The older original TR sizes were 1 /1. Therefore a tyre 185 high would be 185 wide, not so today.

One good thing - with smaller diamater tyres you won't unknowingly exceed any speed limits!

As a matter of interest - I measured the circumference of an Avon turbospeed 185/15 (purchased about 10 years ago) on my TR wheel. It's approx 2.1 metres. A 195/70 should be about 2.05 metres circumference. A 185/65 should be about 1.95 m circ.

It's some time since I had a speedo recalibrated. I haven't looked at my TR speedo, but the last time I did this job (on another Triumph car) there was label on the speedo which indicated that the input speed of 1300rpm gave an indicated speed of 30mph. Don't hold me to these figures. This calibration is independant of the axle gearing and road wheel size because it ties the speedo display to the gearbox rpm. The axle gearing and road wheel 'gearing' come after ofcourse. Maybe the calibration people take your tyre size and final drive into account?

Once again, according to the Triumph manual, a TR6 with carbs had a final drive ratio of 3.7 with PI 3.45. Top gear is 1.0. The o/d ratio was different A type to J type.

 

I hope this helps.

 

Ted

[Richard Crawley]

Triumph (Leyland) workshop manual quotes two tyre sizes 165 or 185 for the TR6 and only one set of road speed / engine RPM figures. (471 rpm/ 10mph in top, no o/d).

 

My local tyre dealer says that his computer conversion system shows the 205/70 is the correct direct replacement for the old 185 size. This size, I believe, will not fit the TR body or the 5 1/2J wheel rim. The second number in modern tyre descriptions is the profile ratio, width / height. You probably know that. The older original TR sizes were 1 /1. Therefore a tyre 185 high would be 185 wide, not so today.

 

 

Welcome to the forum Ted. The 185 section was only fitted to American market cars & was not really suitable for the standard 5 1/2J wheel section. I believe the original tyres were actually 82% (or 85%) section so were not 1/1 ratio. A 205/70 tyre may be the nearest equivalent of the original 185 rolling radius but it’s dangerously incompetent for your local tyre dealer to suggest fitting a 205 section tyre to a 5 1/2J rim; it's just too wide. In the 70’s/80’s, it was common to see TR6’s fitted with XJ6 tyres as they were really the only wide section, radial tyres available. They weren’t too bad when fitted on the 6 or 6 ½ inch alloys available at the time but they totally screwed the gearing, fouled the body & made a car on standard rims car handle like an unset jelly; I fitted some & took them off after a week!

[PJM]

I had my speedo repaired and calibrated by JDO. Full instructions are on the web site to calculate the gearing ratios etc. but briefly you jack up the back wheel and rotate it counting the turns of the wheel and the turns made by the speedo cable drive, provide details of the tyres, make and size and a couple of days later you get your calibrated speedo returned. Mine reads bang on against a GPS. A satisfied customer.

[Ted M]

Welcome to the forum Ted. The 185 section was only fitted to American market cars & was not really suitable for the standard 5 1/2J wheel section. I believe the original tyres were actually 82% (or 85%) section so were not 1/1 ratio. A 205/70 tyre may be the nearest equivalent of the original 185 rolling radius but it’s dangerously incompetent for your local tyre dealer to suggest fitting a 205 section tyre to a 5 1/2J rim; it's just too wide. In the 70’s/80’s, it was common to see TR6’s fitted with XJ6 tyres as they were really the only wide section, radial tyres available. They weren’t too bad when fitted on the 6 or 6 ½ inch alloys available at the time but they totally screwed the gearing, fouled the body & made a car on standard rims car handle like an unset jelly; I fitted some & took them off after a week!

Hi Richard,

 

I worded the message badly. He didn't reccomend fitting 205's - just that they gave the nearest rolling circumference (according to his online refernce chart) to the old style 185's. He and I both knew that they are far too big to fit the wheels. In the end I have fitted 185/65 simply because 195/70 are difficult to come by.

 

Ted