Tr3A Speedo accuracy

[Alan Kitchen]

I have recently had my car set up on a rolling road. One side issue that arose was that it came to light that the speedo was consistantly 16% optimistic (across the range of speeds), i.e. 30mph on the speedo was only 26mph road speed (and 80 was only 69).

 

The car is fitted with 15" radial tyres, which is as far as I can establish are the same size as those originally fitted.

 

Can any one advise: 1. Why this is happening and 2. how can it be remidied.

 

I am not sure whether this amount of error may be illegal.

 

thanks,

 

 

 

Alan Kitchen

[stuart]

That sounds like a typical TR speedo to me. Best to have it recalibrated. If you do a search on the forum it will give you the numbers for several people who can do it.

Stuart.

[Andrew Smith]

.........it came to light that the speedo was consistantly 16% optimistic (across the range of speeds), i.e. 30mph on the speedo was only 26mph road speed (and 80 was only 69).

 

Only 16% Alan?

 

Blimey, mine’s 33% out at 30 i.e. reads 40 and waves around all over the place at anything over 70mph – I’d say you've got an accurate one there - hang on to it!

 

Cheers

Andrew

Edited June 11, 2007 by Andrew Smith

[ianc]

There should be a 4-figure number in small characters immediately beneath the total mileage odometer, and that is the original calibration of the instrument. The number will be about 1164 for a car with 3.7 axle and originally fitted with 155 or 165 section radial tyres (or even cross-plies), which would have been higher profile than most tyres used by TR people these days. Apart from possible error in the odometer itself, other contributory factors are: axle ratio, tyre size and aspect ratio. A 19 x 65 tyre is almost the same diameter as an original (1950/1960) 165 x 15, but if the tyres are differnet, the axle ratio has been changed, or a different speedo fitted, things can be awry.

Ian Gibson wrote a technical article on this subject very many moons ago (it might have been whilst I was Technical Editor, which would be up to 1986), and it should be on the CD.

Ian Cornish

[james christie]

There should be a 4-figure number in small characters immediately beneath the total mileage odometer, and that is the original calibration of the instrument. The number will be about 1164 for a car with 3.7 axle and originally fitted with 155 or 165 section radial tyres (or even cross-plies), which would have been higher profile than most tyres used by TR people these days. Apart from possible error in the odometer itself, other contributory factors are: axle ratio, tyre size and aspect ratio. A 19 x 65 tyre is almost the same diameter as an original (1950/1960) 165 x 15, but if the tyres are differnet, the axle ratio has been changed, or a different speedo fitted, things can be awry.

Ian Gibson wrote a technical article on this subject very many moons ago (it might have been whilst I was Technical Editor, which would be up to 1986), and it should be on the CD.

Ian Cornish

 

Aren't you guys all making things a bit complicated ?. Pull the needle off the spindle and put it back where it should be ! That way you can have the over -or under, reading you want !

 

james

[Andrew Smith]

Aren't you guys all making things a bit complicated ?. Pull the needle off the spindle and put it back where it should be ! That way you can have the over -or under, reading you want ! james

 

Like it, and spoken like a true TR'er of old!

 

There's very little you can't fix on a TR with a hammer either!

 

Cheers

Andrew