Speedo Calibration

[Hodgie]

Folks,
I am in the process of getting the old girl back on the road and one of the issues I had last time she was running was the speedo.
I have fitted a 3:08 pinion and gear and 15" wheels with 50 profile tyres.
I sent the speedo away back in the day to get it calibrated properly (having rolled it x revolutions and measured the distance), but when it came back it was way out.
Does anyone have the answer? Is there anyone who can do the calibration for me from the technical specs provided to eliminate any error in measurement at my end? Or indeed, would a different worm drive sort the problem (assuming i reverted to standard speedo calibration)?
Thanks

[jeffremj]

Hi,

Do a google search for 219001 tr7 and you will get lots of hits regarding this problem. The best answer includes some of my info as shown here with early internet info (June 1997 !!!):

http://www.team.net/TR8/tr8cca/crossref/02_Data/DriveTrain.html.

Specifically:

“The following gears were available from the factory:

Output shaft worm gear

Colour # teeth Part #
White 7 TKC1273
Black 8 TKC1274

Pinion gears (these stick out the side of the transmission and attach to the speedometer cable)

Colour # teeth Part #
Orange 20 219001
Green 21 219002
Red 22 219003 or 219003A
Black 23 219004
Blue 24 219005
White 25 219006

Assuming stock tyre sizes (185/70-13) the following applies to vehicles as delivered from the factory:

TR7 (3.9 rear axle) : Pinion: black (23) Output shaft: white (7)
TR8 (3.08 rear axle): Pinion: red (22) Output shaft: black (8)

The rest of what follows hasn't been verified mathematically, but back in June 1997 there was a protracted e-mail discussion with the main contributors being:

Fred Smith:

“Other Gear options that apparently work:
3.45 rear axle | Pinion : Green (21) Output shaft: White (7)
3.45 rear axle | Pinion : Blue (24) Output shaft: Black (8)
3.08 rear axle | Pinion : Orange (20) Output shaft: White (7)
3.9 rear axle | Pinion : White (25) Output shaft: Black (8)”

Mike Jeffreys: (Note Mikes term "driven gear" is same as my use of "pinion gear")

“My 'rule of thumbs' are:
1) that each tooth on a driven gear gives a 5% change
2) when you go from a 3.9:1 to 3.45:1 axle this is about a 10% change
3) when you go from 3.45:1 to 3.08:1 axle this is about a 10% change
4) when you go from 3.08:1 to 2.85:1 axle this is about a 5% change

So,

1) a standard TR8 gearbox driven gear has 22 teeth,
2) a TR8 gearbox driven gear with a 3.45 axle should have 24 teeth,
3) a TR8 gearbox driven gear with a 2.85 axle should have 21 teeth,
4) a TR7 gearbox driven gear with a 3.9 axle should have 23 teeth,
5) a TR7 gearbox driven gear with a 3.45 axle should have 21 teeth,
6) a TR7 gearbox driven gear with a 3.08 axle should have 19 teeth but the lowest value available is 20, so 5% under-reads, just like my TR7V8 - using 205/50x15 tyres will also help bring the speedo closer to reality,
7) a TR8 gearbox driven gear with a 3.9 axle should have 26 teeth - have to use 25.”

[REX]

I had the same problem when converting my first TR7. As both the axle and tyres were to a new spec,I chose the route of re-calibrating the speedo. I used a company called Speedy Cables. The results were very satisfactory. For more info go to http://www.tr8archive.co.uk and click on TR7V8 conversion and then speedometer. I hope this will help.
Regards Rex

[john 215]

Hi,

I went the same route as my learned friend Rex did and used Speedy Cables, although have not bothered yet when I went from 3.08 to 2.84:1

http://www.speedycables.com/


Cheers John