tr7 v8 speedo calibration

[TOM Fisher]

Hi All

My first post on here. I recentlypurchased a nicely converted TR7 V8 with the 3.9 EFI engine fitted. all going well but I suspected the speedo was under reading.  My Tom Tom tells the truth and the car is 10% out. I just got a speeding ticket for 57 mph in 50 which has added insult to the problem. The easiest method but a bodge would be to put the correct numbers on the speedo face. I had the mechanical speedo on my TR4 calibrated at Speedograph here in Nottingham. Is there another fix for this? Cheers.

[yorkshire_spam]

Depending on the gearbox there are a number of speedo drive gears available with different numbers of teeth - these can be used to improve a % error. Eg. if it's under reading by 10% and currently has a 22 tooth drive gear then switching to a 20 tooth drive gear should get it pretty close.

To correct a fixed "offset error" (ie always reads a fixed number of mph above/below) then adjusting the needle position is the way forward.

I'm not familiar with the TR7 V8 gearbox, is it an LT77 or an R380? (Or another?)

 

Edited November 26, 2021 by yorkshire_spam

[TOM Fisher]

That sounds just like the answer I was hoping forI will investigate the drive gear to see what is in it, not sure the gearbox but its 5speed I presume Rover. Thanks very much.

[Nick Jones]

Is the odometer out by a similar amount? And do you care about that?

If yes, and you do care then either you need to try and source a different drive gear or get the speedo recalibrated.

If the odo is about right, or you don’t care, it should be possible, if a bit fiddly, to pull the needle off the spindle and reposition a few degrees round.

Could also be helpful to know the root cause - maybe a taller diff or a wheel/tyre combination with greater rolling radius.

Nick

[Beans]

With V8 conversions there are a few things to check. As mentioned the cable drive gear in the box and wheel tyre combination.
But more important what final drive is fitted. This has to match the gearbox!
Check the stamped number on the underside of the diff housing, this will point you towards the CWP ratio:

[Howard]

Two soloutions are (1)Speedograph Richfield will make a special speedo drive cable with a correction gear box - I use it on my 7 very successfully for many years since the five speed gearbox fitted probably came from a Sherpa van.

(2) A cheap and simple fix is a GPS speedo - this is the one I use GPS Speedo which is held in place on my dash using a magnetic self adhesive mount & is only £23.99. I use it since it switches easily between MPH & KPH which is great for driving in Europe. (Only one point to note is that the unit is delivered as adjusted to match a standard car speedo which overreads by about 7% so you need to reset it to 100% to get a true speed reading)

Cheers

Howard

[Beans]

... and check the gearbox number.
My TR7's have only a marginal deviation on the speedo's.

[TOM Fisher]

Thank you all for this great info.

[TOM Fisher]

Thanks to everybody the problem is solved, the existing gear bearing had been stuck in with gasket silicone so took along time to fiddle out, the reident gear had 23 teeth I replaced it with a 21 tooth and a new rubber gasket. The Speedo is now reading 10% faster so spot on now.

[yorkshire_spam]

Great result @TOM Fisher and thanks for taking the trouble to report back so people know the outcome!

 

[Guppy916]

On my GT6 I used https://speedycables.com/instruments/gauge-repairs-conversions/

to recal the speedo, been great ever since

Edited December 8, 2021 by Guppy916
two of the same photos