TVR Speedometer Correction ?

[electron]

So I've decided to put the TVR on the road to try to iron out a few minor bugs ...

It has an XJS 3.6 Manual Salisbury diff which from the tag on it I make a 3.54.

I'm running 15" Supalites with Yokohama 195/55/15's

So the questions are ...

1) What is the standard diff in a TVR 3000M 1974 ?

2) Any pointers to a wheel rolling radius calculator on the web ?

3) Does the panel think I need to get the speedo recalibrated ?

If yes any clues where ? :-)

Thanks

Chris

[tvrski]

just take a mobile gps with you, or measure time while cruising with a constant speed between a known distance and calculate, if it's not more then 10% off, i wouldn't bother recallibrating. all imho

[B16 RFF]

electron said:

1) What is the standard diff in a TVR 3000M 1974 ?

That would be a 3.45 TR6 diff in a '74 M.

Buy, beg, borrow or steal a GPS speed camera warning device that will give you an accurate speed readout at a constant speed. Compare to the speedo reading.

Paul.

[electron]

Thanks for the replies.

One other suggestion was to pop down to Halfords and splash out on a cycle speedometer. My classic motorcycle friends have done this to save weight/complexity.

I came across www.speedograph-richfield.com/ which has the manual calibration form and this is the route I'll go down as it looks like I'll get my original speedo recalibrated correctly

I'll post the results.

Thanks Again

Chris

[tvrski]

good idea but a cycle speedometer must also be calibrated (or set to the right wheel circumfence)

[davidy]

Chris

The TVR speedo wasn't accurate in the first place!

You can work out the rolling radius differential yourself quite easily =>

(Wheel Diameter (mm) + (2 x (Tyre Width * (Tyre Profile/100)))) * 3.142

Then take a standard TVR Wheel Tyre 14" 185/82 (or Turbo 14" 195/70)

Calculate one as a percentage of the over and that will give you an approx over-read/under-read

TVR used 3.45:1 diffs in early cars and 3.31:1 in later cars (I'm sure they didn't recalibrate speedos!)

With your diff ratio and the smaller rolling radius of your tyres you will accelerate faster, but have a lower top speed and your speedo will over-read. (and so will the mileage)

I would take the GPS advice, get some figures in each gear for mph/1000rpm (assuming your rev counter is reasonably accurate) and ignore the speedo and work it out in your head as you drive. If its for light road use then this will be fine and saves the cost of recalibrating the speedo. If you are on strict limited mileage (and liable to exceed the mileage) or it becomes a pain, get the speed recalibrated.

Hope that helps

davidy

[Daza]

ETB Instruments also used to do reclaibration.

They even put electronic internals inside my rev counter for the Zetec conversion.

Darren

[B16 RFF]

davidy said:

I would take the GPS advice, get some figures in each gear for mph/1000rpm (assuming your rev counter is reasonably accurate) and ignore the speedo and work it out in your head as you drive. If its for light road use then this will be fine and saves the cost of recalibrating the speedo. If you are on strict limited mileage (and liable to exceed the mileage) or it becomes a pain, get the speed recalibrated.

I think its the law that you have to have a working speedo that reads within +10% and -0% of true speed throughout the range.
I doubt if they would consider using the rev counter to be an accepable alternative.
Personally, I would want to be stay within the law and would consider using the rev counter to be a bit of a bodge as a long term solution.

Paul.

[davidy]

Paul

It may be the law, but who is going to know? I always used to used an 1000rpm to mph ratio, you just could not rely on the speedo

davidy

[B16 RFF]

davidy said:

Paul

It may be the law, but who is going to know? I always used to used an 1000rpm to mph ratio, you just could not rely on the speedo

davidy

Each to their own, I just would prefer to have a working speedo and not have to worry about it.

Paul.

[tvrski]

i've been driving without a speedometer for a long time, then after trying to fix the cable for the second time i found out the speedometer had seized! i put some oil in it and managed to unseize it, not yet sure if it will give accurate readings though.