Why is my Speedo SO inaccurate?

[jeffremj]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Simple math says that you are going 86% of the speed that your speedo says you are going (68/80=.86). You need to get a speedo gear with 86% of the teeth that you currently have. If you have 25 teeth now, you need 21 or 22 (25 x .86 = 21.5) and that will get you pretty darn close.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">You are the wrong way around. The speedo is running too fast, so you must add teeth to slow the speedo.

[TR doNUT]

I sent my speedo clock to "Speedy cables" in London who can recalibrate the clock correctly. All you have to do is remove the speedo clock to get at the cable, mark the tyre or put the valve at 6 o clock position, push the car one turn of the wheel and at the same time count the turns on the speedo cable. The more accurate you are the better. Your clock will be accurate regardless of what gear ratio or profile tyre size so long as you do not change them, like what I did ! I have done this with three cars of mine and very happy

[RJS]

You are very right jeffremj. I had it right in excel when I was playing with numbers, and then I reversed it when I typed out the post.

[HowardB]

For what its worth, I have a rover 5 speed gearbox in my car that was not originally a TR7 box - possibly from a light truck. It has always over read by about 20% so not much use. The difference between the TR gearbox and the one I have was not in the nylon speedo drive gear, as I tried changing that, but seems to be inside the box.

I get around this two ways:-

1) I fitted a correction gearbox from speedograph which is not problem free. It gets through speedo cables faster than I would like - usually 1 every two/three years.

2) I also use a GPS speedo which is v accurate, but not so good at changes of speed and no use at all in the San Bernard tunnel

Cheers

Howard

[jeffremj]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by HowardB</i>

For what its worth, I have a rover 5 speed gearbox in my car that was not originally a TR7 box - possibly from a light truck. It has always over read by about 20% so not much use. The difference between the TR gearbox and the one I have was not in the nylon speedo drive gear, as I tried changing that, but seems to be inside the box.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">Indeed, there is the gear, which is different and a lot harder to change! I guess you were using a Rover gearbox with a 3.9 axle - this would give a massive over-read - 20% being a figure I worked out [:)]