Why is my Speedo SO inaccurate?

[bonnietiler]

I always thought that it read fast, those midwives overtaking me on the B roads was a clue...ahem...
But I fired the old satnav up today and it reads VERY fast, so that at an indicated 70 I'm at about under 60mph and at an indicated 80 ...maybe 68, I know my tyes are little low on profile but according to the internet checking thingy only by 2% (60 profile rather than 70)
Could the speedo box be a problem.
Any help much appreciated
Cheers
Bonnietiler

[HDRider]

I would check what the odometer is correct first to determin if it is a speedo problem or a ration problem.

If the odometer is correct then resetting the speedo needle will cure it all. If teh odometer is wrong also by approximatly the same percentage I would look to see if the rear end has been replaced by one with a different overall ratio. If that is the case you will either have to;

Live with it

Replace the axle gears with the correct set

or Change the speedo drive gear in the transmission.

My 1976 Coupe had the wrong gears in it as someone had replaced the 3.27 center section with a 3.9 unit from a Spitfire. The car ran fine but 65mph was something like 4,000 rpm. Quite annoying for more than a few miles. I opted to replace the gear set with a proper ratio set.

Edward Hamer
Petaluma CA

[bonnietiler]

Probably better to just live with it maybe?

[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 bonnietiler</i>

I always thought that it read fast, those midwives overtaking me on the B roads was a clue...ahem...
But I fired the old satnav up today and it reads VERY fast, so that at an indicated 70 I'm at about under 60mph and at an indicated 80 ...maybe 68, I know my tyes are little low on profile but according to the internet checking thingy only by 2% (60 profile rather than 70)<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">Need more info - actual tyre and wheel sizes, axle ratio and gearbox type (4 or 5 speed).

[TR Tony]

If the diff ratio has been changed with the V8 conversion then the speedo drive gear would need to be changed as well.

There is a worm drive gear on the output shaft of the LT77 gearbox, under the rear cover, TR7 with 3.90 ratio diff is white with 7 teeth, TR8 with 3.08 ratio should be black with 8 teeth. This mates with the drive gear on the end of the speedo cable, for the 3.08 axle this should be Red with 22 teeth. If you have a black drive gear on the cable end then that is a TR7 gear with 23 teeth.

There are quite a few other colours & combinations of gears - there is a very helpful chart of the various gears & ratios on page 40 of the Roger Williams book How to Improve TR7, TR7-V8, TR8.

Tony

<font size="1">1981 TR7 FHC Cavalry Blue
1980 TR7V8 DHC Jaguar Regency Red - sadly sold!
1977 TR8 FHC EFI Factory development car Inca Yellow</font id="size1">

[Neil_W]

Maybe easier to put on an overlay dial or in my case go to Spec Savers [8D]
Good news it is showing higher than your real speed so PC Plod won't pulling you over.
If I could remember to take 10 mph off any reading every time I drove - maybe just leave alone or drive with sat nav stuck on window on fast days out.[^]


http://www.ebay.co.uk/itm/SPEEDO-OVERLA ... 43c5a1a0be

[HDRider]

You can aso get a ratio device that fits onto one end of the drive cable. Most Speedo shop can supply these but they are not cheap.

Edward Hamer
Petaluma Ca

[FI Spyder]

It would bother the heck out of me. Don't know if you have them where you are but on s stretch of road were a lot of people have died over the years were I go to club meetings there is a big speed reader and I compare there. I would get the right gear ratio's to get it close. The gear on the speedo cable where it goes into the trans is easy to get to and change.

- - -TR7 Spider - - - 1978 Spitfire- - - - 1976 Spitfire - - 1988 Tercel 4X4 - Kali on Integra - 1991 Integra - Yellow TCT

[john 215]

Hi,

When I put 3.08 CWP set in my old FHC V8 I had the speedo recalibrated by Speedy Cables. Should get my DHC done now I have a 2.84 CWP in the rear of that.

http://www.speedycables.com/calibration.html


Cheers John


LIVE LIFE A QUARTER OF A MILE AT A TIME!

1976 Speke FHC Beauty Now with an overdrive conversion

1979 3.5 FHC(STATUS PENDING!!)

1982 2.0 DHC NOW A 4.6, BUILT NOT BROUGHT !!!!

[trekcarbonboy]

I was off by about 5mph. Originally a 4 speed car that was changed to 5. After installing new, slightly taller wheels and tires, it's dead on.


Craig '75 2.0 FHC

[jeffremj]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">There are quite a few other colours & combinations of gears - there is a very helpful chart of the various gears & ratios on page 40 of the Roger Williams book How to Improve TR7, TR7-V8, TR8.<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">Thanks for the reminder, here is the info:



Basically, if you are over-reading by 5%, use the next value larger value, e.g., if you have the black one, fit the blue one - I did this on my TR7 16V. If you are 10% over-reading with a black, fit a white. The blue is still available at Rimmers:

http://www.rimmerbros.co.uk/Item--i-219005A

[bonnietiler]

It was a TR7 but is running a 3.08, so where do I start then?
I realise having the right sized tyres would be a good place. I have 15" with 185/55 MGF wheels coming this week. That look pretty much bang on on the ratio front...they might come off again if the ride is bone hard though!

[jeffremj]

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">
It was a TR7 but is running a 3.08, so where do I start then?<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">This confuses me, my TR7V8 with a TR7 5 speed and a 3.08 axle under-read and I had to fit an orange to get near the correct value - 70mph on speedo is 72mph via gps.

<blockquote id="quote"><font size="1" face="Century Gothic, Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">I realise having the right sized tyres would be a good place. I have 15" with 185/55 MGF wheels coming this week. That look pretty much bang on on the ratio front...<hr height="1" noshade id="quote"></font id="quote"></blockquote id="quote">I had 195/50x15 which is similar, but 205/50 x 15 is the best for looks and correct speedo.

Finally, there was a different internal gear on the Rover 5 speed, which offset the pinion range.

Further though has brought me to this conclusion - you have the Rover V8 gearbox and axle. This expects a 195/70x14 tyre but you are using something around 10% smaller, so your speedo will read around 10% to fast. So, just check the colour of your pinion and buy one with 2 more teeth to give around a -10% speed change.

[Hasbeen]

The 8, & 3 of the 7s I have owned all did exactly that. They read 100 when traveling at 91 Km/H. I assumed they were set that way.

Only my current 7 is basically correct. I say basically, as at 3000 RPM the speedo actually waves from 98 to 102.

Don't you think it is nice to have a car so friendly that it continuously waves to you?

I was not unhappy to have a fast reading speedo, as I'm sure it makes me drive a little slower most of the time, probably preserving my licence.

Hasbeen

[RJS]

I have gone through this twice (way when I first got my car, and then again when I changed out the diff ratio). It is not that hard to figure out, but first you need to know what gear you have.

As I remember, there is only a single bolt holding the speedo cable to the transmission. Remove the bolt and clip, and then pull out the gear. Clean it up and figure out what you have (then put back together if you are going to drive it). It takes longer to jack up the car and get your tools out than to get the speedo gear out.

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.

Good luck,
Rob