MGB GT speedo issues

MGB GT speedo issues

Author
Discussion

MGsimon

Original Poster:

8 posts

114 months

Saturday 20th September 2014
quotequote all
Hello all, iv just brought myself a well looked after 78 gt in vermillion red.



I was taking her over to the paint shop last night to get a few small surface rust and paint bubbles sorted before the winter comes in a I noticed on my sat nav that the speedo is reading rather high.

When doing 30 it says 40 and when doing approx 75 it reads 90ish.

HELP

williredale

2,866 posts

151 months

Saturday 20th September 2014
quotequote all
Could be the wrong speedo fitted. What does it say on its face?
Or it could be wrong sized tyres. What do the sidewalks say?
Or something else! But those two are easy to check.

Also, is the odometer reading correctly?

MGsimon

Original Poster:

8 posts

114 months

Saturday 20th September 2014
quotequote all
williredale said:
Could be the wrong speedo fitted. What does it say on its face?
Or it could be wrong sized tyres. What do the sidewalks say?
Or something else! But those two are easy to check.

Also, is the odometer reading correctly?
The car is in the paint shop ATM so can't check those things I did notice to the car doing a mile the sat nav said 0.8 of a mile.

nta16

7,898 posts

233 months

Saturday 20th September 2014
quotequote all
Simon,
unless there are special circumstances farting around with cosmetics on a classic that is new to you is totally the wrong place to start, the priority is all wrong

if your main interest is how the car looks fine but if you want advice on getting the car good to actually drive then I and others can help but a lot of stereotype MGB owners only want the car to look good and for very limited summer use so the advice is wasted on them as they'll ignore it

MGsimon

Original Poster:

8 posts

114 months

Saturday 20th September 2014
quotequote all
She is in for some welding work and rust being sorted around the windows, which I feel making her water tight is a concern. She drives and runs fine the speedo issue is the only problem iv come over so far ( famous last words )

nta16

7,898 posts

233 months

Sunday 21st September 2014
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that's fine

as for how well the car goes, with respect, have you driven many good mechanical examples to know how good yours is, many even long term owners haven't and think of their under par example as 'they're all like this/do this'

you're obviously have a preference for first hand experience rather than learning from the mistakes of others, which is fair enough, it's your car, time hassle and money

good luck

MGsimon

Original Poster:

8 posts

114 months

Tuesday 23rd September 2014
quotequote all
Iv driven others and other people who have owned Bs in the past have even said mine drives better.

But this is my only car so it won't be getting put away for the winter etc just dealing with the rust and sealing underneath so no more welding will be needed in the near future.

Only asking what could be causing the issue with my speedo

v8250

2,724 posts

210 months

Tuesday 23rd September 2014
quotequote all
Simon, if your speedo is reading too high it's only going to be one of a number things that can cause a mis-reading. Do the following and by process of elimination you'll crack it.

1. confirm wheel and tyres size :. ensuring correct rolling radius...though by looking at your car they look correct +/- the odd mm.

2. speedo cable - make sure both the gearbox end and the speedo end are secure and well lubricated - specifically, be sure the cable is not twisted through it's length as this will cause premature failure and give a duff reading; though the reading is normally lower rather than higher. also, if fitted, check the speedo angle drive.

3. if the speedo cable is old, to be frank, change it for a new one. they're pennies to buy and easy to change.

4. inspect the speedo drive output of the gearbox. check the condition of the drive gear, it should have clean sharp edged gears where the cable enters. clean area and re-grease before installing the new cable.

5. if speedo is still reading too high, it's the speedo that is faulty. you now have two choices 1. send yours away for repair, or 2. buy a new one. realistically it's easier to simply buy a new one. fit this, make sure all is working fine and then...you have the opportunity to take apart the original speedo for some fun, see if you can find the fault...it may be it just needs a good clean and light re-oiling inside.

follow the above and you'll find the fault. good luck...

ps if you get really stuck there are some good MG guys locally to you in Bucks. contact the local MGOC chaps and for sure one will come to the rescue!

nta16

7,898 posts

233 months

Wednesday 24th September 2014
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Simon good to hear you'll actually be using the car, you're in a tiny minority, and that relatively it runs well (although owners saying yours runs better than theirs isn't always saying a lot)

Driver's Handbook will give you size of wheels and tyres (165/80/14), as said your look about standard, so if you have the correct speedo you should be that far out

if your tyres are a different size the calculator two-thirds down this page will help with speedo differences (and the rest of the page and other three pages contain loads of useful info) - [url]http://www.carbibles.com/tyre_bible_pg4.html/url]

Expatloon

215 posts

156 months

Friday 26th September 2014
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The error doesn't seem big enough but just for confirmation check the colour of the square plate over the OD solenoid, it should be blue.

If it's black then it means that the gearbox has been replaced at some time but with pre RB one which would have a black label and a speedo drive ratio of 1280 turns per mile (TPM) as opposed to the 1000 your speedo is designed for.

As I say though that would produce an error of + 28% which is considerably more than you're seeing.

MGsimon

Original Poster:

8 posts

114 months

Monday 13th October 2014
quotequote all
Thanks guys. I will be doing this as soon as it's back. Also iv found out she now needs some new fuel lines.

Here is the rust that has been sorted.

v8250

2,724 posts

210 months

Monday 13th October 2014
quotequote all
MGsimon said:
Thanks guys. I will be doing this as soon as it's back. Also iv found out she now needs some new fuel lines.

Here is the rust that has been sorted.
Simon,

1. when your garagiste replaces the fuel lines make sure he uses kunifer brake/fuel line, not plain copper or steel.

2. when the chrome waist pieces are renewed a) replace with stainless steel, not chrome, and b) be certain they water seal the fixing holes; this is where the rot/surface rust begins. get them to do this with an rtv sealant after painting/at the point of fitting waist piece rivets. if they don't do this rust will re-appear.

ps this are classic MGB rust areas

Expatloon

215 posts

156 months

Wednesday 15th October 2014
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MGsimon said:
a few small surface rust and paint bubbles sorted before the winter comes in
In hindsight something of an understatement !

Where is that bottom right a photo of ?

Dbest92

300 posts

132 months

Wednesday 15th October 2014
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Expatloon said:
MGsimon said:
a few small surface rust and paint bubbles sorted before the winter comes in
In hindsight something of an understatement !

Where is that bottom right a photo of ?
Looks like the lower rear quarter by the back bumper? Defiantly worth getting done properly, looks a nice car smile

MGsimon

Original Poster:

8 posts

114 months

Sunday 19th October 2014
quotequote all
Expatloon said:
MGsimon said:
a few small surface rust and paint bubbles sorted before the winter comes in
In hindsight something of an understatement !

Where is that bottom right a photo of ?
Yea was bit of a understatement glad I got it checked out. The bottom right picture was of under the rear bumper where it attaches to the car. Was rotted though both bits of metal, lucky it didn't get on to the chassis

MGsimon

Original Poster:

8 posts

114 months

Wednesday 19th November 2014
quotequote all
Hi all, I now have my car back and first thing I did was to check the tyre sizes. They are 185/70R 14!

Would this course of my issue ?

ATE399J

729 posts

236 months

Thursday 20th November 2014
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MGsimon said:
Hi all, I now have my car back and first thing I did was to check the tyre sizes. They are 185/70R 14!

Would this course of my issue ?
By my calcs this would account for just under 2% speedo error.
165 full profile (82%) gives a rolling rad of about 12.33" and 185/70 gives about 12.10".
The smaller RR would over read (is that right?).
P.

MGsimon

Original Poster:

8 posts

114 months

Friday 21st November 2014
quotequote all
I'm reading approx 30 when doing 20 if that helps.

v8250

2,724 posts

210 months

Friday 21st November 2014
quotequote all
MGsimon said:
I'm reading approx 30 when doing 20 if that helps.
Simon, your speedo' reading error will not be coming from that difference of tyre size. To check use this website http://tire-size-conversion.com/tire-size-comparis... I've checked and you'll see a maximum of 06.mph difference at 70mph. As I mentioned previously, go back and check your speedo' gear drive AND the speedo' cable. Make sure the cable is lubricated correctly and not twisted when installed.