Gt6 wheel/tyre size - speedo not reading correctly

[Red Devil]

Have just taken my mk2 gt6 for a spin as I haven't really driven it before. Really enjoyed with my 6 year old daughter smiling along all the way also. Anyway, I couldn't help but notice that as I approached a speed indicator device in one village it had me doing around 25mph when the speedo was just over 30. In the next village they also had one so I got it to read 30 on the device and this was just under 40 on my speedo.

 

I have Minator 13J rims fitted with 165/70 R13 tyres. I'm assuming the speedo is working correctly and therefore the wheel/tyre ratio isn't correct. What size tyres should I have fitted to get the speedo reading accurately? I probably do need to get new tyres so a good opportunity to get this right.

 

Many thanks

 

Steve

[Clive]

http://www.mintylamb.co.uk/gearspeed/

 

use that and input you data. It will give you an idea of what is what. The tyre size is a little smaller than std, so yes will over read, but your inaccuracy is much more significant (ie your combo will be about 2-3% out, but speedo is 20+% out!

 

Possible problems:

Incorrect speedo/diff combination. The speedo is paired to the diff ratio on most Triumphs, and over the cars history lots of stuff may have got swapped about.

Does the car have overdrive? if so what type. J types need the correct speedo gear, just about every one I have had has over read speed. By about 20%, both in cars I have rero-fitted it and those that came with it. 

 

You need to investigate what diff ratio you have, what overdrive (if any) and the speedo identification numbers. Report back with that info!

[Red Devil]

I do have an overdrive but D type. I guess it is better for me to go slower than too quick but like you say with the tyre size it shouldn't be as far adrift as it is. If it is the speedo how on earth do I set about trying to get the right one as they appear secondhand I've noticed but rarely say which car/gearbox they are for!!

 

I have noticed that the 175/70 tyres get me even closer so maybe I should fit these next time but I'd like to understand what is happening with the speedo. I also guess that there is more chance of the wheels rubbing if I increase the size as I have noticed some comments about this previously.

 

Of course the speed indicator devices could be incorrect also but to pass two of them and have the same affect is odd given usually they appear quite accurate. I'll have another drive by to check again!

[Clive]

You really need a GPS/satnav to do an accurate check, at a steady speed ideally at 30, 40 etc etc.

My 30% chance of being right is that you have a 3.27 diff but a speedo calibrated for a 3.89.

 

There are places that can recalibrate your speedo. But first, check your diff ratio and speedo numbers.

 

Changing the tyre size to 175/70 will increase the inaccuracy by about 1mph at 30....you would nned to fit something like 10inch mini wheels to get the reading corrected!

[Pete Lewis]

Triumph had a speedo for each diff ratio , and not much in the speedo pinion options

 

speedo could read 10% +4mph in the old days so you could do 60 and read 70 they cant read slower

 

the turns per mile are printed next to the oddometer roll look for something around the 900 to 1200 type of number

 

away to check what you need , is mark 52.8 ft track on the road,

stick a card needle to the inner speedo cable push car the distance and coint the turns

 

x the result by 100 and thats what you need on your speedo

Places like speedy cables or jdo can reset you own instrument

 

If the error is consistent you can romeve the glasss rim remove the mechanism hold the disc drive and move the needle on its spindle, never take the needle off the hair spring will unwind and never to be reset

 

Move it a few mph might just give you acceptable readings

[John Bonnett]

It would be interesting to hear what revs you are doing at  a true 30 or 40mph in 4th gear.

[JohnD]

Or, you can ride the motorway with a friend counting the 100 meter posts.  So many in a minute, or how long to pass, say ten, and then it's mere arithmetic.

 

Sad to say the 100m pots are not well maintained, so test drive first to find a run of upright ones.

It's all to get you out in your Triumph!

John

[Red Devil]

Interesting drive tonight and not 100% scientific tonight I know but I downloaded an app on to my phone which is supposed to monitor speed based upon gps. I have to say it totally backed up my experience yesterday but with a interesting twist at the higher end. Currently if the speedo reads just under 40 it seems to be really 30mph, similarly just under 50 is 40mph. However, as I got up to about 56 it seems like it was close to 50mph and then at about 63 it was reading 60mph. This is with the overdrive engaged in 4th gear and I forgot to check exactly but approx 3000 rpm.

 

Hence, I think I'm going to live with it being out at the lower end in favour of the fact it almost seems to correct at the higher end. One day I'd like to get it sorted but it isn't my priority right now I guess so long as I know what it is doing albeit a little frustrating. I did enjoy going out for a drive in the evening sunshine though which is what it's all about for me!

 

Thanks

 

Steve

[Pete Lewis]

Time for some sticky tape speed numbers stuck to the glass

 

pete

[John Bonnett]

That's interesting. On my car fitted with 155 80 13s , 3.89 diff in overdrive fourth 60mph is the same as yours; 3000rpm. Rather than mess about trying to find a speedometer to suit you could get yours calibrated by Speedy Cables. 

[Dick Twitchen]

Steve, 

As a backup to Pete's sticky tape used the Minty figures to calculate rpm/speed for 30, 50 and 70mph in 4th overdrive and put little green roundels on the tacho bezel against the appropriate revs.  Interpolate as required.

Dick 

[mishmosh]

my herald running standard 411 diff and d type over reads on standard wheels by 15 % . My Bond on Mk IV wheelswith oversize tyres  and J type with 411 diff under reads by 10 pc. I swapped them over . Sorted.  Not ideal though ! The Bond really needs a different diff ....