The Triumph Dolomite Club - Discussion Forum

This has been bugging me for a while now so finally felt the need to post and get some feedback...
When I collected my Dolomite 1500 from Wales and brought it back to Norfolk, my daily driver Merc covered just over 300 miles. A friend who was driving the Dolly covered 350 miles. He was also under the impression that he was cruising at about 60 mph for much of the time. However, he was following me all the time and at no point did our mini convoy exceed 55 mph. It was quite a long journey!!
At some point in it's life the Dolomite has been fitted with an overdrive gearbox - and very nice it is too. It does not appear on the original sales invoice so I have to assume it is a replacement by a previous owner.
Would this explain why the speedometer & odometer both over-read, and if so, can it be easily (read cheaply) corrected? If not, I'll stick with the current gearbox as it really is very good.

What you desire can be achieved by swapping the speedo DRIVEN gear in the gearbox, which is quite easy to do. there are a number of different gears to choose from, colour coded as well. But working out WHICH one will correct the speedo is harder, I would imagine a 1 tooth difference would be sufficient. So first you need to find out how many teeth are on your original.

Alternatively, as the error is within the 10% allowed by law, you could just live with it and drive to suit it! Which is what I do with the Carledo, my choice of Nova speedo, Omega gearbox, Sprint 3.45 diff and 15" wheels has left me with a speedo that UNDER reads by 40% so when the clock says 60, i'm actually doing 100 (on track of course) At first it was a pain, but some careful checking against a satnav showed the error to be consistent so a clock 18mph is a true 30, 42 is 70 and so on. After a few months I got used to it and now I no longer even think about it, doing the conversion automatically in my head, much the way I did for years with my LHD Volvo Amazon, which had a KPH only speedo.

Steve

It should be easy and relatively cheap to fix. Assuming the diff hasn't been changed then all you need to do is fit a different drive wheel in the box where the angle drive fits in. Overdrive Repair Services in Sheffield can advise.
Take out your existing wheel and count the teeth accurately. Note the colour as well.
Then call Overdrive Repair Services, tell them the over-reading figures you've recorded, the colour and number of teeth of your drive wheel and hopefully they'll have a solution.

Check out this page, Speedo Pinions:
http://www.overdrive-repairs.co.uk/buy- ... rts%20List
Just noticed all sorts of goodies on that page including exchange solenoids, angle drives and solenoid spanners!

Non original tyre size can also have a significant effect on speedo accuracy. So can tread depth, so worn tyres with 5mm of tread gone will increase speedometer error, albeit by a small amount.

However, most car manufacturers err on the side of caution with speedometer readings, especially I believe modern Japanese so it is not unusual to find a 100kph reading equates to an actual 95-96kph. However, the error rate may not be equal across the range of readings so 50kph may be almost spot on.

At least I now know why so many people sit in the outside lane of the motorway doing only 95!

We have the luxury in NZ in having various sections of motorway with distance markers from 0 to 5km, specifically for checking speedometers. I've found also that iPhone GPS speedometer apps can be extremely accurate, assuming the road is flat and straight.

My standard Sprint on tyres with the original profile (175.70x13) has a speedo reading about 5% fast (reads 105 when actually doing 100kph). The other with auto 'box on 185.70x13 is almost spot on. Nissan 350Z reads 4% fast, but the XF Jag is less than 2% fast.

Incidentally the speedometer for a manual overdrive and an automatic Sprint are calibrated differently and are not interchangeable. There is a small number at the bottom of the dial face that indicates the calibration, so it may be worth seeing how yours compares to another 1500 that came originally with an overdrive gearbox.

In NZ the traffic cops have a tolerance above the limit before they normally issue a ticket (no guarantee though). This is usually 9kph, but in school holidays and public holidays it drops to 4kph. Knowing the indicated speed that produces an accurate 108-109 for the car you are driving is therefore important to some of us!!

Geoff

For some reason later build 1500 dolomites have a different rear axle ratio to the earlier cars. I have been advised that 3.63;1 ratio dif were fitted from chassis number WK13577DL for 1500HLs and from WG3741DL for the 1500. The earlier cars had a 3.89:1

If you go to the wanted section of this discussion board and read the recent article "Dolomite 1500 manual to O/D conversion" it includes an old Dolly mixture article that I wrote about fitting O/D to a 1500 and in it I cover how you can identify the various O/D units.

I also believe you need to change both the pinion and the speedo drive to get the correct speedo reading and to stop the teeth being damaged. I have never done this so I do not know how easy it is to change the speedo drive gear.

An alternative method is to get the speedo recalibrated.

In the moss-europe spitfire parts catalogue in gives the following info for the O/D speedo drive. The spitfire has 13 inch wheels like the dolomite

NKC46 Gear speedo drive 3.89:1 axle ratio
NKC45 Gear speedo drive 3.63:1 axle ratio (I believe blue)

NKC49 Pinion, speedometer drive 3.89:1 axle ratio
AEU1578 Pinion, speedometer drive 3.63:1 axle ratio (I believe red)

During the type approval of the vencer the speedo had to have a 10 percent misreading to the upper side. It is obligatory for car manufacturers to have their speedo calibrated that way.

Jeroen

gmsclassics wrote:, especially I believe modern Japanese so it is not unusual to find a 100kph reading equates to an actual 95-96kph.

The wife's Lexus over reads hugely, more than one might expect from a Toyota. I can only assume it helps fuel consumption! My BMW is spot on.

Thanks for the responses. I had an idea that the options were to have the speedo recalibrated or to change the speedo drive pinion. ORS's website implies that replacing the drive pinion involves dismantling the overdrive, which I'm not keen to get into as it's working well.
I may decide to live with it, as it over-reads so don't think I'm going to be breaking any speeding laws!!

Rimmer also sell some of the bits, more than twice the price of Overdrive Repair Services,

Simple, take off the gauge glass, pull off the pointer and put it back on again slightly higher/lower than before!

The speedo drive gear on the gearbox shaft is always the same in an overdrive box or non overdrive.

The difference comes in the cog that takes this out to the cable and this must have the number of teeth to match the diff ratio.

You may well find the cog in the non overdrive box can be used in the overdrive one but Ive never tried this. The speedo's are all 1000 turns per mile as shown on a small printed figure in the dial bottom.

Its easy to change the speedo drive gear as it just pulls out but it may also need a new O ring seal.

Jonners