I think my speedo reads under after conversion to O/D

[Bumpa]

My 1850 HL (1977 model) was built with a single rail non-O/D gearbox. I have fitted a single rail O/D gearbox. I haven't changed the axle ratio, but have fitted Sprint wheels with 175/70 x 13 tyres. When my wife was following the car in her VW she thought I was breaking the speed limits, whereas my speedo showed I was under the limit.

Should I have changed the drive gear in the O/D gearbox? It is my understanding that the same speedo was used in both types, so I assumed the drive gear would be the same. If I need to change, which one do I need? Part numbers?

Thanks
Mike

[yorkshire_spam]

Did the O/D gearbox come from a car with the same final drive ratio?
IIRC Speedos are the same, but the number of teeth on the speedo pinion drive varies according to final drive ratio.
Cheers,
Sam

[oily66]

Most moderns over read (its to slow you down) re try it with a sat nav, see what that results in.

[yorkshire_spam]

Forgot to say... car will under read if tyres/wheels with a higher effective rolling radius are fitted, for each extra tooth on the pinion drive gear you adjust the speedo by about 5% (very roughly)

Cheers, Sam

[Bumpa]

I bought the O/D gearbox from ebay and was told it had come from a Dolomite 1850 like mine. Certainly it was a direct replacement. According to Rimmers catalogue, the same final drive (3.63:1) was fitted to all manual 1850s, overdrive or not. Only the automatic had a different ratio (3.27:1).

Changing to Sprint wheels and tyre size should have made the speedo over-read by 3%, but I'm convinced mine under-reads. I will have to try it with my TomTom to verify what I feel.

Now here's where logic lets me down. If I put in a drive gear with one less tooth, will that make the speedo read higher or lower? I THINK it will read higher, but my brain hurts pondering this issue!

[tony g]

The taller tyre diameter will cause under reading but you are going faster, so 30 might show as 26 etc

Tony

[xvivalve]

The plate on the side of the overdrive codifies what car it originated from...if you want to be more certain...

[Bumpa]

The taller tyre diameter will cause under reading but you are going faster, so 30 might show as 26 etc

Tony

Not sure if that's right. According to the tyre size calculators on the web, the original tyres (155/82 x 13 Diameter 584mm) are taller than the Sprint tyres (175/70 x 13 Diameter 575mm). Therefore the new tyre is going to rotate slightly faster for any road speed, making the speedo over-read. According to the calculator I used the difference is only 1.5% so pretty insignificant.

I have been out with the TomTom installed and have found that the speedo is only about 2 or 3 mph under-reading at a true 60 mph, so about 5% error. I can live with that as long as I know about it.

Alun, next time I'm under the car I'll have a look for the overdrive plate and report what I find. I would like to know if it is a proper Dolly gearbox.

[GTS290N]

Make sure you check the speedo at different speeds! Mine under reads slightly at 30, gets better as the speed increases and reads correctly at 80. Of course all moderns are under-reading at that speed.

[tony g]

Ah ok my bad

Tony

The taller tyre diameter will cause under reading but you are going faster, so 30 might show as 26 etc

Tony

Not sure if that's right. According to the tyre size calculators on the web, the original tyres (155/82 x 13 Diameter 584mm) are taller than the Sprint tyres (175/70 x 13 Diameter 575mm). Therefore the new tyre is going to rotate slightly faster for any road speed, making the speedo over-read. According to the calculator I used the difference is only 1.5% so pretty insignificant.

I have been out with the TomTom installed and have found that the speedo is only about 2 or 3 mph under-reading at a true 60 mph, so about 5% error. I can live with that as long as I know about it.

Alun, next time I'm under the car I'll have a look for the overdrive plate and report what I find. I would like to know if it is a proper Dolly gearbox.

[Mad Mart]

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph.19 Sep 2012

[Carledo]

Because of the hotchpotch of Triumph and Vauxhall parts on the Carledo, the speedo under reads by a consistent 40% as checked by satnav at 10mph increments, all the way to 110! But it's SO far out, you could NEVER think it was telling the truth and I have simply adapted my brain to translate what my eyes see into reality and no longer have to think about it. I suppose I COULD use the KPH scale as MPH for a rough guide, but the figures are small and in a semi illegible orange font, I can't be bothered. It must work as i've yet to get a speeding ticket in the car! Technically illegal I guess, but if I never get caught, no one will know! (except you lot now!) I did think about getting a new face made for the clock giving the correct speeds, it would give me the chance to have a speedo that reads to 200mph. But since the PCB on the current Vauxhall Nova speedo is dying fast and is NLA, i'm having to go back to traditional Triumph instruments, we'll have to see what that throws up in the way of errors! I have a 1000TPM Sprint speedo and a Sprint 3.45 axle, the tyres are 195/50/15, which are a pretty close match to 175/70/13 in terms of rolling radius. But the wild card is the Carlton speedo driven gear, I have no idea what the ratio is, but the original diff it was mated to was 3.7/1 and there were no ratio choices, so no choice of driven gears either!

Steve

[Galileo]

Fit a Smiths Flight speedo, then you can calibrate it to whatever you want. Mind you, costs roughly £200 so not a cheap option!

I've got used to using the tacho, 1500rpm in 4th is 30mph, 2600 in OD is 70mph etc. made nice and easy as 1000rpm is 20mph in 4th (approx and Sprint).

[Carledo]

Fit a Smiths Flight speedo, then you can calibrate it to whatever you want. Mind you, costs roughly £200 so not a cheap option!

I've got used to using the tacho, 1500rpm in 4th is 30mph, 2600 in OD is 70mph etc. made nice and easy as 1000rpm is 20mph in 4th (approx and Sprint).

I have to get a Smiths electronic speedo for the Dolomega as it has an electric drive on the trans which interacts with the trans and trip computer too. So I MUST keep it! Merlin Motorsport at Castle Coombe do them for around £170, at least I don't need to buy the pickup which is another £50! But even so, i'm delaying the inevitable in the vain hope of finding a used one for more sensible money! I certainly wouldn't buy another for the Carledo whose Omega manual box came without any drive at all (manual Omegas draw a signal from the ABS) fortunately for me, the holes were all present, just blanked off and I installed the Carlton drive and driven gears with a Nova cable and speedo. I think this mismatch is what gave me the huge error, since the Carlton drive takeoff is from the gearbox output shaft, ie ahead of the final drive, and the Nova is driven from (effectively) the o/s driveshaft, ie the OTHER side of the diff! Using the Dolomite instruments could well make the speedo much more nearly correct! It would be cheaper to get a Sprint speedo recalibrated than buy another electronic one!

Steve