Speedo inaccuracy

[IWantAWedge]

Well, I noticed yesterday that my speedometer is rather inaccurate, seems to be about 10-15% out once you get going, reading low. i.e. when the speedometer was reading 70, my tomtom was showing 85. Also, the final 10miles home only registered 8.9 miles on the odometer. Now I can live with this, and just adjust my speed accordingly to avoid having pictures taken of the car, but is there an easy fix for this, some way of calibrating the instruments, or is it more involved than that.

My initial thoughts were that the wrong size tyres were fitted, but a check of that proved that the correct 205/60/R14's were fitted all round.

[Wedg1e]

Funny you should say that because I always understood TVR speedos to be calibrated in 'fibs'...

My Tasmin's speedo used to be a mile (or rather, several miles) out, yet the 390's is jaw-droppingly correct according to Mr. Garmin... 100mph on the speedo is 97mph on the Satnav and even at 125 indicated it's only 5mph optimistic. Mind you I have seen the needle off the scale at 160 (on a private test track, m'lud) which, even if the car could reach its claimed 146mph top speed, implies some sharp non-linearity once past 120.
Essentially these speedometers work by a spinning magnet on the end of the drive cable, inducing a magnetic field in a close-fitting cup around the magnet. The cup is attached to the pointer and acts against a return spring, so other than the tension of that spring there's not much to adjust. Everything else depends on the scaling of the speedo face digits and the speed at which the drive cable spins, which is determined by a take-off gear in the transmission. On the Rover (LT77) box you can change the gear to get different ratios but I'm not sure if the Ford 'box is similarly tamperable.

[RCK974X]

All Ford 'boxes up to and including type 9 (5 speeder) had a wide choice of speedo gears, and cover diff ratios from something like 4.5:1 to 2.8:1 for 13 inch wheels, or pretty much everything you might find.

I think with newer stuff though, it's easier to recalibrate the speedo for electronic senders.

NB. in case it's any use, as far as I know all Ford mechanical speedos were calibrated at 1000 turns per mile.

[adam quantrill]

My 400SX reads 9% too high when on standard 15" rear tyres but with 16" rims and a slightly increased rolling radius it's spot on.

So apart from the speedo drive you could consider 15" rims on the back to change the ratio slightly.

[Rockettvr]

First time I drove my wedge I couldn't work out why I seemed to be steaming past all the other traffic on my local A road even though I was only doing 60. Turns out 60 on my speedo is about 80 mph true speed !! Someone diagnosed a "sticky" speedo - I thought it might free up as it hadn't been used for a couple of year prior to me acquiring it but 1000 miles on it's no better. It's a 280 5 speed with smiths clocks. It's a little worrying under reading by so much with all the speed cams around. Guess some WD40 is required - if I can get to the back of the dash that is!

[Wedg1e]

Rockettvr said:

Guess some WD40 is required - if I can get to the back of the dash that is!

Oo, you don't wanna do that... I tried it years ago on a Simca (!).... what happens is that the oil fills the gap between the magnet and the outer cup, creating a viscous coupling. As soon as you start moving, the speedo neeedle goes off the end of the scale!

[Rockettvr]

Bugger! Not sure what the cure is then other than a total strip down of the speedo ? Need to sort it as I'm bound to end up getting a ticket at some point as It reads so far out

[Danny Hoffman]

My 350i's speed is spookily accurate like Ian's (not tried to hit 145mph though so no idea if it would show 160.......)

Is it possible your diff has been changed, aside from a faulty speedo I can't think what else could be the cause as you have checked the tyre size?

[JMF894]

I was checking my speedo against the garmin just this weekend and at 80 it was absolutely spot on

Jimbo

[hallsie]

JMF894 said:

I was checking my speedo against the garmin just this weekend and at 80 it was absolutely spot on

Jimbo

on a private road just round the corner I assume?



Stu

[JMF894]

hallsie said:

on a private road just round the corner I assume?



Stu

80 km/h Stu



Jimbo

[grahamw48]

If you're nervous of speed cams (as I am) use your Satnav, if you have one, alongside your speedo. Just as well in my old 400SE, as the speedo didn't work at all.

[njhucker]

There is no mechanical link to the speedo pointer. The pointer is connected to a piece of alloy, which is pulled around by a magnet which is next to it. The speedo can be calibrated by increasing/decreasing the magnetic force ( which is done using an electromagnet) or by increasing/decreasing the hairsping force. The pointer is normally an interference fit on the shaft. Say when you sat nav is telling you that you are doing 60, your speedo says 55, you can do the following to give a reasonable calibration:
Take off the speedo case.
Turn the alloy disc so that the pointer is reading 55 and hold the disc there.
Reposition the pointer on the shaft so that it now reads 60.
Your speedo should now be fairly accurate.
Now this is fine for the speedo, but if the speedo is reading incorrect because of a change of gearing, tyres etc, then the odometer will not be correct as that is directly driven, and can only be calibrated by adjusting the gears inside, or the gearing of the speedo cable drive gear/pinion.

[njhucker]

Forgot to mention that if your gearbox is ford. They do different pinions with different numbers of teeth, and the pinions are colour coded. i found this out when many years ago the pinion went on my Taimar and a TVR dealer gave me the wrong one.

[Wedg1e]

Danny Hoffman said:

My 350i's speed is spookily accurate like Ian's (not tried to hit 145mph though so no idea if it would show 160.......)

Is it possible your diff has been changed, aside from a faulty speedo I can't think what else could be the cause as you have checked the tyre size?

What's even more bizarre on my car is that (as I only realised this year) the diff ratio is 3.31:1 rather then the more usual 3.54:1...! I guess whoever ordered my car from new wanted a bit more acceleration

[grahamw48]

Ian, your final drive would give higher gearing therefore more sluggish acceleration, surely ? (But better fuel economy).

[Danny Hoffman]

I seem to remember every TVR road test always seemed to be with a car the factory had fitted an optional lower ratio differential, I wonder if this was to improve the all important 0-60 times........

[adam quantrill]

njhucker said:

Say when you sat nav is telling you that you are doing 60, your speedo says 55, you can do the following to give a reasonable calibration:
Take off the speedo case.
Turn the alloy disc so that the pointer is reading 55 and hold the disc there.
Reposition the pointer on the shaft so that it now reads 60.

Yeah but - at 120 it would have shown 110, and you've only moved it round 5 mph, so it would then show 115.

Also when doing 0 it would read 5.

[njhucker]

A 3.31 diff instead of a 3.54 will give easier cruising, rather than better acceleration. Normally with all things being equal, the more powerful the car, the lower the diff ratio. Jags for example used to use 3.54, 3.31 on the xj6 & xj12 & 3.07 on the XJR

[grahamw48]

Also 4+ on the 2.8 Auto...one of which I once owned. Not the quickest Jag around.