17 replies to this topic

[Msmith3327]

I have just had the Final drive changed for a 3.1 from a 3.4, the other gears haven't changed.

Now I know that my speedo is now wrong but is my rev counter now wrong also??

I have been looking at the calculator on Guessworks website and the revs seem to fit the mph at low revs but the higher I go in rev range the bigger the difference between what my speed is and what the speedo show. Is this correct?

Sorry if this doesnt make much sense, but it does in my head.

[BoboGib]

The Rev Counter will more than likely take it's signal from the coil so should be near enough correct.

the speedo takes it's drive from the gearbox, so if you have had a new final drive fitted, chances are you'll have to change the speedo drive to match.

[Msmith3327]

As the speedo works correctly in 1st, 2nd and 3rd gear, if i change it to ensure 4th is correct wont it mess up the others?

[Dan]

What gear the car is in has no bearing on the speedo at all. The error is proportional so the faster you are going the greater the difference.

[Msmith3327]

Thanks for the heads up guys, it has cleared the fog from my brain.

[Cooperman]

The rev counter measures the engine speed electronically, so the final drive has no bearing on it.
The speedo drive is measured from the gearbox just before the nfinal drive, so the percentage inaccuracy will be the same in each gear. The faster you go in terms of road speed, not in terms of engine revs, the greater will be the error. Going from 3.44 to 3.1 FD ratrio, your speedo will under-read your true speed.
If you take the indicated speed, divide it by 3.1 and multiply the result by 3.44, you will have the true corrected speed, subject to the original speedo error of course. In percentage terms it's approximately 11% under-reading, so add,say, 10% for a quick indication of actual speed.

[Msmith3327]

Hmm I am still confused now, I will have to note down the revs to speedo on the way to and from work. From memory I am sure rev counter said 2000, speedo said 30mph, rev 3000 speedo 45.

I also need someone to follow me and keep tabs on my speed.

oooooh the joys of Mini ownership lol

[Guess-Works.com]

or get a GPS which will tell you how fast you're going.

[rwerry]

On 12" wheels with a 3.44 final drive you would be doing 16.45mph/1000rpm. = 49mph @ 3000rpm
With a 3.105 final drive the same car would do 18.25mph/1000rpm = 55mph @ 3000rpm

[Msmith3327]

I reckon a mate in his car for a pint is cheaper than buying a TomTom

[MRA]

Subject to your mates speedo being correct..........

And you tacho (rev counter) could quite easily be 1000 rpm out.

[TopCatCustom]

IMO Sat Nav's read too fast- my lorry tacho has to be calibrated on a rolling road every 2 years, whilst talking to the operator he said they are calibrated to be perfect- nothing else, unlike cars which are almost always too slow on the speedo.

I know in theory a sat nav can't be wrong (apart from not knowing when you are going on an incline/decline which would affect the reading) but it is about 2-3 mph out at 50mph in my truck. Best way- follow a lorry with a friend driving!

[Chris_R]

Surely the method of calculating your speed based on revs quoted above will only be relevant for 4th gear, if you're not in 4th you'll need to apply a different percentage to the speedo reading for the revs you're at to get an accurate result??

By far the easiest resolution must be to have the speedo drive changed to the correct one for the final drive ratio, or to contsantly run a GPS

[cooperrodeo]

This is where I mention the measurements done on my car.

I have 10" wheels, 3.44 diff, 1280 turns per mile speedo and correct speedo gears in the gearbox. Working backwards from the figure of 1280 turns per mile, this gives a figure of 16.53 mph/1000rpm and also an effective radius of the wheel of 243mm.

A rolling road session indicated I had nothing like 16.53mph/1000rpm. The reality is that the measured effective radius of the front wheel when laden is more like 230mm. (You have to roll the car a bit just before measuring as it settles down even further when left to stand).

The result is that I actually have 15.65 mph/1000 rpm and so the mileometer reads 5.6% high. I had already tweaked the speedo needle on the bench so it read correctly at 70mph (assuming 1280 turns per mile), so that also reads 5.6% high at 70mph when in the car.

In fact I rely on my rev counter for speed (using the 15.65 mph/1000rpm figure) as it has been calibrated at 3000rpm and 6000rpm using mains frequency.

[Cooperman]

Plus, of course, the fact that as tyres wear the speedo will over-read some more.