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I spent half the weekend changing the 4.1 open diff on my NA8 for a 4.3 Torsen and was expecting the speedo to be out after the change. To my suprise the speedo seems to be spot on (I am guessing the tacho is correct, and I used a gear calculator to check speed at different RPM). I can only guess that the speedo was reading slow before (odometer was about right before the change based on km posts on a long trip last weekend).

So, has anyone else had issues with speed accuracy before, and can it be tweeked to read correctly and still have the odo read correct too?

Also, if most of the NA speedos read low, how do you poor buggers in the Eastern Block survive all of those fixed speed cameras?

ps. Torsen diff rocks!

diff sure does rock.
just sit under that magic mark. In NSW they warn you. unlike in Vic.
here you get around 3 wraning singns. before the fixed camera.


I've gone from a 4.3 open to a 4.1 Type 2 torsen.

My MX-5 speedo over reads by 5% (105 indicated = 100 true). Checked it against 5km of measured road distanced and not off the odometer. Astina was spot on.

changing the diff ratio won't affect your speedos accuracy, it only changes the ratio of revs at the same speed up or down......................the only major factor in speedo accuracy is if you change the rolling diameter of your wheels & tyres

AJ wrote:changing the diff ratio won't affect your speedos accuracy, it only changes the ratio of revs at the same speed up or down

Mmmmm . . . I haven't had my coffee yet . . . and I'm not getting any younger . . . are you sure? . . .

JBT wrote:My MX-5 speedo over reads by 5% (105 indicated = 100 true).

likewise...

My GPS receiver tells me that 60kmh indicated on the speedo is 58kph.

Funnily enough, the Calais is exactly the same.

What size tyres do you use?

I went from 4.1 to 4.3 in my NA8 too, and it always feels like the speedo is over-reading. If I sit on the freeway at an indicated 115km/h I get overtaken all the time

AJ wrote:changing the diff ratio won't affect your speedos accuracy, it only changes the ratio of revs at the same speed up or down......................the only major factor in speedo accuracy is if you change the rolling diameter of your wheels & tyres

Too much birthday bourbon AJ?? The speedo drive is taken from the gearbox on most cars, and is calibrated based on the original final drive ratio. Changing the final drive ratio from 4.3:1 to 4.1:1 will result in the actual speed of the vehicle being faster than the indicated speed. Given that nearly all cars speedos are optomisitic by around 5% minimum, the change will most likely result in increased accuracy of the speedo. Going the other way like Babs has done will result in the car being slower than the indicated speed. The problem can be overcome by changing the speedo drive gear to one that matches the final drive ratio if you are worried. I use the GPS to check true speed and drive accordingly.

Babalouie wrote:What size tyres do you use?

I went from 4.1 to 4.3 in my NA8 too, and it always feels like the speedo is over-reading. If I sit on the freeway at an indicated 115km/h I get overtaken all the time

Babs, not sure if you meant JBT & myself, but my drive train, wheels & tyres are all stock NA8 - indicated 115 is actually 110 for me. I believe JBT went 'plus 1' with his wheels & tyres.

I checked with stock 14\" wheels and tyres as well as with 15\" with 195/50 tyres and got the same result both times.

I should have added that an increase in the rolling diameter of your wheel/tyre combination on the rear will result in an increase in actual speed, while a decrease in overall diameter will have the opposite effect (but will allow the car to accelerate quicker - hillclimb cars often use smaller dia rear wheels for this reason).

I run 195/50/15, but this was taken into account by the gear calculator. If the speedo is correct now then it must have been reading slow with the 4.1. On the weekend i will go and find some km marker posts and check the odometer. Will check the speedo at the same time with a GPS. The thing that really worried me is that I drove ~1000 km last weekend on a double demerit point weekend sitting on just the wrong side of the limit. If my speedo was out, then I was real lucky not to get pinged.

If you went from 4.1 to 4.3 without changing your speedo drive gear, then the speedo will be over reading (indicated > true).

195/50/15\" = 185/60/14\" as far as rolling diameter goes.

The RPM vs indicated speed won't change unless the speedo drive gear is also changed.

Is there a corresponding speedo cable gear reduction we can buy when going from a 4.1 ratio diff to a 4.3 ?

My current speedo with 4.1 diff already reads fast (115km/hr indicated when travellling 110km/hr), going to 4.3 diff would make indicated speed even faster.