Speedo calibration with bigger tyres

[Big-Stu]

Thought I would start this topic on speedo errors when changing tyre sizes. These adjustments were done to the speedo on a 1992 F75 diesel Rocky. I don't know if Ferozas and other Rocky models have these adjusters on the rear of the speedo.

The concise guide to calibrate your speedo in 25 easy to follow steps:(if you got any other hints please add them)

Get a hold of a GPS or a friend with an accurate speedo and a mobile phone.

Drive your vehicle at 60, 70, 80, 90, 100, 110 kph (up to 180 kph if you’ve got a diesel Rocky) Using the GPS or the reference speedo write down your indicated and actual speeds. So now you’ve got something to work with.

Rip the speedo out of the dash. Easy to say, hard to do.

Get yourself a fully charged battery drill and clamp it in a vice with the chuck facing upwards. Get a set of them screwdriver bits to suit your drill and find one that fits neatly into the rear of the speedo. If it doesn’t fit neatly you will end up stripping the drive at the rear of the speedo. Generally the phillips bits seem to work best.

Rest the speedo on top of the drill, don’t push down to hard, and run it up to full speed. I’ve done a few speedos and most will show somewhere around 70 to 90 kph. Make sure you consistently get the same reading every time you start and stop the drill, remove the speedo etc. You must be able to consistently get the same speed reading. This now becomes your reference speed that you can easily reproduce on the bench. It’s also handy to have a 2 speed drill so you can get a couple of readings.

So if your drill reference speed is 80 kph and in your vehicle it showed 72 kph you know that the speedo needle has to be moved up by 8 kph.

From here onwards I accept no responsibility for broken speedos or speeding fines.

Before you do anything else to the speedo physically hold the needle at the 80 kph mark and carefully mark the position of the round movable disk that the speedo needle is attached to, and the position of any adjustment screws. If you stuff it up these are your starting positions.

To change the speedo reading you will need to do a combination of the following:

Adjust the spring tension for the speedo needle. After this you will probably find the zero position is wrong. This is where the Daihatsu zero adjustment comes in real handy !! (info based on the F75 diesel Rocky) From memory I think this is all I had to do to my F75 speedo.

Physically move the needle position. Once again it will have to be rezeroed. I’m not sure how accurate this method will be – maybe someone else can comment.

Just remember the bigger tyres change the ratio. From your actual vehicle readings you should see values like 5kph slow at 50 kph and 10kph slow at 100kph.
So try and get a few reference speeds with your drill so you can see what your adjustments are doing. Obviously 60 kph and 100 kph are the important ones. (and of course 180 kph for the diesels)

One last thing, remember the odometer will be still be out by about 8% for the 31 inch tyres. If all else fails you can get ratio adjusters that fit between your gearbox and speedo cable but they cost a fortune. This way is much more fun isn’t it.

And if you manage to get a diesel Rocky up to 180 kph your speedo is a long way out of calibration or it’s the Feroza I see at Willowbank drag strip every so often.

Stu

[RockyF75]

My Diesel Rocky consistently hits 180kph ..... but only when i'm doing a smokeshow trying to get up a rock-ledge

Thanks for posting that info, i will now definatley pay someone else to calibarate my speedo (it sounds too hard and riskiy for me )

[BundyRumandCoke]

Big Stu
I think I can shorten your proceedure down to one or two steps.

I agree with the first step, get a GPS

2nd, mount it where you can watch it when driving, and set it to Speedometer.

Your done. Much quicker, and even more accurate.

[runnin4life]

the weird thing is my speedo isnt that far out with my 31's i went along with my mum she had her hand held uhf and i had one and when she was doing 90km's my speedo said about 92km's


so i dont think its as bad as my old man in his 80
he's got 31's and when it says 100 he is doing 90 so its out by a fair bit

lol
any ways
cheers
elliot

[runnin4life]

the weird thing is my speedo isnt that far out with my 31's i went along with my mum she had her hand held uhf and i had one and when she was doing 90km's my speedo said about 92km's


so i dont think its as bad as my old man in his 80
he's got 31's and when it says 100 he is doing 90 so its out by a fair bit

lol
any ways
cheers
elliot

p.s. dont use mobile fones wilst driving its an offence and dangerous