Speedo Adjustments

David try this.

David, I also have 80 series & my speedo is out about the same amount! I am using STD split rims, tyres, speedo drive gears, diff ratio & my speedo is still out. I sent the instrument cluster away & had it tested-it reads spot on, no faults with it. I have now received from TISCO INSTRUMENTS, OSBORNE PARK PERTH W.A. a small black electronic ratio box. it has 4 wires-12volt, earth, signal in & signal out. All you have to do is turn a small screw while driving along untill the correct speed is achieved. Can use GPS. It is adjustable for all size tyres. Cost $120.00 I havent fitted it yet though, been too busy with work. Hope this helps.

I have 285/75/16 Cooper ST's on my Patrol and I find that my speed varies from around 5% to 10% to what my GPS says I am doing. This fluctuation in speed stopped me from getting a Marks Adapter kit to correct the difference in tyre size.

To find a cool thingy just type in tyre calculator into google

As long as your speedo is out by a known amount ie: Percentage(checked by GPS etc) and consistent, why bother adjusting it- just allow for it in your driving. Thats worked for me for many years.
I've had many cars where the speedo is not accurate (most of them in fact) but I've always checked them (used to use the measured klm posts they have for that purpose b4 GPS)
We currently have four different cars in the family (sadly only one 4wd) and all have different but consistent speedo errors. We know what the errors are and drive accordingly- easier than trying to fix them all! eg: my son's Gemini UNDER READS by about 4kmh at 60 and by about 7 at 100! Even my wife knows exactly what the error in her speedo is so no excuses if she gets a ticket!
Having said that, I never cease to be amazed by people who've never checked theirs, and then are "astonished" when they get a ticket!
Cheers

Get in touch with these guys, they can probably help you.
Ringwood Speedometer Service
693 Whitehorse Road Mitcham 3132
Ph & Fax: 98742260

I put bigger tyres on my 100 series and now the speedo reads to within about 2% but the oddometer is over 10% out.

I need to get my oddometer calibtated to the tyres then alter the speedo to suit?

Hi David, we spoke at the 4WD swap meet about the speedo driven gears available from Toyota for 80 Series Landcruisers. The gear on my petrol auto was a 32 tooth standard, this was swapped for a 31 tooth gear from toyota (available same day in Perth) suitable for my 33in BFGs it has almost corrected the speedo properly (about 4.5% higher indicated speed) as evidenced by using two magellan GPS units.

For toyota owners using taller than standard tyres, not correcting the speedo has wider implications for transmission shifting, fuel economy, and engine life. The pulse generated by the speedo sender actually has several functions. Apart from the position of the speedo needle, the signal is used as an input for the auto transmission CPU, and the engine management CPU, it must also agree with a separate ground-speed sensor in the transmission. Here's where it gets tricky, if for example the speedo indicates that your speed over the ground is say, 10% lower than actual, the signal recieved by the engine CPU has the engine thinking that the vehicle is in a condition of greater load than it really is. The CPU regulates functions such as fuel injection timing and duration. In this case the CPU would be fooled into thinking that the vehicle was travelling up a long gradual incline (as the airlow MAF meter would input high volume signals to the CPU) , even when you are on the flat! Obviously fuel ecomomy is compromised to some degree, and engine life decreased due to less lubricant fluid film in the upper cylinders due to washing with excess fuel.

Futher inplications for auto owners includes possible driveline damage from kickdown occuring at much higher road speeds than was intended for the transmission. This happens as the transmission CPU thinks you are travelling at a lower speed than you really are and considers a downshift appropriate when you plant the foot for overtaking or hill climbing. Smoother up-shifting will also result from correcting the speedo.

Steve