How to determine original tire size and adjust speedometer?

aragno

Active member
I purchased a 110 with non-OEM wheels/tires, I suspect.
I suspect this due to my speed being read by various radar on the road-side vs my speedometer. It is off by about 5mph.

I will show 100kmph (62.1mph) and the radar will show 57mph.


Once I know the difference, how do I adjust my speedometer to match my current tire size.
 

evilfij

Well-known member
Factory tires were either 7.50x16 or 235 85r16 and they are about the same size. What do you have now?
 

evilfij

Well-known member
I don’t think anything is wrong. Speedometers run a little fast and 235 85 R16 is a little bigger than 7.50x16.

I don’t believe there is any way to adjust the speedo.
 

aragno

Active member
I don’t think anything is wrong. Speedometers run a little fast and 235 85 R16 is a little bigger than 7.50x16.

I don’t believe there is any way to adjust the speedo.
Nearly 10% seems fairly significant and I believe that’s also creating inaccurate odometer values too.

I’m running a R380 gearbox.
 

4RF RDS

Well-known member
You can change out the speedo drive gear to compensate for the inaccurate reading they are colour coded (see below). Not sure which one you’d need. I am in the same boat and have been for years. I had OEM tires which were 7.50 16s and I changed to 235/85 16 and speedo is inaccurate by about 10 %. I have not yet gotten around to swapping the one in the LT230 yet.

This change will work on a cable driven speedo or an electronic one. You could try contacting Ashcroft and ask them which gear you’d need. (They sent me a bunch when I built up my Series III 88 with a 300 Tdi and LT77s and Lt230 many years ago). 👇🏻
06803BC2-B42D-4516-A8EF-155D767E4A2D.jpeg


Good luck.
 

UnfrozenCaveman

Well-known member
Just swap the speedo for a Kph one ... then you can do the mph conversion AND estimate your actual speed at the same time :ROFLMAO:

...gotta have a hobby on those long drives ... can't hear a stereo over the flapping canvas anyway...
 

Robert

Well-known member
GEMS trucks are neat because they show the same ~10% error on the speedo but the data out of the OBD port is correct.
 

erover82

Well-known member
But there is not a gear available for the LT230 to swap.

Moving up a tooth in gears should get him closer to what he's looking for. That said, it's normal for vehicle speedos to run a little fast due to manufacturer avoidance of liability.
 

evilfij

Well-known member
There are like 5 different ones you can get. These 4 and then the NAS one which is white, but I believe still has 20 teeth
View attachment 26079
There are no gears to make the speedo run slower is what I meant. At least I think that is how it works. All the ones I have seen in defenders are 20 teeth (I knew about the 205 tire 90 gear, but did not know about the others). I will say on both my discos, the speedo ran fast but the odometer was near dead on. Maybe he has a t-case out of a 90 in his 110 with the 22 teeth gear, but I doubt it.

You can buy an electronic speedo (roversnorth has them) I assume that is adjustable but I don’t know.

I still don’t think it’s actually broken or has anything wrong.

Edit: I think I have the speedo tooth thing backwards.
 

rocky

NAS-ROW Addict
That “error “ isn’t unusual. Speedos are allowed to over report your speed not under report it.
Plus those police signs aren’t always accurate. Download a GPS speed app on to your phone and test.
 

aragno

Active member
That “error “ isn’t unusual. Speedos are allowed to over report your speed not under report it.
Plus those police signs aren’t always accurate. Download a GPS speed app on to your phone and test.
I tested with 2 GPS speed apps. They matched all the roadside radar setups (4 different ones).

While I appreciate the conservatism of the speedometer, 10% is a bit much and even my 1951 Chevy truck doesn’t have this type of error.
 
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