Thread: Speedo Out

Originally Posted by d2dave

Having just fitted 265/75/16 to the disco I know the speedo will now read slow.

Until my son comes home with his GPS I don't know by how much.

Can anyone with the same tyre size tell me what the speedo shows when doing 60, 100 and 110?

My speedo reads about 6- 7 ks slower right through the range with same size tyres .
I just make sure im 7 ks under the posted speed . I also have a speedo app on my phone i use when i have the tyres fitted to keep it at the posted speed

It will come back closer once you fit the 4.11s though Dave, but the other way, 33s will bring it back to standard.

A Truespeed speedo correction unit will fix things in the meantime.

Couldn't find that switch Dave.

Baz.

Originally Posted by Offroad Dude

My speedo reads about 6- 7 ks slower right through the range with same size tyres .
I just make sure im 7 ks under the posted speed . I also have a speedo app on my phone i use when i have the tyres fitted to keep it at the posted speed

That can't be right. When tyre size changes it is a percentage. So if your speedo was out, say 5 kays at fifty, it will be 10 out at a 100

Originally Posted by Redback

It will come back closer once you fit the 4.11s though Dave, but the other way, 33s will bring it back to standard.

A Truespeed speedo correction unit will fix things in the meantime.

Couldn't find that switch Dave.

Baz.

I thought with D2's that the speedo was read from the ABS at the wheel, in which case diff ratio would not matter.

Thanks Baz for looking for that switch. I have sourced a cruise control switch since I found out from Erich that the outer cover can be changed.

Originally Posted by d2dave

I thought with D2's that the speedo was read from the ABS at the wheel, in which case diff ratio would not matter.

Thanks Baz for looking for that switch. I have sourced a cruise control switch since I found out from Erich that the outer cover can be changed.

I think and I could be wrong, it does make a slight difference, also if going to 4.11s you might loose your cruise control, most do from the info I got from Slunnie, some don't, this might be why.

Baz.

33s on stock gearing means the ABS sensor is picking up a different number of rotations of the wheel for a certain speed, compared to stock tyre sizes.
4.11 diff gearing returns the rotations/speed back to stock values, hence the ABS sensor reads the correct speed.
At least that's my understanding of it, probably wrong but it's all a learning curve
With 265s you're about 8% out IIRC

Originally Posted by Disco Muppet

33s on stock gearing means the ABS sensor is picking up a different number of rotations of the wheel for a certain speed, compared to stock tyre sizes.
4.11 diff gearing returns the rotations/speed back to stock values, hence the ABS sensor reads the correct speed.
At least that's my understanding of it, probably wrong but it's all a learning curve
With 265s you're about 8% out IIRC

Changing the diff ratio just changes engine RPM for a given speed. Bigger tyres, lower diff ratio so the engine doesn't struggle.

If the speedo is read from the ABS sensor it does not matter what diff ratio is used.

A larger tyre is spinning slower(so therefore the axle/CV joint as well) than a small one at a given speed regardless of any ratio's, diff or transfer case.

If the tyre increase is 8%, it stands to reason that the speedo will then read 8% slow

So 92 on the speedo should be 100

Originally Posted by Redback

I think and I could be wrong, it does make a slight difference, also if going to 4.11s you might loose your cruise control, most do from the info I got from Slunnie, some don't, this might be why.

Baz.

I can't see how a diff ratio change would affect cruise control. Isn't it also read from the ABS sensors?