14 replies to this topic

[Nick04]

My speedo is +10 mph.
I've got an early a+ 1000 engine, "1984 i think".
The car is a 1100 special, so has 1275 clocks.
What speedo drive do i need to make it work right???

[MiniGrin]

Have you recently chained the wheel/tyre size?

[Nick04]

No, brought the car like this, the engine has been changed at some point!!

[Classic1275cc]

pretty sure I read I more than a dozen places on here that that wheel sizes makes next to no difference,

i have the same problem with a mini that has a metro engine and the common 2 clock mini clock, when the Speedo says 30 I’m really doing 35, and when the Speedo says 40 I’m doing 55, and as you go faster gets more inaccurate

[Nick04]

Mine is alway +10!

[cal844]

wheel weight makes a difference, i have standard 12 inch rover minilites and 12x6 revolutions, and the speedo reads 1 mph more with the revolutions compared to the minilites!

[L400RAS]

wheel weight makes a difference,

balls does it.  Speedo works on purely final drive rpm

 

, i have standard 12 inch rover minilites and 12x6 revolutions, and the speedo reads 1 mph more with the revolutions compared to the minilites!

are they both using the same profile tyre? 165/60/12?  I bet the rover minilights are 5" and if the revos are 6" the tyre will be slightly more stretched, thus tyre wall profile shallower thus circumference smaller, thus for the same physical speed, the revos will be rotating more often, thus the speedo shows quicker...

[Cerberus]

Not sure how you could tell if it's reading 1mph more on a mini speedo anyway.

[cal844]

use a calibrated speedo (ie speed gun) drive at 20mph on the speedo, and see what pings up on the gun!

 

wheel weight does make a difference as the revolutions are slightly heavier, and it is a 165/60/12 A539, the tread isnt stretched!!

[Cerberus]

Still have the same problem, how can you tell your at the exact same point on the speedo each time.  Could easily have 1mph error on the speedo.

 

 

 

If the speedo drive is turning at a rate to give 20mph, heavier wheels won't make it change, the engine is still making the speedo drive turn at 20mph.  The inertia of the wheel will be higher but it can't affect the reading of the speedo drive which is before the wheels.

 

I could see if the tyres weren't exactly the same, as then the rolling radius would be slightly different, whether it would be enough to change it by 1mph I don't know.

[L400RAS]

wheel weight makes a difference!

Can anyone else verify this, as I cannot see wheel weight affecting the speedo reading.

[HarrysMini]

Wheel weight does not affect the speedo! End of!
As was said above, the speedo is driven by the final drive speed. I'm not going to go into loads of detail as there are loads of threads about speedos on TMF, probably one in the FAQ section. Have a look there or search.

However, the size of the wheel does affect it. The rolling radius of wheels on a mini is about the same with 10s 12s and 13s as different profile tyres compensate for the different wheel sizes so it wouldn't make much of a difference.

[L400RAS]

Wheel weight does not affect the speedo! End of!

 

cheers Harry.

R.

[Guess-Works.com]

use a calibrated speedo (ie speed gun) drive at 20mph on the speedo, and see what pings up on the gun!

 

wheel weight does make a difference as the revolutions are slightly heavier, and it is a 165/60/12 A539, the tread isnt stretched!!

 

I think I've heard all of them now....

 

Technically speaking wheel weight could actually make a difference. Basically, the more weight in/on the car, the more the tyre will be deformed when it contacts the road. Guess what, this is why car manufacturers provide multiple ratings for tyre pressures based on load.

 

The difference in weight from one wheel to another is so infinitesimally small compared to the actual weight of the car and its contents it will make sod all difference to the deflection of the tyre, and hence speedo reading.

 

I suspect that if you took a dump before you got in the car you make make more difference to the car's total weight.

 

Tyre pressures make a notable difference to speedo readings....

[Guess-Works.com]

Back to the OP...

 

You need to find out what final drive you have in the replacement engine's gearbox...

 

Once you have that you can calculate the speedo drives required....

 

Out of the factory the 1100 special will have had a 3.4 final drive, 10" wheels and a GT clock set... the gearbox would have had 6/17 drive gears.

 

http://www.guess-wor.../Tech/ratio.htm