Tyre/Speed Changes

[Panrixx]

Having nothing else better to do, due to the poor weather here in Cornwall, I decided to ponder the difference in speedo readings different tyre sizes would make.

Having read a number of threads about speedo conversions and members getting false speedo readings, I wondered what affect different size tyres might have.

Ignoring that fact that a loaded vehicle would not have perfectly round tyres, and if my maths is correct, a 265 70 R15 tyre will have a circumference of 93". A 31 10.5 15 tyre works out at 97.4".

If I was doing 60mph with the smaller tyres they would revolve 40877.42 times in that hour.

If I was using the larger tyres 40877.42 revolutions would equate to 62.84mph.

33" tyres would have a circumference of 103.686" and 40877.42 revolutions in an hour would equate to 66.89mph.

To cut a long story short a 10% larger tyre gives 10% higher speedo reading, obviously!

If any one can blow a hole in my maths, or logic, please enlighten me .

[drdan]

Where does the speedo measure its revolutions? If it is determined by the number of revns of the axle at the wheel then using a larger tyre will mean that for a given axle rotation speed (or engine revs/gear ratio etc...) the car will move forward a greater distance with progressively larger tyres.

Thus (I think?) your speedo will continually read 60 mph (for example) but you would be going further each hour by a factor of the increased tyre size.

Using your numbers above:

93" circ. - reading correct
97.4 " circ - actual speed = 97.4/93 x speedo reading = 1.05 x speedo
103.7" circ - actual speed = 103.7/93 x speedo reading = 1.12 x speedo

So for set speeds of:

speedo

30 40 50 60 70 80 90

actual speed with 265 70 R15

all correct

actual speed with 31 10.5 15

32 42 53 63 74 84 95

actual speed with 33"

34 45 56 67 78 90 101

If I'm right (which is very much open to question!!!) then if you're running 33" tyres then you need to be careful of speed traps, especially on the motorways as when you think you're doing 80 you're actually doing 90 and when you think you're doing 90 then you're over 100 mph! Not that anyone would drive over the very sensible 70 mph speed restriction on UK motorways!

Please feel free to correct the above. It all depends on how the speedo measures the vehicles speed and I'm not well versed in such things!

Cheers.

[Slugsie]

OK, some good maths there, no faults obvious.

However, a couple of things to bear in mind. All Speedos read slow, and it can be by as much as 10% (thats the legally allowed variance IIRC). My 2.4 (using GPS speed measuring, and running standard 265/70/15 tyres) would indicate 70mph on the dial, but would actually only be doing 65mph.

Basically, if you want an accurate speed measurement either...

Run a GATSO at high speed, wait two weeks, and examine the photo

or

Buy/borrow a GPS, and do a spot of calibration

[Panrixx]

I beleive that the KZN130 models use some kind of pulse from the engine (Gary might clarify how this is actually done).

Therefore, the rear wheel speed will have a direct ratio to the engine speed.

[Wild Thing]

I beleive that the KZN130 models use some kind of pulse from the engine (Gary might clarify how this is actually done).

Therefore, the rear wheel speed will have a direct ratio to the engine speed.

The speed pulse senser is in the gearbox in much the same place that a cable is it has a rotor inside with magnets and coils so when the magnet passes the coil it creates a pulse which is passed up the wiring to the speedo head where it is transfered to the speedo coil which moves the needle, hope this is clear enough
The pulse senser in the engine is for the rev counter but works in the same way on a diesel, on a petrol engine the rev pulse sensor is incorporated into the electronic ignition system

[joker??]

OK, some good maths there, no faults obvious.

However, a couple of things to bear in mind. All Speedos read slow, and it can be by as much as 10% (thats the legally allowed variance IIRC). My 2.4 (using GPS speed measuring, and running standard 265/70/15 tyres) would indicate 70mph on the dial, but would actually only be doing 65mph.

Basically, if you want an accurate speed measurement either...

Run a GATSO at high speed, wait two weeks, and examine the photo

or

Buy/borrow a GPS, and do a spot of calibration

used my tomtom @ talex both gave the same speed of 75 the clock said 80

[Gamedawn]

However, a couple of things to bear in mind. All Speedos read slow, and it can be by as much as 10% (thats the legally allowed variance IIRC).

Ain't that the truth. At work we've got a 54 plate Hilux on factory fitment tyres and the speedo is 9 - 10% slow according to gps. Takes a bit of getting used to doing 55 through a 50 gatso but we've not been flashed yet

[Panrixx]

The speed pulse senser is in the gearbox in much the same place that a cable is it has a rotor inside with magnets and coils so when the magnet passes the coil it creates a pulse which is passed up the wiring to the speedo head where it is transfered to the speedo coil which moves the needle, hope this is clear enough
The pulse senser in the engine is for the rev counter but works in the same way on a diesel, on a petrol engine the rev pulse sensor is incorporated into the electronic ignition system

Thanks for that Gary, makes sense to me now. I take it that the odometer works in a similar way. If you wanted to change the odometer from Km to Miles would that just mean another electonic gizmo (tech term) to do the conversion?

[Panrixx]

Ain't that the truth. At work we've got a 54 plate Hilux on factory fitment tyres and the speedo is 9 - 10% slow according to gps. Takes a bit of getting used to doing 55 through a 50 gatso but we've not been flashed yet

I'm trying to get my head around this!!

If you can do a registered 55 through a 50 speed trap, does'nt that mean the speedo is fast and not slow? I suppose it depends which way you look at it.

[Wild Thing]

Thanks for that Gary, makes sense to me now. I take it that the odometer works in a similar way. If you wanted to change the odometer from Km to Miles would that just mean another electonic gizmo (tech term) to do the conversion?

You would need a new speedo head

[Slugsie]

I'm trying to get my head around this!!

If you can do a registered 55 through a 50 speed trap, does'nt that mean the speedo is fast and not slow? I suppose it depends which way you look at it.

Er, yes, erm, I meant to say that you are going slower than the speedo says.

[NFC]

Speedometers are allowed, IIRC, to read anywhere between actual speed and actual speed + 10% at speeds above 30mph.

They may not read slow above 30mph.

If you're really looking for something to do, calculate the change in speedo reading due to tyre wear between a new tyre and one that's reached its legal minimum.

[Panrixx]

Speedometers are allowed, IIRC, to read anywhere between actual speed and actual speed + 10% at speeds above 30mph.

They may not read slow above 30mph.

But they will if you fit larger tyres!

If you're really looking for something to do, calculate the change in speedo reading due to tyre wear between a new tyre and one that's reached its legal minimum.

It will be marginal. In any event, a worn tyre will mean the speedo is reading more than (below 10%) your actual speed so meets IIRC.

As a rough calculation assume 10mm tread depth less 1.6mm legal limit = 8.4mm x 2 = 16.8mm less diameter when on the limit.

A new 265 70 R15 tyre is 752mm (29.6") in diameter. A worn one 735.2mm. So the speedo will read 16.8/752 = 2.2% higher than the actual speed.

Please feel free to check my math/logic.

[TonyN]

All the math is fine, but its don't mean much in reality.

Linda's truck has stock gearing for 265's, now running 33's. We borrowed Robs TomTom in Cornwall the other week and the speedo is less that 1mph out at 30mph, and 2 mph out at 70.

The original speedos are notoriously inaccurate, no-one really knows if the speedo conversion is spot on with its adjustment and lots of trucks are running different size tyres to factory.

Its a wonder its anywhere near!

Thanks for that Gary, makes sense to me now. I take it that the odometer works in a similar way. If you wanted to change the odometer from Km to Miles would that just mean another electonic gizmo (tech term) to do the conversion?

If the speedo chip has been wired up properly, it should feed the odo and speedo with the corrected readings as they both get them from the sender on the t/case.

If you've just had the speedo face changed, then yes you'll need the chip to get the odo working in miles.

[Panrixx]

Good point Tony,

So is the only real way to tell by using GPS? How accurate are they?

You could follow another car but how accurate will it' s speedo be? I know the Corvette's speedo is spot on as it was commented upon during the SVA test. Apparently, it's because they have the factory fitted run flats on.

Any on know what the Gatso (etc) limits are set at or are they variable?