Different size tyres : speedo error

[Dolby]

I know that adding larger/smaller tyres gives you an incorrect reading on the speedo and effects gearing - but by how much?

I've worked out the rolling diameter of my tyres - and the larger tyres only have a 1cm extra rolling diamter (unless my calculations are wrong). Surely 1cm makes a negligable difference?

 

[tco21]

Simple. Don't wear a speedo while driving.

 

[Dolby]

Oh!

Actually found this http://www.csgnetwork.com/speedocalibcalc.html

 

[HapticSimian]

see here

 

[Smooth Criminal]

Oh!

Actually found this http://www.csgnetwork.com/speedocalibcalc.html

Enter Stock Tire Size
195/55-R15

Enter New Tire Size
205/40-R17

When the vehicle speedometer indicates 120 MPH or KMH
The vehicle speed is actually 120.06045340050377 MPH or KMH

That's not entirely accurate. On stock wheels the vehicle speed is 117 at an indicated 120 (tested on a buddy's car, same as mine, that's stock standard). On my car it's 116 at an indicated 120. Used the GPS reading to determine the actual speed.

 

[ponder]

That's not entirely accurate. On stock wheels the vehicle speed is 117 at an indicated 120 (tested on a buddy's car, same as mine, that's stock standard). On my car it's 116 at an indicated 120. Used the GPS reading to determine the actual speed.

You do realise GPS is also not that accurate? I would rather trust a calculation. Other things like tyre pressure can also change the actual tyre size.

 

[HapticSimian]

You do realise GPS is also not that accurate? I would rather trust a calculation. Other things like tyre pressure can also change the actual tyre size.

More accurate than a car's speedo, though.

Also keep in mind that any car has its speedo calibrated to read a little low in stock form. Where speed traps are concerned, rather a little over than a little under.

 

[ALFAHOLIC]

NOTHING is accurate! even if you doing 120kph your most lilkely doing 116kph...also, as mentioned by Ponder, GPS speeds are also inaccurate, the really accurate stuff are very expensive.

Don't worry about 5kph difference, the car looks good on the 19's, and that's better than a measly 5kph!

I absolutely hate it when I go and ask for tyre prices they tell me no you can't go that high or you can't go that wide...flip man...5kph is nada!

 

[Smooth Criminal]

You do realise GPS is also not that accurate? I would rather trust a calculation. Other things like tyre pressure can also change the actual tyre size.

Yep, but you're using the same frame of reference in both vehicles, and it's closer to true speed than the speedometer reading.

The point is that the site predicts that the indicated speed and the true speed is identical, which is false. Also, different manufacturers calibrate their speedometers differently so it's difficult to predict a measurement of true speed across all vehicles based on an indicated reading.

 

[geezer]

Maybe a bit off topic, but for those that have gone from say a 14" to a 17" wheel, what was the effect on the fuel consuption? My head says that the consumption must increase.

 

[Keeper]

hmm....

Where's that thread about the truck doing 120km/h going faster than a car doing 120km/h, because it has bigger wheels?

 

[HapticSimian]

Maybe a bit off topic, but for those that have gone from say a 14" to a 17" wheel, what was the effect on the fuel consuption? My head says that the consumption must increase.

Your right foot is going to play a MUCH bigger roll than rim size...

 

[kkza]

The larger the wheel the larger the outside diamater,
so if you go from say 14 inch to 17 inch as mentioned above, your axel/spedometer is still doing the same number of rotations, but the car is moving forward a little bit more each rotation than it would on a smaller tire, while this 2/3cm might not seem like a lot, think about how many rotations your car does on say a jhb/dbn trip, i think it would add up to a substantial amount, and it should decrease fuel consumption too, although probably a negligible amount

 

[HapticSimian]

The larger the wheel the larger the outside diamater,
so if you go from say 14 inch to 17 inch as mentioned above, your axel/spedometer is still doing the same number of rotations, but the car is moving forward a little bit more each rotation than it would on a smaller tire, while this 2/3cm might not seem like a lot, think about how many rotations your car does on say a jhb/dbn trip, i think it would add up to a substantial amount, and it should decrease fuel consumption too, although probably a negligible amount

Remember that when fitting a larger rim, you'll combine it with a lower profile tyre, keeping the outside diameter as close to unchanged as possible.

 

[Keeper]

Remember that when fitting a larger rim, you'll combine it with a lower profile tyre, keeping the outside diameter as close to unchanged as possible.

yeah....expensive rims mean even MORE expensive tires

 

[ponder]

hmm....

Where's that thread about the truck doing 120km/h going faster than a car doing 120km/h, because it has bigger wheels?

Yeah, that was a classic

What's heavier, 5kg of lead or 5kg of feathers?

The larger the wheel the larger the outside diamater,
so if you go from say 14 inch to 17 inch as mentioned above, your axel/spedometer is still doing the same number of rotations, but the car is moving forward a little bit more each rotation than it would on a smaller tire, while this 2/3cm might not seem like a lot, think about how many rotations your car does on say a jhb/dbn trip, i think it would add up to a substantial amount, and it should decrease fuel consumption too, although probably a negligible amount

You are forgetting that the profile of the tire changes so it's roughly the same size. the 14&17 inch is the rim diameter, not the tyre diameter.

 

[flaker]

have always found that fitting a good brand ( regret dunlop excluded ), size as per manufacturer, gives best ride & good wear. saves tons of moneys

 

[sly]

have always found that fitting a good brand ( regret dunlop excluded ), size as per manufacturer, gives best ride & good wear. saves tons of moneys

Very true. Did 80000km on my '06 corsa, regulaRly checked tyre pressure, wheel balancing, alignment at every service. The goodyears served me well..

 

[Fazda]

That's not entirely accurate. On stock wheels the vehicle speed is 117 at an indicated 120 (tested on a buddy's car, same as mine, that's stock standard). On my car it's 116 at an indicated 120. Used the GPS reading to determine the actual speed.

Unfortunately there is no formula where "one size fits all" in order to calculate the inaccuracy.

When we were doing regularity rallies where your odometer is blanked out and all calculations and instructions are based on you doing dead accurate speeds, we found that we had to re-calibrate our speedos before each event, and then on the longer events like Joburg to Swaziland, the readings were already becoming inaccurate as we started nearing the end, simply due to tyre wear.

Originally we would test the speedo in 5 km/h increments from 10 km/h up to 120 km/h over a measured kilometre, and get 100% accurate readings that way...then on an event, we would use the 200 m boards on the main roads to double check speeds. We HAD to do it because you can be told to do 73 km/h for ten minutes and twenty five seconds, and then turn left....if there were two roads to the left, say within 100 m of each-other, you were totally buggered if you were actually doing 75 instead of 73!!

Anyway, I digress...best way to do it is to check with a GPS as that is probably more accurate than a stop watch and line of site on 200m boards. For every day driving, it is most certainly the best option, and a variation of 1 or 2 km/h in either direction shouldn't make any difference.

What we DID find on the rallies, was that there was absolutely no guarantee that you could take two Golfs, for example, with same size tyres, and expect them to both have the same speedo error.

So, that being said, don't look at Car Magazine road test and say, oh, I drive a BMW M3 and it has a speedo error of x%...the car that was tested had that error...not yours....

Hope that all makes sense.

 

[dabbler]

I used to check my speedometer calibration on an open road where you can drive at close enough to 120 km/h as possible (according to your speedo) and then measure the time it takes to travel 10 km, using the milestones (kilostones ?). At exactly 120km/h the time taken to travel 10km should be 5 minutes, 0 seconds. Your actual speed is calculated by s=d/t ie. 10km/time.

The only problem here is you need an open road/freeway with very little traffic, where you can maintain driving at 120km/h for 5 minutes at a time. Is that still possible?