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Go to http://www.miata.net/garage/tirecalc.html (unfortunately metric tyres only) but a conversion is simple... go to the AU BFgoodrich site,they have a good table for tyre diameters.
But the average 31 is around 797mm in diameter making very close to a 265/70R16 and the 33's are around 852 in diameter close to a 285/75R16.
Put it in the calculator and the magic figure of 7.3% too slow for your speedo. So when yoru speedo is 100km/h you are actually travelling 107.3 km/h.... close to being booked!
Remember though that speedos are generally calibrated to overestimate your speed, so generally what you "gain" is only that which you'd lost in the first place.
That's generally...... dont take anyones word on it.
Sorry mate. The unit is a kit, if you have basic soldering skills, and hour to assemble. It has 2 rotary switches, one adjusts the speedo on 10% increments and the other in 1% increments.
I assume that you have an electric speedo not a cable one?
The output of your speedo sensor goes into the unit, the output goes to the cable to the speedo (basically find where the speedo comes into the cab an cut it),plug it in an away you go! So all you need is the wire to the speedo and a 12 volt power source. What the unit does is to adjust the frequecy of the pulses from the speed sensor.
Once in place the best bet is to have a GPS and start driving. Tinker with the rotary switches until you have the correct speedo reading. The switches are maked 0 to 10 so when you have it set correctly for your 31's for example, write down the setting. When you put on your 33's do the same.
If you alternate between wheel sizes, just rotate the rotary switches tothe settings that you have recorded.
I have had one since I put the 33's on and I cannot do without it, both for the speed reading and also for measuring my fuel enconomy (so the odometer is correct).
There are prebuilt ones on the market but I believe they cost a motza. Still cheaper than a speeding ticket and the points that go with them.
You could use the speedo on a GPS device and ignore the vehicle speedo. It is not dependant on the tyres, gearing etc. I have one built into my CB radio , Uniden GPS 105 and it is very good. Where you mount it is crucial. Mine is on the dash in a pod and is in line of sight.
I am the Nightrider! I am the chosen one. The mighty hand of vengeance, sent down to strike the unroadworthy!
Major prob with this is tunnels etc. Try crusing in the cross city tunnel or harbour tunnel in Sydney without a working speedo.... Bling..bling.. it is not the toll booth operators that are going taking the revenue off you!!
RoadNazi wrote:You could use the speedo on a GPS device and ignore the vehicle speedo. It is not dependant on the tyres, gearing etc. I have one built into my CB radio , Uniden GPS 105 and it is very good. Where you mount it is crucial. Mine is on the dash in a pod and is in line of sight.
So even "The boys" dont trust vehicles speedos. Hmmmmmmmmmmmm...
George Carlin, an American Comedian said; "Think of how stupid the average person is, and realise that half of them are stupider than that".
RoadNazi wrote:You could use the speedo on a GPS device and ignore the vehicle speedo. It is not dependant on the tyres, gearing etc. I have one built into my CB radio , Uniden GPS 105 and it is very good. Where you mount it is crucial. Mine is on the dash in a pod and is in line of sight.
So even "The boys" dont trust vehicles speedos. Hmmmmmmmmmmmm...
I trust no one, and even have doubts about myself.
but............ I have confidence with my speedo with it being only one kph difference with the GPS.
I am the Nightrider! I am the chosen one. The mighty hand of vengeance, sent down to strike the unroadworthy!
RoadNazi wrote:but............ I have confidence with my speedo with it being only one kph difference with the GPS.
what is the legal limit for a speedo to be out - is there one?
Bruce..are you referring to the ADR's allowance in speedo calibration????..I think it is 10% difference. That is the only standard I am aware of......it is not an offence to have an inaccurate speedo..but often the result is an offence.
I am the Nightrider! I am the chosen one. The mighty hand of vengeance, sent down to strike the unroadworthy!
Since I went from 35x10.5x15 Simexs to 315x75x16 Xterrains my speedo has gone from being 100% accurate to reading 5% over, IE: Speedo reads 105 = true speed 100
[quote="Wooders"]If ya want a 4x4 camry go ahead & buy a Patrol or Cruiser.[/quote]Rangie with 80s LC diffs, Isuzu 4bd1, Twin ARB lockers, 8000lb Hi mount warn, 315x75x16 Procomp XTerrains
Major prob with this is tunnels etc. Try crusing in the cross city tunnel or harbour tunnel in Sydney without a working speedo.... Bling..bling.. it is not the toll booth operators that are going taking the revenue off you!!
Cheers
Rainsey
yes but you should know by then what the gps says compared to what the speedo says so you can figure out what speed you are going at those times when the GPS reading isn't there
Prob not the most accurate way, but i've measured my troopy, my old 45 ute, and our old 62 wagon on 35's against the 5km long speedo checks on the freeway, and all three speedos told me that i had travelled 4.5kms in the 5km speedo check, meaning that you have to read at 90% of the speed you would like to drive - works for me. Whenever i know there's a speed camera, fixed or you see it in time, i usually go to 10k's under the speed limit, which up to 110km/h will be pretty safe.
Landcruiser UTE 1996 FZJ75 - SPOA, 35"s,
extended chassis, poor mans extra cab, 4.56 diff gears, Lockright front, chinese air locker rear...
Prob not the most accurate way, but i've measured my troopy, my old 45 ute, and our old 62 wagon on 35's against the 5km long speedo checks on the freeway, and all three speedos told me that i had travelled 4.5kms in the 5km speedo check, meaning that you have to read at 90% of the speed you would like to drive.
Landcruiser UTE 1996 FZJ75 - SPOA, 35"s,
extended chassis, poor mans extra cab, 4.56 diff gears, Lockright front, chinese air locker rear...
grimbo wrote:yes but you should know by then what the gps says compared to what the speedo says so you can figure out what speed you are going at those times when the GPS reading isn't there
mark it on your tacho if you have one (if not cheap insurance), with a white permo marker if you have one, and for those few mins, just go slower.
Maggot4x4 wrote:Since I went from 35x10.5x15 Simexs to 315x75x16 Xterrains my speedo has gone from being 100% accurate to reading 5% over, IE: Speedo reads 105 = true speed 100
So in fact it is going from 100% accurate to 0% accurate, with a consistant error throughout the range....
George Carlin, an American Comedian said; "Think of how stupid the average person is, and realise that half of them are stupider than that".
A bit of Trivia. I, for no more than not having a life, was reading the NCOP regs for speedo accuracy. NCOP 4 Section LB Transmission page 5/LB 14
And I quote ".. the speed indicated by the speedometer should be no less than the vehicle's actual speed and no more than 10% above the actual speed.."
So it looks like Maggot has got the only legal speedo out of all of us!!
