Speedo out on my SW20 Mr2.

[mr2lover]

when travelling 60 on my cars speedo the road angel says 66kmh.

The cars one is slower then it should be. I have too road angel and they work fine in teh skyline.
All I can think of is the mr2 rolling diameter is out? or could it be a faulty speedo?

I have 18 inch wheels front and rear.

[wiso]

no i'd say with 18"s the rolling diameter is out.

what size tyres you got on those 18"s

you can't say there speedo's are out cause remember mr2 come with 15"s, not 18"s changine wheels always modify's the results

[mr2lover]

On the rear I have 265 with 40 series rubber
on the front I have 215 with 35 series rubber.

Actually the 1990 ones came with 14 inch wheels

[mr2lover]

Cars that I previously had like skylines and S15 never changed the speedo when I changed them to 18 inch wheels.

[The Witzl]

you have increased the overall rolling diameter of your rear wheels by having 40 series profile, and being a RWD car, the speed sensor is thus affected and will measure inaccurately.

Get the original wheel/tyre combo that your car had from the factory, and plug the numbers into one of about eleventy billion tyre calculators on the internet to get the correct size tyre to use.

[LeeRoy]

my corolla is the same and its got 17's on it with 40 profile tyres and it came with 14's with 60 profile (i think).

when u put 18's on a car that came with 14's even though you run low tyres as Witzl said the overall rolling diameter is now larger.

Simply put, cause the diameter is bigger, the engine and all associated parts inside the g/box dont have to move as fast to cover the same distance that it did with the 14" wheels. Now because they spin slower and cover the same distance and then engine revs lower and g.box spins slower also, the little gears where the speed reading is taken also spin slower. Cause they spin slower the speedo in your car will give a lower reading at the same speed.

go to one of those sites suggested above and get a reading on how different they are and then you can find out how out the speedo is exactly

just throw the stockies bac.......actually what am i talking about, leave the 18's on and just drive a couple of k's slower

-LeeRoy

[Alchemist]

Or there are speedo convertors out there....that adjust the signal appropriately.

[peedman]

i think ur allowed a 10km/h difference in ur speedo and ur actual speed limit

[Alchemist]

i think ur allowed a 10km/h difference in ur speedo and ur actual speed limit

It's not really a good idea to have a varience in your speedo any more than factory. The Police give a small percentage either way of the speed limit for speed cameras(2% I think) varience, each manafacturers speedo will be a little different, then what it out with 18" wheels....not a good idea..... just drive 5Km/h under limit, use your road angel or get signal convertor

[sideshow]

i can supply fit and calibrate electronic speedos using a speedo adjuster box and my gps
its very accurate

[mr2lover]

It's not really a good idea to have a varience in your speedo any more than factory. The Police give a small percentage either way of the speed limit for speed cameras(2% I think) varience, each manafacturers speedo will be a little different, then what it out with 18" wheels....not a good idea..... just drive 5Km/h under limit, use your road angel or get signal convertor

what is this signal converter you speak of? Where can I get one from and how much?

Thanks

[The Witzl]

Jaycar Electronics, $25.00 for an unbuilt kit.

[davedave]

SW20 < 1995 = mechanical speedo.
<converter nazi>No converter for you!</converter nazi>

http://www.miata.net/garage/tirecalc.html
205/60/14 -> 265/35/18 = speedo reads 6.8% too slow (100kph is actually 106.8kph)

[mr2lover]

Specification Sidewall Radius Diameter Circumference Revs/km Difference
205/60-14 123mm 301mm 602mm 1890mm 529 0.0%
265/40-18 106mm 335mm 669mm 2102mm 476 11.2%

so at 100kmh is actually 111.2 damn

[mr2lover]

And at 60kmh its actually 66.73 kmh. So the ratio of difference does get larger on higher speeds?