If I was to go for bigger wheels and profiles, it'll throw the speedo off.
Is there any way of re calibrating it electronically or is it mechanically fixed?

I have 18" wheels with 225/40 Toyos on my ST205 and going by the GPS speed my speedo is reading more accurately than it was with the OEM 16" wheels.

I know this doesn't answer your question directly but large wheels may be a non-issue for you.

_________________
Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp

but what if I deliberately want to change it? can the stock speedo be adapted?

Very easy Ray.
If you know your speedo is out by 'x' miles, remove from car and using very fine needle nose pliers gently grip pin under indicator needle and adjust needle position buy moving needle eighter way. If you know what I mean.

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98 Celica GT4 ST205
Full forged rebuild with Wiesco pistons, Crower rods, mildly ported head, 264/272 HKS cams, FMIC, PFC Pro, GT35 BB turbo, turbo back 3" system.
Currently running 1.6bar/520+bhp

Remove clear plastic cover from instruments and lever off the speedo needle. Find a straight, traffic free, level stretch of road and drive at a steady 60mph (or other known speed of choice) as indicated by your gps, and refit speedo needle to indicate the appropriate speed. Hey presto - a 'recalibrated' speedo.

After completing the above your speedo error from 'true' speed (when travelling at speeds different from the setting speed above) will be dictated by the change in rolling diameter from the o/e tyre size which you can find here: http://www.alloywheels.com/Tyre_Calculator. For ST205 o/e tyre size is 215 50 R16 I think. i have 225 45 R17 tyres and I find that at speeds up to about 70mph the speedo corresponds very closely with gps reading (when travelling at a steady speed).

You could build an electronic gizmo that would modify the frequency of the input signal to achieve an even greater level of accuracy but I wouldn't have any idea about how to do that.

Remember that manufacturers tend to supply vehicles with speedometers which tend to display a speed greater than your actual speed - you need to be aware that the direction of error might change even if the accuracy is improved.

I thought about moving the pin but then I thought if im running bigger wheels, does it go off not +/- xmph butin ratio? Sat at 10mph clock say Im doing 15mph. At 20mph the clock says Im doing 30mph and so on?

Correct

Something like this may work

http://www.k-tuned.com/blog/products/sp ... converter/

Yes you are completely right, which is why the first step is to know how much different the circumferance of your intended tyre is from o/e. To give you an idea of the differences from o/e:

225 45 R17 is 1.94% larger
225 40 R18 is 2.56% larger
225 35 R19 is 2.92% larger

The response of the speedo appears to be pretty much linear (from my experience) - do your own calculations to work out the actual error in indicated speed vs true speed after setting the needle as described above, then make your decision as to the need for the electromic gizmo.

Thanks for the input guys.

Set to be accurate at 50MPH then the error is minimal between 30 and 70 which is the range that matters.

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If at first you don't suck seed, try drier grain.

I have to deal with this problem myself, and my speedo is off around 6.7% due to tire size. I was just going to purchase a GPS speedo and install it in a pillar pod. With my current setup I can barely see my stock cluster anyhow, so a more visible display suits me better anyhow.

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Coldiron (Cold - Iron)
1992 Celica Gt4 RC Conv. St205 swap

http://www.noritoy.com