Is the transmission a W58?
The speedo gear inside is probably different for the celica. When they design the systems the tire size transmission and differentials are taken into consideration. Im sure the diff is taller and the wheels are larger 14 to 16?
Go to the dealer and see if the output gear is different between the supra and celica.
Then only other option I can think of would be to go to a speedo shop and see if they can add a box to the outside to compensate for your setup. I had one on my K5 blazer but I took it off as it was leaking on a trip to Tahoe. When I removed it the speedo was off 1/10 for every mile slow.
Speedo shops mostly deal with trucks that go from standard tire size to super swampers or the like. Toyota trucks fall under this catagory so you should be able to get it done but be prepared to pay a couple of hundred or more.
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