What gear did you have originally and what did you go to??
If you check your door panel the ax. codes are these...
m=2.73
z=3.08
e=3.27
Depending how much change you made..is how many teeth you need on the speedo gear.
I went from 2.73-4.10 and cannot correct the gear completely w/ out a different cable head or something. (thats what i'm told anyway)
Temp sounds like a bad t-stat maybe...should stay in about the bottom third of the range.
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4.10's,long tubes & 75 shot...Goes 12.50's
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http://www.burnouts.webprovider.com