Speedo recalibration

[red4wd]

I am upgrading my 83 K10 with a 350 and 700r4 out of an 89 Blazer my problem is the Blazer had 2.76 ratio axles and I have 3.42 in my truck.  To add another twist to this I will be upgrading from the stock 235/75R-15 to a 33x12.50-16.  What I am looking for are the gears that will be needed to get the correct reading on my speedo.  The calculations I have done actually put the final drive ratio with the 33" tires and 3.42:1 ratio approximately the same as 3.08:1 with the 29" factory tires.  Is there any one out there with any knowledge that could comfirm or debunk my theory?

[chevy754x4]

I have seen some listing on ebay that have adapters to correct the ratio for cable driven speedometers.  Some of these guys claim if you give them your driveline specs and tire sizes they can make the correct one for you.  Worth a look.

[Captkaos]

There are 3 different GM drive gears 15 (gray), 17 (red) and 18 (blue or green).
GM used the 18 and a 35 driven for 2.73 gears (which is what you probably have)
If this is the case, and you switch gear ratios to 3.42, you would have to swap the driven gear to a 38.  The GM part number for the 38 is 1359272.

You can get more info here if yours has anything different:
http://73-87.com/7387garage/drivetrain/speedo.htm