My speedometer is a little off...

[mpyusko]

Mine reads 78 when I'm actually going 70. Same deal on mine: the ECU reports correctly, within 1-2 mph of GPS. I have 265/75/16 over the stock 245/75/16, so there is a bit error on that.

Anyway, I'd be interested in changing the air core motor if someone can provide a part number. I haven't seen many GMT 400 trucks at the wrecking yards the times I've been there. Then you got to worry about mileage being different on the cluster...

BTW, mine runs right under 2500 RPM (2450?) at 70 with 0.75 OD, 4.10s and those tires. I've got other trucks with the same tires, transmission, and gear ratio, and they also run at the same RPM at 70 mph.

I ordered a new air core motor off eBay. I'll see how it works and update here.