Crazy Speedometer - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Pyrotechnic · 05-14-2007, 08:54 PM

I just bought a 1977 GMC Sierra Grande. Pics to come as soon as I get the truck all cleaned up.

I drove it home yesterday and when I got out on the highway the speedometer was reading 95 and was pegged at 100 when I was passing people. I was following my dad the highest he said he went was 75.

I looked at the option sticker in the glove box and for the transmission, the code is M40 which translates to a turbo 400, but looking under the truck it's obvious that it's a turbo 350. I'm led to believe that this transmission isn't original, which could mean that the speedometer gears are wrong.

What do you guys think the issue is ? Is my speedometer just screwed ? Are the speedometer gears in the transmission way off ?

05-14-2007, 08:54 PM	#1
Pyrotechnic Registered User Join Date: Feb 2007 Location: Austin, TX Posts: 3,930	Crazy Speedometer I just bought a 1977 GMC Sierra Grande. Pics to come as soon as I get the truck all cleaned up. I drove it home yesterday and when I got out on the highway the speedometer was reading 95 and was pegged at 100 when I was passing people. I was following my dad the highest he said he went was 75. I looked at the option sticker in the glove box and for the transmission, the code is M40 which translates to a turbo 400, but looking under the truck it's obvious that it's a turbo 350. I'm led to believe that this transmission isn't original, which could mean that the speedometer gears are wrong. What do you guys think the issue is ? Is my speedometer just screwed ? Are the speedometer gears in the transmission way off ?



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