Speedometer Questions

[Stanford]

Here's the deal. When I put my crate motor (stock 275hp.) in my '71 a few years back and i decided that I also needed a new gauge cluster. I bought it all as a packaged unit. I noticed that my MPHs are off a little, well last I knew it read 10 mph slower than what I was going. So it read 20 when going 30. The other day I followed my buddy up to salina so he could get some new tires mounted on his bike. When we got there he asked how fast I was driving. I told him that i was trying to keep it around 75, and he told me we were running about 83. On the way back we got a clean stretch of interstate and we went side by side and had the same result, when my speedo reads 65, and im thinking im going 75, he said that his bike read 83-85. I know that his may not be correct, but i do know that its got to be closer than mine.

Now that I have board you with the story, I was curious if anyone knows what is causing this. Is it something simple or is it going to be a complicated fix. I've also noticed the speedo bouncing around in my '68 4x4. The tranny in the '71 is a th350 and the '68 is a sm465. Any thoughts or tips would be greatly appreciated.

Thanks,

Stanford