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

Stanford · 03-03-2004, 04:38 PM

Here's the deal. When I put my crate motor (stock 275hp.) in my '71 a few years back, 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

*rickspickuppart* · 03-03-2004, 05:05 PM

what size tires does it have?

The 4WD guys on here probably have more experience w/ this than I do, but bigger tires will make it read slow.

Shawn

Stanford · 03-03-2004, 06:13 PM

Both trucks have stock tires. Im more worried about my '71 because thats my daily driver and im afraid one of these days im going to get stuck with a pretty big ticket.

Stanford

Huck · 03-03-2004, 06:53 PM

It's as simple as changing the speedo gear that your cable goes into at the transmission. It comes right out the side and you stick in a different gear ration to speed up the cable. Can't tell you which to go to no knowing what you have but if you go to any transmission shop, they have the gears and can tell you what you need if you bring in the old one. No big whoop---

03-03-2004, 04:38 PM	#1
Stanford WILLY FOR PRESIDENT!! Join Date: Feb 2004 Location: Wichita, Kansas Posts: 190	Speedometer Questions Here's the deal. When I put my crate motor (stock 275hp.) in my '71 a few years back, 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 __________________ *"He's a high tech REDNECK."* George Jones '67 C/10 FOR SALE!! $600, runs, strait grill, good parts, or beater truck '68 GMC K25, 4x4, current project '71 Custom/10, Crate 350, My Baby!! '97 Pontiac Grand Prix GTP

03-03-2004, 06:13 PM	#3
Stanford WILLY FOR PRESIDENT!! Join Date: Feb 2004 Location: Wichita, Kansas Posts: 190	Both trucks have stock tires. Im more worried about my '71 because thats my daily driver and im afraid one of these days im going to get stuck with a pretty big ticket. Stanford __________________ *"He's a high tech REDNECK."* George Jones '67 C/10 FOR SALE!! $600, runs, strait grill, good parts, or beater truck '68 GMC K25, 4x4, current project '71 Custom/10, Crate 350, My Baby!! '97 Pontiac Grand Prix GTP

03-03-2004, 05:05 PM	#2
rickspickuppart Account Suspended Join Date: Jul 2003 Posts: 1,284	what size tires does it have? The 4WD guys on here probably have more experience w/ this than I do, but bigger tires will make it read slow. Shawn

03-03-2004, 06:53 PM	#4
Huck Senior Member Join Date: Jun 2001 Location: Shelbyville, KY Posts: 3,261	It's as simple as changing the speedo gear that your cable goes into at the transmission. It comes right out the side and you stick in a different gear ration to speed up the cable. Can't tell you which to go to no knowing what you have but if you go to any transmission shop, they have the gears and can tell you what you need if you bring in the old one. No big whoop---



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