rear end gears

[Pont406]

I have 3.08 gears in my 69 c10. Speedo is off a little. It reads about 10 mph slower. On the freeway I am doing about 3500-3700 rpm in the fast lane. I want to lower my rpm some. I looked at summit and the lowest gear set for a 12 bolt is a 3.08. Anyone knows who have a 2.73 or so gears for a 12 bolt? Thanks

[mvfd70]

what transmission do you have? You may be better off getting a tranny with overdrive. If you get the rear gear to high the truck will be real sluggish taking off. I'm planning on running a 3.73 with a 5 speed manual trans. I'm figurin that should be just fine for me. There are others here that have lots more experiance with what you are wanting.

[Jtrux]

In my blazer with the same gears and a sm465 4 speed tranny with 32's I do 2500 all day long at 70. Yours doesn't seem right, that seems kind of high to me, are you going by the glove box or have you actually visually seen and verified that you do in fact have 3.08 gears. It's possible that if you're looking at just the glove box they could of been swapped. I know I plan on swapping mine cuz it's a dog on the highway when going up hill.

[WorkinLonghorn]

There's something WAY wrong with those figures. Just how fast do you drive? I had a 1500 with 3.08 gears (and 33" tires) and 3500 RPM would have been 112 MPH.

[Longhorn Man]

I totally agree, my old truck had 3.07's (3.08's are car gears) and at 2500 RPM I wqas doing between 60- 65.

[WorkinLonghorn]

Even with 26" tires you'd be going 88 MPH @ 3500 RPM.

[Pont406]

I took the diff cover off and it's a 3.08 ratio. I am running 29" tires. 400 trans. I do pass a lot of traffic at that rpm. Spedo shows 70-75 mph. Mine main consern is a temp. Normally it's 160 but at 3500 rpm gauge goes up to 190. I am afraid it will overheat in the summer. Maybe the tach is off. Monday I will get a timing light and drive with it to check the tach.

[Cannon007]

LOL i'll bet you do pass a lot of ppl, sounds like your speeding cause your speedo is off, slow down and look into geting an Overdrive tranny. The new overdrive trannys also have torque converter lock up which is like another overdrive but not sure how they work on a retro fit? The Overdrives tranny 4l60 or 4l80 will give you a lower first gear punch too

[Wrenchbender Ret]

Either your tach is off or your trans is not shifting to 3rd gear.

[Longhorn Man]

that is a 3.071428571 gear. 43 divided by 14... 3.07
At 3652 RPM you'll be doing right at 100 MPH
2375 is right at 65 MPH
An overdrive is pretty much unneeded unless you insist on cruising 40 MPH over the speed limit... assuming your tach is right.
I suspect your tach is wrong, thee is no way you're running a hundred miles an hr, and NOT realizing you are hauling ass... not to mention the LAPD would notice it too.

[Pont406]

I was ready to remove speedo gears and check for color code. While down there I found an adapter on the speedo cable from an old cruise control. I had the trans out before but didnt pay attention to it. Anyway I removed that adapter and drove it. Now at 1500 rpm it shows 40mph. I think the speedo is working now. At the same time while I was down there I found that the frame on left is starting to rust. You can see surface rust in the pic.

[Longhorn Man]

You guys in so-cal are spoiled.
Don't worry about that surface rust. Left unattended in your climate, it'll last another 40 years before it becomes an issue.

[Shak]

Your tach must be way off. By my calculations, at 3500 rpm, you would be going just under 100 mph. Besides, you live in Los Angeles. The freeways are too clogged to go that fast anyhow!