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Speedometer and Odometer off

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dixiechevy01

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Anyone know of a way to re calibrate or get the speedometer and odometer to read right on a lifted Blazer?

Its an 87 K5 blazer with a 4" lift and 35" tires, has a th400 in it. The speedometer is off a lot, if I'm don't 45-50mph its reading about 35mph and it gets worst the fast you go. The odometer reads about 7 miles when I've traveled 10 miles
 
Odometer and speedometer are driven the same way...drive/driven gears in the transfercase for 4WD. Those need changed.

I believe TCI (the trans folks) have a GM speedometer gear calculator.

I never got way into it, but certain gears only work with certain of the drive sleeves (the piece that holds the driven gear in the transmission) so something to consider even if you get the right gears. Only place I've ever seen a listing of the different drive sleeves is the GM parts manual. Not sure which trans, if not all, had that peculiarity.
 
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Well thanks for the info and the website guys. Yea I don't really wanna put new gears in yet. I like being able to drive highway speeds. The 54RA2 is the one I need but I'm sure what percent I need. The speedometer is off a little she you first start driving then it get worse the faster you go. Lille when it says 45 I'm doing about 70then if it sat 25 I'm doing about 35
 
It's proportional. If you are off 10% (as a for instance) at 25 it will be 2.5MPH off. At 60, it will be 6MPH off. You know it's not the speedometer if the difference is proportional.

If the speedo reads 10MPH (for instance) above or below any actual road speed, there is a problem with the speedometer itself.

Just depends how far beyond the original axle gearing/tire size/speedometer gear set yours is to what the percentage off is. Since you know 45 is really 70, you can figure that out with good old math. 45 divided by 70 is 64%, so your speedo is off 36%, which isn't exactly correct based on your 25/35 statement, but close.
 
I've been driving my m1009 like this for years. 3.08 gears and 37" tires. I just make sure to keep it at the speed limit. IE: at 55mph I'm actually doing about 65mph
 
Ok so do you think there could be anything wrong with the speedometer it's self since its not a consistant percent? The speedo sensor or the t case does have some kind of adapter on it not sure what it is. If the speedo is off the odometer will be off to right? And yea I been driving it like this for awhile just be nice to know what speed I'm actually going
 
You need to figure out how fast you are actually going, a GPS is usually the easiest way and accurate enough for these purposes. If you have a highway with distance markers you can time yourself and do a little math.

Are you sure the percentage isn't constant? From what you describe it gets worse the faster you go, which would be indicative that it is off by a set percentage. There are a variety of calculators on the net that let you see how far off you will be and what your displayed versus actual speed is. For instance, I regeared from a 3.08 to a 4.10, and went from a 31 to a 33 inch tire, and my speedo reads about 26% high. At 50 my speedo reads about 63, and at 100 (hypothetically) it would read 126.

If the speedo is off the odometer will be too.
 
Just have a friend drive next to you while you are on the cell phone with them. Then have them tell you when they are at 40, 50, 60 etc.
 
if you have the gear reduction box on the T case that the cable hooks up to, just connect it directly to the transfer case and drive it with a GPS or other accurate speed device. Note how far off you are and then you calculate how far off it is, in decimal and get the closest reduction box to your figure.
 
Ok I'm gonna have to look at it again. I'm pretty sure it has a gear reduction adaptor on the t case. Ill look at it tonight and see if it is and if it is Ill take it off and just hook the speedo wire up and see what it does
 
Well I took the gear reduction adaptor off and it reads about the same. I went past one of the speed check things in a work zone and it said I was doing 60 and the speedometer said about 45
 
Changing the speedometer gear is standard procedure when you've changed the axle ratio or the tire size. First of all, what axle ratio does the truck have now? Really, you NEED gears appropriate for the tire size or the performance of the vehicle will never be right. But if you want to fix the speedo now and then fix it again when you get the right gears, that's fine. It's not expensive or difficult to change the speedo gear, as long as the right driven gear is available.
 
Well I think it's 3.73 gears but I'm not sure. The guy who had it before did alot of stuff to it and i don't know if he changed put the gears at all. It had the 4" lift and 35s when I got it so maybe he did. I know the gears aren't that low though. It has the gear reduction box on the t case for the speedo gear. I'm gonna look into getting another one
 
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