Speedometer is off need help

[bruce119]

OK I got this new 93 XL running pretty good worked out a lot of bugs. I do have another 93 XL donor truck I had for 10 years it is the same exact truck made same month same factory only 9,000 apart on the vin number.

Any way I was checking fuel mileage and it is only getting about 12-13 mpg my old truck got 18 or better. That was my first clue something was off. When I pass through radar zones there all over the place around here. It shows I an going about 3 mph fast than I actually am. For instance when my speedo says 25 the radar is 22. I checked this a few different ways I set the cruse at 60 mph timed it and my mileage came up a bit short than 1 mile. I also checked mile markers on the highway and again my odometer comes up about a 1/3 short of a 1/10 mph. My gas gage doesn't work so the first time I filled up I almost ran out. I filled up at about 220 miles and it took 20 gal. I never put that much gas in my old truck. I didn't think those tanks were that big.

Any way not off that much and it is steady. But with it off I can't get an accurate gas mileage. It will help me not get a ticket and The millage is actually higher than it shows.

Any way to tune it up it does have 235 75 15 tires. Now my old truck did have a speedo issue when sometimes when it was on the highway it would sore from cruising at 60 up to 80 than slowly come back down. But this is different it is steady no other fluctuations.

Ideas
Thanks
Bruce from Florida

 

[hardpike]

Lots of issues here. You dont have a good baseline to start off with. However, tires is the first thing to look at. Any change in size will cause a diffrence in the speedo. I also try to varify speeds when I see the radars on the side of the road.

 

[bruce119]

Lots of issues here. You dont have a good baseline to start off with. However, tires is the first thing to look at. Any change in size will cause a diffrence in the speedo. I also try to varify speeds when I see the radars on the side of the road.

I pretty much did that. Tires are not new but they are 235 75 15 that is what it calls for. And this area is loaded with radar signs I must pass about 5-6 a day just in one town there at every school and I have been through others same thing. It seems the speed/odometer is off about -3 mph.

My biggest concern is trying to figure gas mileage can't do it if the miles aren't accurate.

Now I haven't got into this too deep yet. But I think the speed/odometer run off a cable. Would this be a product of perhaps something like a worn gear at the end of the cable at the hookup point at the trany. I am thinking worn sloppy gears could throw it off. Kind of like a smaller or larger sprocket on a bike making it faster or slower.

What kind of fix would this be. I got a Chilton book I should take a look. But figure I start with a question see if anyone could put there finger on it.

thanks

 

[hardpike]

could be a gear, check out this web page
http://www.explorer4x4.com/speedo.html

does not look hard to change out the gear, and it looks like a >$10 fix. I would give it a shot

 

[YBCold]

somtimes the actuall speedo wears out or the cable. had to replace my speedo but i had to buy the entire cluster

 

[bruce119]

somtimes the actuall speedo wears out or the cable. had to replace my speedo but i had to buy the entire cluster

Yea I understand that and I think my old X was wearing out. But I would think like my old one it would do erratic stuff my old X when going about 60 sometimes would sore to like 80 then slowly return. My new X spedo problem is steady about 3 mph slower than actual speed. That's making me think a worn gear.

Hardpike thanks for that link. Worth a try it's cheap enough. What I think I need to do is pull it and see if there is any sign of wear. I would hope to see wear than that mite be the problem.

thanks

 

[Maniak]

If the speedo is always 3 mph off at any speed, then its the speedo head.

If its 5% off all the time (3 mph at 60 mph and 1.5mph at 30 mph) then its a tire size/gear ratio/speedo gear issue.

About the mininum you can change your speedo by changing the driven gear is 5%.

You need to borrow a GPS (or an Iphone with a GPS app ) and make sure its 5% off all the time.

~Mark

 

[bruce119]

I got a GPS w/laptop I didn't use that yet. BUT I have been through those radar sighns at differant speeds and it's allways about 3 mph. So when the radar says 23 my speedo is 25 radar says 42 my speedo says 45. And I am only getting about 12-13 mpg. I beleive that is due to a false odometer reading.

 

[Maniak]

Ok, in that case, its not the speedometer gear.. BUT.. here's another monkey wrench....

The speedomter and the odometer can be off different ammounts. Last time I had our X on a dyno at a speedo shop the speedo was 22% off and the odometer was only 15% off.

The speedomter head was rebuilt and then they read within 1% of each other.
The 22% off the speedo/odometer were off was then taken care of with bigger tires and a driven gear change (we regeared but didn't have the new, bigger tires on when we put it on the dyno).

~Mark

 

[bruce119]

Well I guess I need to take that GPS for a ride and get a little more accurate data.

Now my OLD 93 X (donor truck) was accurate to best of my recollection speedo & odometer where correct. With the exception of every once in a wile when on the highway it would sore from say about 60 mph to 80 mph then slowly come back down.

I don't know the history of the New 93 X how long the speedo was off or even if the previous owner new it.

I can swap parts but I know there is a problem with the old one and don't want to put a bad part in. What would cause the speedo to sore at about 60 mph to about 80 mph then slowly back down to normal and only once in a wile. This is my old truck. Keep in mind we have 2 different trucks that are just about identical.

thanks for the info it helps.