Confusing Speedometer Issue

[Eagle_Scout2002]

I tried searching first and did not come across these issues. I have a 1999 Ranger with the 4.0, manual trans, and 4x4. It has the stock 3.73 gears in it. I have changed tires from the stock 245/75R16s to 235/85R16. My speedometer is reading a little wierd. At 45 mph it is dead on accurate. Over about 47 mph I am going faster than my speedometer is reading. Example is at 55 I am going 58. Below about 42 mph I am going slower than what the gauge reads. Example is at 35 i am going 31. With the change in tire size should it not read slower than actual speed through all speeds??

Also it does one other thing wierd. With the tach set at 2000 rpm, sometimes the speedometer reads dead on at 55 mph and sometimes it is almost 60 mph. This is on the same flat streatches of highway. Anyone seen that issue before? It just seems to vary day by day what the reading is. Well thanks everyone.

Ray

 

[killj0y]

you might need to change the worm gear if it is electro-mechanical. Not sure if yours is digital.

 

[killj0y]

Check out this thread maybe it will help. http://www.therangerstation.com/forums/showthread.php?t=102083 Another idea is to find the speed sensor and perhaps make sure its working or isn't dirty depending on if it is a pulse generator.

 

[RavoHimself]

as for why the number changes around i have no clue. but i can tell you that having one number that is exact is normal. when they set up a speedo on a brand new car. they usually pick 60 mph(i think) to be exact. the further you go away from that the further off it will be. so (in stock scenario) at say 40 you would really be doing like 38 and at 80 you might be doing 84. changing tire size will move your "exact" number but the logic is still the same

 

[killj0y]

I know I'm runnung 31s with stock and while using a speedo app on my phone my speedo reads 5 to 15 mph too slow depending on the speed I'm going, increases as I go faster. About 5 mph differance at 40 on the gauge and 15 at about 65 on the gauge.

 

[TimR]

my 99 has the VSS on the rear end. I'd start with that.