Mph is off help please

[Redneckmudslinger]

Have 88 gmc jimmy it's got 35s with 6 inch lift and my mph is off by 10 how can I fix this so my mph is right thanks

 

[Carcinogen]

What drivetrain?

 

[Redneckmudslinger]

Not sure what drive train but I known it has a turbo 400 transmission if that helps any

 

[paratrooper307]

It should have a speedometer cable, you will have to change the drive gear or driven gear to correct the error. If it has a VSS I think you have to reprogram.

 

[B.N.Z.MTNS]

Have 88 gmc jimmy it's got 35s with 6 inch lift and my mph is off by 10.

At what speed are you off by 10. When you increase your tire size from factory your speed will be off by a percentage, not a solid number. If it's off by 10% then at 30 MPH you would be closer to 33 but at 70 MPH you would be closer to 77 and so on...

I just figured out what percentage my speed was off by and compensated while driving. At roughly 17% - 18% off I would cruise at 55 - 60 MPH on my speedo and I was running with the rest of traffic on a posted 65 highway. After I dropped gear ratio in my axles I was closer to factory percentages so I don't have to deal with it too much now.

 

[Redneckmudslinger]

Already replace the driven gear still off when I go 50 mph I really going 60 there's a little box that connect at the end of the speedo cable before it connect to the transmission what is that is there gears in there I have to change

 

[badmix]

Heh..mine is off by 20mph , its been that way for 8 years, I just lived with it. I know at 85mph , im doing 65mph. I wouldnt know what to do if/when it ever read correctly. lol

I think ive got the proper plastic speedo gears in a drawer somewhere but didnt want to tell tailshaft off to replace some plastic gears.

 

[meanboyjr]

Mine is off approximately 12%, and because I'm lazy, I just use my smartphone with a speedometer app to make sure I'm within legal limits. The GPS is fairly accurate.

 

[Redneckmudslinger]

Well there is a gear ruduction box connect to the speedo cable and the it continues in the transmission I think that's my problem but what can I do about this box can I bypass it and just connect the cable to the transmission or is there another way of doing this

 

[Redneckmudslinger]

My off by 10 like if I going 50 I really going 60 so what % is that

 

[dyeager535]

If you've got the reduction box, you'd have to either factor it in, or remove it and get the right gears.

Personally I'd want to remove it, just one more thing to potentially go wrong, but I'm not sure if GM ran those because it was simpler to use the same gears in all the trans/t-cases, or because no speedometer gear combo was available that would be close enough.

10% of 50 is 5, 20% then is 10, so if 50 is really 60, your speedometer is 20% slow. But again, with the ratio box, you'd have to factor that into the gears. I'd pull the box and drive it, see what percent it's off then, and see if gears are made that will correct it.

 

[Redneckmudslinger]

Ok so how could I take the box off and by pass it like if I take the box off could I just hook the cable up with no reduction box

 

[dyeager535]

Yep. Should be able to remove the box and thread the cable in the same way.