When driving at indicated 75mph true speed is 70mph. Tire size is correct. Bought used with 120k. Noticed it had brand new trans fluid. Appears to be original trans. When referenced it appears to be 1 tire size off. A 31/10.50/15 tire from a 235/75/15 would correct speedo problem. So 1 gear ratio off being indicated. When going thru the trans fluid change (home done I'm guessing) could the sensor being indexed incorrectly be causing this problem? I'm thinking the original sensor failed and while replacing it he lost most all the fluid. Guessing he didn't index correctly and just filled it back up with new fluid. Does this sound like rational thinking? Or am I way off?..

 

How did you "notice" it had brand new transmission fluid? Original 20 year old tires? "Appears" to be original transmission? "When referenced"? What does that mean? Have no idea what you are trying to explain.

 

I would just adjust your driving a bit or get a little bigger tires... it's probably not further off than a large number of vehicles on the road anyhow - and at least it's not of the other direction :-). I've no idea what you're talking about regarding indexing the speed sensor... isn't that speed sensor on top the differential in a 98 anyhow?

 

FYI, speedometers are not off by 5 MPH unless the needle itself has been moved. Rather...with wrong sized tires, gears, or the speedo gear they are off a percentage. In your case yours is running about 7% fast. For example, at a true 35 MPH (35x.07=2.45) your speedometer is reading about 37.5 MPH.

 

I never said "original tires". I said original tire SIZE. When I bought the van it was parked in a guy's unlit garage. It was pouring rain. I pulled the trans dipstick just to check levels. Looked ok. After bringing it home I went over it better (and in better light) and saw it was brand new fluid. The reason I said it appears to be original transmission is nothing in the drive line visually appears to have been recently replaced. No new or clean bolts, lines or parts. When referenced means when referenced to a speedo I know is correct. Reason I asked about indexing is because it was brought up on another thread about replacing the sensor gear for a large tire swap.. Which I may be wrong is located at the tail of the transmission. Thing that really gets me to question the whole thing is this 20 year old van looks brand new in and out. The guy gave me a notebook which had the dates of every thing ever done to that van and which envelope in a small shoebox full of envelopes by year they were in. I'm talking very detailed.. Conspicuously missing is the trans fluid change and the fact that if the speedo was off by 1% he was the kind of guy to look into it. That's also why I think it was a home job. Thinking he changed every fluid, belt and hose he possibly could then just hung the for sale sign on it and just missed indexing the sensor correctly. OR I'm just in hopes of a easy fix!!