Weird issue with my 97 B2500 van, it has 30/9.50/15 ATs on it which has a 30" diameter. The OEM tire size is 235/75/15 and is a 28.9" diameter. So the tires SHOULD be spinning more slowly and the speedo SHOULD read about 2-3 mph slower than actual but as you probably are guessing, the opposite is happening and my speedometer is reading almost 4-5 mph FASTER than actual when I gauge using my phone GPS speedometer app. So it's acting like I am running smaller tires than OEM when I am actually running larger tires. What the hay?


The only variables I can think of is the rear end which is original but the tranny was rebuilt a few years ago and possibly they didn't rebuild the tranny and just swapped it out for one they had on the shelf that was set up for a different rear end ratio. That's my best guess.


Question being... is there any other reason for this? I can look up the rear end code and order a new speedo gear to match may ratio just to be sure or I can just ignore it but I want to be sure because I want to put smaller tires on it which will make my speedo roll even more incorrectly. I hate the idea of my drivetrain spinning at 90 mph when I am only doing 78 or whatever.

 

Ok, yep, that's a weird one....

Your speedo is driven by the VSS in the tailhousing of the trans. Might have the wrong gear in it, easy to check, pull it out, check the color of the gear, and compare that to the chart on transmissioncenter.net.

 

1. Did you ever have the original tire size on the van and test the accuracy like you did with the larger ones?

2. In the tailpiece of the tranny is a little gear that drives the speedometer sensor. These gears come with different numbers of teeth to adjust for different rear axle ratios and tires sizes. It's possible when the tranny was swapped that this gear was not transferred over. There is a number stamped on the gear which is the number of teeth on it. You have to remove it, see what number is on it, then get the next size or two up.

 

I will assume by the replies that I didn't miss anything and the speedo gear is the culprit as I thought. Thanks for the transmissioncenter.net link.

 

Best guess without actually laying hands on it.

 

Love the slots, With the white letters facing out. This is the way it should be!