New Tires

[HatGuy]

I just got some 31x1050 15" tires put
on my Blazer now the speedometer is
off. I have a 350 TH and 373 gears.
What gear do I need to use it the
transmision?

Thanks

[vtblazer]

Shouln't be all that far off.

Go down the interstate and check it against the mile markers.

Time yourself doing a mile and just figure it from there.

It'll be come habbit to "adjust" what your reading on the speedo.

I'm also running 33's and I'm only about 5mph off.

[jhow66]

Remove your speedometer housing to see what color drive gear (one on the trans output shaft). Then go here to figure what driven gear(one that hooks to your speedometer cable) you need to correct error.
http://www.tciauto.com/tech_info/speedo_gears.htm
(scroll down to T350)

[71swb4x4]

You should be pulling your speed from the transfer case. Not sure how it differs, but it does have that piece that comes off the transfer case, maybe you can replace a gear in there?

[LUV2XCLR8]

Originally Posted by jhow66

Remove your speedometer housing to see what color drive gear (one on the trans output shaft). Then go here to figure what driven gear(one that hooks to your speedometer cable) you need to correct error.
http://www.tciauto.com/tech_info/speedo_gears.htm
(scroll down to T350)

Cooool link

[71RestoRod]

Do you have a drive ratio adaptor on the transfer case? It will be a small 'gearbox' mounted between the speedometer hookup on the transfer case and the speedometer cable. If you do, take it off and connect the speedometer cable directly to the transfer case.

Then, as VT suggested, drive down the highway and take note of the mile markers. Drive for five miles and measure with your odometer how far the truck thinks it went... then call a speedometer shop, give them the info and they will set you up with the correct drive ratio adaptor.

For the record, with 33" tires and 4.11 gears, mine was right on the money with the adaptor removed (Bonus!!). They sometimes set up unmanned electronic radar signs around here (your speed is...xx). There happened to be one in my neighborhood the day I performed the mile marker test. I made several passes (some in the green, several in the red!!) and the speed was right on in every case.

I'm thinking that with 31 in. and 3.73s you should be pretty close without the adaptor. It all depends on the actual tire diameter. Of course, if the speedometer is not calibrated well, all bets are off for measuring MPH...