I just replaced my tires and now the speedometer is off just a little.
On these newer speedometer how hard is this to fix?

I went from P235 75 15 from P235 70 15 it off by 3 mph at 55 mph not a lot but at 65 it off by 4-5 mph

 

mine has done a similar tire change im suposed to have the 235-70 15s stock i went to a set of 235-75-15s didnt notice much now i have a set of 30s and im going faster then the speedo reads. on your drivers door there should be a sticker that will tell you what size tire it came with from the factory my 99 ls has the soft ride supension so 235-70-15s are stock

 

The change in speedo readings is all because of the OD of the new tire. Assuming the old tires were stock size the speedo was calibrated for that OD tire. The new tires being either taller (speedo will read slower than actual road speed because a taller tire rotates less than a shorter tire) & the opposite is true for a shorter tire compared to a taller.
Now a new tire of stock size may make you think the speedo is off but that's just because you've become used to the tread wear (smaller OD) of the old tires but it wouldn't be too much of a difference.

Read about aspect ratios etc http://auto.howstuffworks.com/tire2.htm it should help better understand what's going on with them.

 

Installing tires other than the original size will affect speedometer & odometer readings, transmission shift points and ABS operation. It can be corrected, (recalibrated) using a capable bi-directional scan tool. Any GM dealer can do it for you.

 

off topic but i cant make a post can someone please help? i am new and cant seem to fingure it out greatly appreciate the help

 

A dealer can only change it to a stock size tire. If the size was not an option they wont be able to do anything. You would either have to buy a speedo recalibrator or send the pcm in to be tuned

 

that's what I was wondering if a dealer could adjust it.

 

Ummm you just did post... but take your question to Forum Help & Suggestion Center - Blazer Forum - Chevy Blazer Forums
A moderator will be able to help you there.

 

I have the same issue as well.
Need to change shift points & recalibrate speedo for larger tires.
Does anyone know if Hypertec Max Energy Programmer works for the 1989 4.3?