my speedometer is off what to do?

[jake wells]

ok so im running 225/70R15 tires.... my speedometer works
just fine but there is one problem its says im going 35 mph but im going 45. or im doing 30 and im really doing 40.
could the problem be my tire size? i would like to know the correct tire size would be for my truck. i have the original spare but its size 8.25-15 doesn't help me is there a conversion chart for tires? i would really like to fix my problem.

Jake wells

[unclewatts]

One way to fix this problem without changing out your tires is to change your speedometer gear. They're less than $10 and www.classicparts.com carries them. They have a tech dept that can recommend what gear you need but you have to know how many teeth you got now and what rear end ratio you're running. BTW, 225/70 r15 is not that much different than the original stock tire size.

[Palf70Step]

That tire size is around 1.5" shorter that stock, so you will be off by about 5MPH (faster than speedo says). Considering other factors of how far off they can be originally,plus did someone change the rear end gears or tranny will aslo add to the inaccuracy of the reading. To get it closer as uncle watts mentioned, you can change the gear on the speedo cable, but whoever does the calculations, to do it correct will need the color or gear on the tranny shaft, as well as what your rear end gear ratio is.

[mikey531]

My speedometer is off too so I used my TomTom GPS to find out the speed I was really going. It shows mine being off about 10 MPH with a turbo 350 and 4.10 gears. The speedometer in my 2000 GMC is off a couple miles per hour too. I am going to use this chart when I get the ambition to change mine.

http://novaresource.org/speedo.htm

[cdowns]

check your local yellow pages for a speedometer correction shop and you can have them correct it if you dont want to go thru the hitnmiss of doing it yourself and get it done fast

[Hottrucks]

ok guy heres the short version...my speedo gear was wlking down the shaft making my speedo work today and not tomorrow so I as spending every third day with the speedo out and prying it back inot position that got old really fast but you can see the gear color though the hole where the gear goes into the trans so get that first!! should take longer to jack the truck up then to check the color...take a pair of channel locs and a 7/16" wrench anyway once you get the new correct gear pop the drive shaft and pull the tailshaft off (your not going to loose much fluid maybe a quart at the most..I lost about a pint) I used a 1 1/4" wrench and slid the old one off...just place it over the outputshaft and use the X member to pry against it will be tough coming off there is a little clip that holds it from walking be sure to get a new one of those too....place it in the gear and slide the new one on and make sure it snaps into the little hole...put the tail shaft back on driveshaft in and speedo gear in the side...done..... took me about an hour and I have a 2 piece drive shaft