Speedometer change when getting different tires

I just got 20" rims on my truck. The tires are actually a bit smaller than the stock tires, because they're low profile. The size is nearly the same. Anyway, does anyone konw if the mileage and/or speedometer automatically adjust to the new tires, or do you have to take your truck in to get it fixed?

If the over-all outside diameter of your tires is going to be about the same, it shouldn't throw off the spedo. If you change the outside diameter by more than an inch or so, it will make a difference. I'm not sure what you would have to do to recalibrate it.

Here is a handy site that will help you determine if your speedo will be off or not
http://www.miata.net/garage/tirecalc.html

Originally posted by Melt
Here is a handy site that will help you determine if your speedo will be off or not
http://www.miata.net/garage/tirecalc.html

Cool, thanks. It's not that bad. When I go 60, I'm really going 58.3 MPH. 2.8% too fast.

On all the newer cars and trucks the milage and speed comes from a sensor on the transmission. so no you wouldn't.:alien: :smoker2: :sun: :flash: :smoker: :silly2: :smoka: :frog: :grey: :licker: :devil: :bloated: :bandit: :alien2: :eek: :confused: :mad: :rolleyes: :cool: :D ;) :p :o :( :) :flamer: :eek2: :biggrin2: :cyclops: :rocket: :dogpile: :angel: :eshooter: :hehe: :hehehe: :right: :coolguy: :help: :apoke: :spit: :apuke: :badass: :argue: :aug2: :buck :crying: :huh: :cwn27: :evillaugh :greenchai :bathroom: :jump: :newburn: :monkeypis :sleep: :silly: :stormzap: :thinkerg: :what: :uhoh: :zx11pisse :xelicon: :shocked: :lol2: :ylsuper :toothless :finger4:

Not necessarily true lilwrestler. One thing most people dont understand is that yeah the trans gives the actually reading but just about every computer in the truck is calibrated to certain paramiters. If you change tire size it may effect ABS operation, speedo operation( that ones obvious) and the way the trans shifts. You can take it to the dealer when you do this or before you do and they can reprogram the certain modules that may be effected with the correct tire size. But beware there is only certain tire sizes that they can do so the best thing to do is make friends with a tech and see if he can get the list before you purchase new tires. Ive done a few for friends that have done this and for the most part it works out ok. There is actually a bulletin for this but I cant remember what it is at this time.

I agree with bowtiebandit, it will also depend on your year truck how to go about changing the ratio.

88-91 used a pin burn in on the back of the cluster

92 throug i think 96 used a drac(digital,ratio analog convertor) module that could be orderd from a auth declo cluster center.

97 and up reprogrammed the bcm with a tech scanner