Speedometer calibration question

[MillerPKA]

Got a quick question. I know the larger the tires go over stock and the more its lifted the less accurate the speedometer reading gets. Well I don't exactly have the money to calibrate right now so I want to know...is the reading generally less than actual, more than actual, or completely variable? I'm guessing i may be off by as much as 7mph after the 315s but I want to know if I should be keeping under for safe measure or going over to compensate. Thanks guys

[BruceTS]

I use my GPS to check the speed, at 60 mph I'm actually doing 65 on 315's.....

[Nuthuts96]

My 265/75/16's the speedo is dead on according to the GPS. the worn down 265/70's i replaced was under-- 60 on the speedo was about 57. sounds like you want to go a bit slower than the speedo says...

[4runnerchevy]

Heres a calculator, nothing fancy. http://4lo.com/calc/gearspeedo.htm

I am around 7 MPH faster than my speedo it seems.

[4LowLocked]

Here is another popular calculator if you want to check it out. I just switched from 265/70/16 to 265/75/16 as well and noticed prior to switching (note this is in KMs) at 100 my GPS showed 98 - now after switching, 100 shows at 104 km/hr on gps

heres the link
http://www.miata.net/garage/tirecalc.html

[Rock Slide]

As pointed out above, with larger tires, your stock speedo will read that you are going slower than you actually are.

I once had an '86 4Runner w/ 35's. I used a gps to compare my actual speed to what my speedo read. I found that when my speedo said I was doing 70mph, I was really doing 76mph. The gap is smaller at lower speeds, but will increase at higher speeds.

Once I regeared, my actual speed vs what my speedo read was less than 1mph of each other at all times.

[Seanz0rz]

with 4.10's and 33 inch tires, actual 65 is achieved at 62mph indicated.

[JWBehm]

with 4:88s and 35s i am still 10mph off, speedo says 65 but actual is 75. I am going to fix it within the week tho.