Tire Size

if i change my tires from 205x60x15 to 205x65x15 what will happen to to speed?

The tire diameter would be very slightly larger, but you would hardly notice. Speed is a function of how hard you depress the gas pedal.

A Georgia highway cop trying to nab an unsuspecting New Yorker on his way to Florida might catch you for the very small error in your speedometer. There is a complex formula I won’t bother you with, but some tire companies also publish the “rolling diameter” of their tires when properly inflated. You can take that, and compare it with the old diameter, muliply by 3.1416 to get the difference.

Since your speedometer is not all that accurate anyway, I would not worry about it.

It won’t make enough difference to worry about.

Thank You!

205/60-15 4.8" 12.3" 24.7" 77.6" 817/mi 60MPH 10000mi N/A
205/65-15 5.2" 12.7" 25.5" 80.1" 791/mi 58MPH 9683mi 3.3%

A 3.3% difference, should fit in the wells, but enough of a difference that I’d do some checking with the dealer if it had traction control, ABS, stability control and fitment problems. Some cars have so little clearance or electronic considerations the extra .4 inches needs a little more attention than we could generalize here.
In years past, I’ve jumped sizes much more than that…not now w/o make/model specific guidance.
BTW, if you find no problems with car maker, change the spare too.
I hope your motivation is not to just save money…it could cost you much more w/o checking further.

If you have access to a GPS take it along with you and drive at different speeds and compare that with the speed on the GPS. Eventhough I have the same size tire on my truck that came on from the manufacture I noticed when my speedo reads 70mph the GPS almost always says 66-67mph.