Dang that's a lot off. I am on 15s with 205/50 tires and my speedo is about perfect I think. But I haven't checked with maps.

 

I’ve never used Google Maps to check speedometer accuracy. I use a phone app called Speed Box.

It doesn’t make sense for your speedometer to be off by that much with the stock tire size. What year is your lude? Has your car had a swap to something other than a 3rd gen engine/transmission?

 

I've never owned a car with an accurate speedometer. They're always off by about 10%.

 

Rim and tire size will throw off the speedometer.

The speedometer powered but the transmission gear in our cars and the OEM rim is 14" rims and 195/50/14 tires if I remember correctly.

 

The oem size is 195/60/14.

 

Since the OP has said he has stock sized tires, the questions I was asking were geared more towards whether his car had an engine/trans swap. Maybe Sean can address this since he's done several. I'm assuming you'd need to make allowances for speedometer accuracy if you do an H22 swap. I'm also assuming 4th and 5th gen Preludes have a different stock tire size/diameter than 3rd gens. This is pure speculation since we don't know whether the OP has 3rd gen power train components or not.

 

I wouldn't trust Google maps. In my experience I have seen it off by as much as 5 mph on my 2023 Honda.

If you can find one of those road signs they put up that measure your speed in real time, that would probably be more accurate.

I haven't used the app that luzer recommended so can't speak to that.

Or you can use highway mile markers. Drive at a constant speed throughout the mile. Divide the seconds it took by 3600 and that is your actual speed. You can then compare that to your driving speed to see if the speedometer is accurate.

 

I've verified that the app I mentioned is accurate using the two other vehicles listed in my sig that I know have accurate speedometers.