I have fat fives and 205/50/R15 tires on my crx. It seemed like i was flying by people on the highway so I set up a not too accurate test but had a friend drive in cruise control in front of me and I matched their speed. At 30 I was going 35, at 65 I was going about 71, and at 75 I was going about 82 or 83. I calculated the outside diameter of the stock rims and stock honda spec sized tires and it is only off by a tiny bit more than an inch. I know my speed would be off given the larger diameter but I didn't know it would be by this much. Is anyone else seeing this drastic of a change or is something else wrong?

 

not every 205 is the same height, however most are around 23.1" still that's bigger than a 195 50 15 or 205 40 16 which is what i see most often on a crx, those are around 22.4" the most accurate way i can think of to fix this if you plan on keeping these tires awhile is to find one of those cop radar stations and do multiple passes by it. once you know how far off you are you can either keep it in mind or pull the needle on the speedo and place it exactly where it should be. as in hold the car at 30mph verified via radar and push the needle back on at exactly 30mph. then test 10 over and 10 under to make sure everything is peachy.

 

Cool didn't know you could do that.

 

To answer your original question...

I believe all CRXs came with either 175/70-13 or 185/60-14 stock tires. The calculated outer diameter of 205/50-15 is 1.9 percent larger than 175/70-13 and 1.5 percent larger than 185/60-14, so it will change the accuracy of your speedometer and odometer by that small percentage. For example, since 205/50-15 is 1.9 percent larger, if your speedometer was accurate at 100 mph with 175/70-13, you would then be going 101.9 mph with 205/50-15when the speedometer says 100. In general, on a CRX, you would be better off with 195/50-15 than 205/50-15, since 195/50-15 is closer to the stock outer diameter (0.1 percent larger than 175/70-13, 0.3 percent smaller than 185/60-14).

The numbers apply if all else is equal - which it usually isn't. As noted above, not all tires labeled as a certain size are exactly that size; however, they are usually pretty close, and you can generally assume they are that size for calculation purposes. Also, other factors can affect speedometer accuracy; for example, the accuracy difference due to a difference in tread depth between a tire that is new and one that is worn and ready for replacement is around 2 percent. And speedometers may not be precisely accurate, even with the stock tire size, although they are rarely off by more than 2-3 mph.

Your statements above assume that your friend's car's speedometer is precisely accurate, which may or may not be true. Any of the factors stated above may affect the accuracy of his speedometer, just as they can affect the accuracy of yours. You would be better off using a precise measurement method to figure out how far yours is off.

Bottom line, though, the tire size can only explain differences of 1.5-1.9 percent, so other factors would need to account for any other speedometer error.

 

Not sure that I would try to push the speedo needle back when it is being used. Sounds like an easy way to ruin a cluster.... But I am not sure. I though speedos were controlled with gears?

 

I agree^ I don't think it works that way.

 

They are controlled by gears.

However, you can change what shaft position corresponds to what needle position, by removing and replacing the needle on the shaft as idrivesideways described.

Basically, if the needle is at 20 but should be at 30, you remove the needle, the shaft that it mounts on stays in the same orientation (its rotated the same amount as before) and put it back on in the 30 position. The potential downside is that it might not go all the way to 0 now, depending on how far you "adjust" it and in which direction.

It is a "hack". Only do this is you have absolutely no other means to correct your speedometer.

 

Removing and replacing the speedometer needle won't work, because it won't change the speedometer reading proportionally, it will only add or subtract a certain number of mph to the reading.