Speedo vs. tire size question?

**shadetree** · 07-27-2010, 06:07 AM

I know the speedo on my 250 reads high but has any one checked with a gps as to the difference when using a 100/90 tire instead of the 90/90 which is slightly different in dia. The 100/90 does less revolutions per mile so how much closer to being acurate would this make the speedo?

**bB2NER** · 07-27-2010, 08:38 PM

The 100/90 is closer to right but it took 110/90/18 to be right on for my 09'.

**shadetree** · 07-27-2010, 08:51 PM

I'm runnin a '86 250 do you think thats the same? Thanx for the input, I am curios about how my speed is every time I drive around here.

**shadetree** · 07-27-2010, 08:54 PM

BTW,Where is Camden TN. I used to live in sweetwater and knoxville?

**bB2NER** · 07-28-2010, 03:27 AM

Quote:

Originally Posted by shadetree

BTW,Where is Camden TN. I used to live in sweetwater and knoxville?

12 miles north of exit 126 on I-40 between Memphis and Nashville.

**bB2NER** · 07-28-2010, 03:29 AM

Quote:

Originally Posted by shadetree

I'm runnin a '86 250 do you think thats the same? Thanx for the input, I am curios about how my speed is every time I drive around here.

When I'm running the stock 3.00X18 tire I always go about 5 mph over and know I'm still ok.

**cagie** · 07-28-2010, 10:43 AM

just as a general rule of thumb for the speeod accuracy: the ratio is on a stock 3.00x18 tire (or a 90/90x18, which are slightly less than 24" in dia.) is something like 2.7:1. This will put you about 10% optomistic.

The way you can check it is to loosen the speedo cable from the speedo. Put a piece of tape like a flag on the protruding cable. Turn your front wheel completely around and count how many times the cable goes around. The result is your ratio.

It would be interesting to know if larger. higher profile tires will effect the ratio, but I suspect not much.