speedometer

i just got some 30in tires. And i know the speedo is off by a little bit, can some one tell me the exact numbers. Thanks

scott

do you have a gps?

bburge

nope or i wouldnt be asking

Well, that does make it a little more of a challange.

First off, remember that it is off on a ratio; not a fixed amount. The faster you go - the more off it is. In your driveway : it is dead on correct!

If a factory tire is about 28.5-29 inches in diameter, the 30 will go a little farther with each revolution, so you're going faster than the speedometer says.

The ratio of the 28.5 inch diameter tire to the 30 (probably really 29.5, most tires are like lumber, stated size is .5 inch larger than real) inch tire is 1.0351

So, whatever speed you're seeing on the speedometer; multiply it by 1.0351 and you have a closer approximation of your speed.

I'll leave you to run the speedo readings that work best for you. I live in Southern California, so I rarely have to multiply 2 digit numbers! :rattle:

I sometimes make better time off-road on big rocks that I do on the freeway to work! AND I HAVE MORE FUN ON THEM TOO!

When you get a GPS and want to figure out how to pick the correct speedo gear (number of teeth) I can help you with THAT too. Many people will point you links with tables they swear by, just as many people will point to to the same tables that they SWEAR AT. They fail just as many (probably more) than they work for.

Don't be afraid of a little math, it is your friend...

bburge

Try these out.

http://raingler.com/tech/speedoChart.asp

http://www.offroadtoystore.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=Reference

bburge said:

First off, remember that it is off on a ratio; not a fixed amount. The faster you go - the more off it is. In your driveway : it is dead on correct!

If a factory tire is about 28.5-29 inches in diameter, the 30 will go a little farther with each revolution, so you're going faster than the speedometer says.

The ratio of the 28.5 inch diameter tire to the 30 (probably really 29.5, most tires are like lumber, stated size is .5 inch larger than real) inch tire is 1.0351

So, whatever speed you're seeing on the speedometer; multiply it by 1.0351 and you have a closer approximation of your speed.

Don't be afraid of a little math, it is your friend...

bburge

Click to expand...

As above, calculate the ratio of tire size change. Then remove the speedo gear and count the teeth or look at the marking on it and adjust by the same ratio. Note that if you increase the tire size, you need to decrease the tooth count on the speedo gear. So take your 1.0351 or whatever your actual number is and divide your existing tooth count by that number.
Ex: 26(existing gear)/1.0351=25.12(new gear)
You can probably guess that they don't make 25.12 tooth gear so round that to the nearest whole. In my experience each tooth makes @ 2 MPH change at typical highway speeds but calculating is the best way.

Brian

When I went to 30" tires, I checked with a GPS before and after. I found that I was a couple km/h below guage reading w stock, and dead on with the 30" tires.

It doesn't really change by much.

Let's make it simple.

Going from 215/75R15s to 30x9.50R15s is a 6% change. If your stock tires were 225s, the change will be somewhere between 4% and 5%.

A 5% difference means that at 100 MPH you will be going 5 MPH faster than the speedo reads. So at an indicated 50 MPH you're actually going 52-1/2 MPH. It's proportional through the entire speed range, but below 40 MPH it becomes inconsequential.

Just go out and use a stop watch. at 60 you are doing a mile a minute.