Topic: Speedometer with larger rims question

[relic1864]

If I change my 14 inch rims to 15 inch rims on my 69 Mustang   How much difference will the speedometer read. Will it make the speedometer read the same ...faster...or slower than it did with the 14 inch rims.

[ol dirty doug]

well technically if you find the same tires with the same hight as the ones you are takinf off then it will be the same....   the key part to your question is going to be what is your old tire size and what is your new tire size?  then i can tell you exactally how much it will be off.

[Jeff1967]

I changed my 67' over last year from 14" to 15" and there was no change at all. But I went from a higher profile basic tire to BF Goodrich T/A series.

Jeff

[FASTBACK65GT]

I like this site for determining old/new tire sizes and changes. http://www.miata.net/garage/tirecalc.html Just plug in your old tires/wheels and change the new ones to be 15s and keep changing until you find the nearly exact size of your old tires.

[a_qcode]

Check this site out if you are having questions about your speedometer accuracy.  It does the calc. for you and you can enter different tire sizes to see the impact.

http://www.csgnetwork.com/speedocalibcalc.html

[66GTKFB]

Unfortuntely, neither caculator tells you what new speedometer drive gear you might need to change. In general (looking at my notes), a larger diameter tire (tyre for the guys down under) means less teeth in the transmission's cable drive gear, and a higher gear ratio (as in 3:00-1 changed to 3:89-1) would require more teeth. I use two methods, my GPS receiver (it's a bit more sophisticated than a Garman) that reads direct mph, or a stopwatch and roadside mileage markers. Go exactly 60mph for one mile (more than once for accuracy) and you should clock 60 seconds. More time you're slow, less time you're fast.
Jim

[Soaring]

Even if you are not exactly on the mark, radar is not so accurate to clock you speeding.  You will not be breaking the law by a mile or two over the speed limit.  I wouldn't sweat it.  Changing the speedo gear in the transmission is a pain in the butt, so don't worry about it.