Tire Size Question

[john m]

Hi. I recently(about 2 1/2 months ago) got a 97 Dodge RAM 1500 PU with a 5.9L engine and 4 Spd. Auto tranny with LockOut(or Lockup). At 60mph, the tach is steady at 1500 rpm) . It has a Richard Petty Effects Package and 15" Goodyear Eagle 1 Nascar Tires. The truck originally had 16" wheels and tires when new. Now, it has 15" wheels and tires. My question is: How much have the smaller wheels and tires changed the accuracy of the speedometer? Thanks. John

[scoutinkeith]

well, it all depends on the overall size of the tire.
let me put it this way, a 33x12.50 R15 and a 33x12.50 R16 will have the same overall size, the only difference is that the one on the 15" rim will have a taller side wall. with that said, from the info you gave, it's imposable to say how far, if at all, the spedo would be off.
if you have the actuail sizes of the tires, you can enter them at this site, http://www.dakota-truck.net/TIRECALC/tirecalc.html just enter the sizes of both tires, and it will show you how far off the spedo will be at different speeds.

[scoutinkeith]

actuailly, I just thought of this, if the previous owner is the one that put the different sized tires and wheels on it, there is a chance that they also had the spedo corrected, so the spedo may not be off at all. my suggestion would be to find a local police officer and explain that you think your spedo may be off, and see if he would be willing to check it with his radar. of it that's not an option, just have a friend drive along side you, and compare spedo readings.

[fredjacksonsan]

There are also speedo shops that can tell you exactly what your error is at certain speeds.

-edit- that was a crappy post; the shops in question run your vehicle on a calibrated "treadmill" and compare your speedo to the calibrated speed, then give you a readout on the error. I used a service like this to get out of a speeding ticket, and made a chart showing the error.

[john m]

Originally Posted by scoutinkeith

well, it all depends on the overall size of the tire.
let me put it this way, a 33x12.50 R15 and a 33x12.50 R16 will have the same overall size, the only difference is that the one on the 15" rim will have a taller side wall. with that said, from the info you gave, it's imposable to say how far, if at all, the spedo would be off.
if you have the actuail sizes of the tires, you can enter them at this site, http://www.dakota-truck.net/TIRECALC/tirecalc.html just enter the sizes of both tires, and it will show you how far off the spedo will be at different speeds.

Thanks, that site gave me all of the info that I need. John M