I posted other day about speedometer error if I changed from 20" wheels and tires to 17". Well I made the trade. The tires I have now are about 2" maybe a little more shorter than the 20's. I am getting exactly the same speedometer speed with a given rpm as with the 20's. 60mph=1900rpm, 70mph=2200rpm. How can that be? Looks to me I would have to have more rpm to get the same speed with smaller tires.

 

What are the exact tire sizes - before and after? Did you physically measure the tires to figure out that they are 2" shorter, or are you going by the tire spec?

 

I actually stood them side by side and measured. Straight edge on top of tall tire extended over to above short tire and measure down from there. Of course how level I had straight edge would affect the distance to short tire, but it is close.

Does the speedometer get its info from the tach via the computer? If so tire diameter would make no difference in speedometer reading, it would only make difference in actual speed. The accuracy of speedometer in that case would depend on what tire size was entered.

Another question, my truck has 17's listed on the door. Does that mean that is what was on it from factory?

 

You're turning 1900 RPMs at 60!? I only turn 1600. You should invest in a tuner to adjust the tire size and tune the truck in general.

 

Mine turns 1,980 rpm @ 80 mph. 285/70x17s.

 

17" is the basic stock wheel. 245/70x17 tires. Yours came on 20" wheels. What are the two tire sizes you're comparing?

 

I had 275/60x20 I now have 265/70x17.
How do you know mine came on 20" wheels?

 

Your first post indicated you were changing from 20" to 17". Were the 20s added later?

 

Your stock tire is probably the same as mine: 245/70x17. My 285/70x17s have made the speedometer/odometer 4.3% slow/low. If you have a GPS, it can show you how much the speedo and odo are off.

 

Were the 20's the original wheels. If not, they could have been giving you a false reading, and now your reading the correct speed. Those RPM's seem pretty high. What engine and gearset do you have. When you went to the shorter tire, did the truck "wake up" at all? Dropping 2 inches of sidewall would numerically raise your effective gear ratio and may feel like the truck has a few more cojones off the line.