Speedometer fast or slow?

[Jokester]

Hi,

I changed to an inch shorter tire tonight.

Will my speedometer now read faster than I'm going or slower.

thx,

.bjb

[Carnut]

Well, lessee,

shorter tire should be less frontal area so less wind resistance.

smaller diameter I think means shorter moment arm so torque may or may not be decreased.

smaller contact patch so road friction is reduced, thus less force required for rolling thus improved fuel economy.

also pickup will be increased for better bottom end performance, so may or may not result in more loud pedal use at lower speeds.

oh, and all things being equal, smaller tire will mean faster speedo reading.

[38HAULR]

Changing to a lower profile tyre will have your speedo reading slightly higher than your actual road speed, your engine RPM for a given speed will also increase slightly.   Same effect as changing a diff ratio from 3.00:1 to say 3.23:1  ,going to a 4.00:1 will increase the effect.  Of course your acceleration improves also going up the scale.    This is a prob we have with the proliferation of speed cameras . Guys swap to higher profile tryes or larger diam wheels ,end up with a speedo reading lower than actual road speed and cop fines. Even though our Country,s design rules allow a 10% speedo error ,our law enforcement act differently. Recently my wife copped an infringement in a 100KPH zone.  They allow a % of their own for their equipment,and the penalty reads. "Detected Speed 109KPH.  "  Alleged speed 106KPH.  Fine $142 AUD. Loss of one licence demerit point.   For the record, we wrote to them in this case,apologising for straying over the limit, as my wife had a clean slate ,and they replaced the penalty notice with a "courtesy warning". This is the sort of issue we have with playing around with wheels tyres and diffs without recalibrating speedos......Frank.

[C9]

Measure the diameter of the tire and compare it to the diameter of the old tire.

You have a little under a 3% change as an estimate.

You can compare that to a diff gear change by using percentages between gears.

As well as use the 3% figure to know how much of a speedo gear change you'll need.

Practically speaking about a one and one half MPH difference at 50 so I wouldn't worry about such a small change.

Leave the speedo gear as is and keep in mind it indicates a little faster than actual speed.

[Crosley.In.AZ]

I use a GPS to see how accurate my speed-O  is.

I installed the optional tire size ( larger) on my 2006 Silverado..  speed-O is slow now.

I can not find an economical way to correct my Silverado speed-O via the PCM ... GM dealer wants 85 dollars to hook up their scanner stuff..  Aftermarket programers are in the 200+ dollar range.

Traffic tickets for speed start at 160+ dollars for fine.  :?

[river1]

Traffic tickets for speed start at 160+ dollars for fine.  :?

yup my last ticket would have been $172. i went to traffic school instead at a cost of $165 :?  class was for 4 1/2 hours last night  

later jim

[chopped]

My last speeding ticket was in 1965. I'm thinking the odds are not in my favor.

[58 Yeoman]

When I had to change the tires on my Ranger, they only had one size larger.  My speedo is right on at 65 to 70, but is 5 high at 30, according to the road side radar sign.  I trust my gps to give me the correct speed.