Tire / Speedometer ?

I installed 31" tires on the jeep which are larger than the originals. How much does that throw the accuracy of the speedometer off? I am guessing 5 mph slow????????

 

Anyone know?

 

a er tre

 

I just got new tires today. I kept the best old one for the spare so I can measure both new and old. My old 31" inch tire measures 29.5" like Jim said. The new 31" tires measue 30.5". Interesting.

Anyway, I've made note of speed differences while driving high speed with my wife in her Honda. At highway speeds my JEEP is right at 5 mph off. That is, when my speedometer says 65, I'm really right at 70. Obviously, at slower speeds the difference will be proportionately less.

 

Borrow/buy/steal a GPS unit.

 

or run 60mph and time yourself with the mileposts (60mph will be 60 sec).

 

Not sure this is as linear as your calculations indicate- Please explain. The is a difference of the radius squared to consider. As Worth indicated, different speeds generate different errors in the speedo.

-Mark

 

RPM=(336 x speed x gear ratio x transmission gear ratio ) / tire diameter

speed enter in mph and tire diameter in inches. The 336 is a conversion factor and remains constant.

Example: Driving at 60mph in 4th gear & OD with 33's and 5.38's, my RPM=2,465.

2,465=(336 x 60 x 5.38 x 0.75) / 33

 

Radius squared * pi is for calculating area. Circumference is what is important in this calculation.

 

Here's a calculator.

http://www.offroadexchange.com/speedo.htm

 

Yes, the change is linear in the ratio of radii (or diameters). Only the circumference matters: 2 pi r, not the area of the tire pi r^2.

 

:lurk:

 

:lurk: Why not just make sure your keeping up with most of the traffic but still have some people passing you? I couldn't care less what speed I'm going as long as I'm not the fastest, cause those are the ones the cops like
But then again.. with this F head I have to worry about getting pulled over for going too slowR) R)

 

Thanks for the info. By using the calculator in CD60's post 50mph = 53.48. I thought it was around 5 mph!

 

Now my head hurts.

Basic geometry was a long time ago. Without really thinking, I thought the circumference grew at a different rate than diameter.

So does this mean if the diameter changes by 3.4% as mine did when it went from 29.5" to 30.5", my speedometer reading will change by -3.4% (note the negative)?

 

I agree- and I am incorrect on the radius squared. Is for area and not soo for circumference. So much for quick-look analysis.

Basically I am pointing out that one can not, IMO, with accuracy simply say that their speed-o is "+/-five MPH off" (or whatever number). May be true at 50 mph but certainly not at 0 mph (or 25 or 125, whatever.) Too much energy spent when in reality Executioner really has the right idea.

Thanks for the feedback.

Mark

 

I started to post a reply almost identical to madizell's yesterday, but decided I didn't have the energy to type that much

Thanks for not being lazy!

 

8)

 

why not just swap worm gears to coincide with the new tire size?