Speedo Accuracy and Wheels

[vwfan53]

For the life of me I have confused myself...

Given the center hole, speedo cable mount in spindle, does tire diameter really come into play here?

True, a smaller dia tire will rotate more times per mile, but will the center hub rotate any faster.

I dunno why I am having a hard time with the physics behind this one.

Thanks,
-Paul

[Sandeep]

Yes the center hub will rotate faster. As to how much faster, its a ratio of new tire diameter and original tire diameter.

Sandeep

[Supa Ninja]

In my Supa I'm running the VDO electronic speed. I got cause when I used to drag race with 195/50's in the front my speed was off by almost 20mph. With the electronic speed I'm able to calculate it to whatever size of tires I'm running. I won't have that ability very long cause I'm going to be going to 944 guages so hopefully 205/40R17's will be somewhat close OD as the 944's original tires.

Nick

[yetibone]

Hey Ninja,
Pardon my asking, but how do you drive your speedometer?
Is it still driven off the left front wheel?

I'm still wondering how to make Hummbug's speedometer work
with 944 spindles.


Yetibone

[Dennis]

You can determine the amount that your speedo is off by finding the circumference of your current tire/wheel combo and comparing it to the circumference of the factory tire specs.

[CLKWRK]

I use a GPS, you set it to tell you land speed, that way you know just how much your speedo is off by.....no math involved

Originally posted by Dennis
You can determine the amount that your speedo is off by finding the circumference of your current tire/wheel combo and comparing it to the circumference of the factory tire specs.

Note below the 78 aspect ratio, stock tires in the 70's were 165SR15 which can be translated to an 80 but 78 is more accurate. A larger overall diameter will slow the speedo down and a smaller diameter will speed it up.

135/78R15 = 23.29
145/78R15 = 23.91
155/78R15 = 24.52
165/78R15 = 25.13
195/50R15 = 22.68
205/50R16 = 24.07
205/40R17 = 23.46
215/40R17 = 23.77
225/35R17 = 23.20
235/35R18 = 24.48

[Supa Ninja]

Yetibone,
It's driven by the left front spindle throw a cable to a gate sampler to the speedo. The only two things I don't like about is the price $200 USD and the circumfernce is smaller then the stock speedo but since I'm running a fiberglass dash it doesn't really matter.

Nick

[jmillheiser]

hmm. that should put a 235/40R17 or a 225/45R17 right around stock height.

my eventual plans for my car are to fit 235/40R17s in back which if they are that close to stock height should keep the RPMs in the stock range while driving.

i was planning on running 215/40R17s up front. could probably just run one of those VDOs that you can re-calibrate on the fly.

of course this is a long way off. car is bone stock right now.

73 std beetle

[bren]

use this calculator.. its great.
http://www.miata.net/garage/tirecalc.html

[Racelook]

So that's a real good calculator....

THnxxxxxxxxx

Wiebrand

Tomorrow BUDEL Keverweekend in the Netherlands!!!!!!!!!

another calculator

http://www.dsm.org/tools/tiresize.htm