speedometer accuracy with non-stock tire diameter

[bucky katt]

i'm sure it's been asked here before but i had no luck in searching so, i'll take the chance of being flamed for asking i have a 72 super beetle and i'm getting ready to drop it and put some 135's on the front and i know that the speedometer is going to read alot faster than i'm really going and i'm wondering how to correct this situation. it's been 16 years since i last owned a bug and back then i didnt give a damn about whether it was accurate or not, but now, in my 40's with a spotless driving record i do care. i think i'll be keeping the stock speedo but i might remotely possibly do a custom dash with non stock gauges (vdo possibly) how can it be done (if it can be done) ?

[tlambert]

There are GPS Speedos now. I seen them on a site but cannot remember where. Try doing a google search and you might be able to find it.

They are not VW but they do look nice.

Or you could use a small GPS also.

[Lidpainter]

I had the same question and found this calculator online. http://tire-size-conversion.com/speedometer-calibration/
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[easy e]

This place can make a small in-line gizmo to correct. You mark your speedo cable where you want it. Probably close to speedo. With your marked cable, they cut it at the mark & add in their corrector.

Give them a call to discuss:
http://www.tacomaspeedometer.net
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aka: Evan
Spreadsheet for Bus RPM, based on gearing & tire size (Excel format)
Searchable, click-navigable 1958 Bus Parts List

[911pickup]

Speedhut makes a GPS speedometer:

http://www.speedhut.com/gauges/GPS-speedometers

Another on the SAMBA site:

http://www.thesamba.com/vw/classifieds/detail.php?id=1327649

For a much cheaper solution you could go for a drive and use a GPS to compare speeds with your speedo. Pick 2 or 3 speeds you usually drive at (i.e. 30 mph) and mark the GPS speed on your speedo with tape or a felt pen.

[2-lo]

Download a free GPS speedo app for your Iphone/smart phone. They work perfect. I only use mine to see the inaccuracy from a tire size change once i know that I know what speed i'm doing from the stock gauge.

[slalombuggy]

If your running 165s on the rear a 135 will read about 10% too fast. I just drove beside a friends car at speed to check the difference

brad

[easy e]

Rear has nothing to do with speedometer reading.... only driveability (RPM's / torque).

Front wheel is connected to speedo & has everything to do with reading fast/slow.
_________________
aka: Evan
Spreadsheet for Bus RPM, based on gearing & tire size (Excel format)
Searchable, click-navigable 1958 Bus Parts List