engine and drivetrain Confirmed, W&N speedo - 3.90 version off 10%, but so wha

[Guest Anonymous]

So I did the math, feel free to correct, but I think this is thte way it works out;

The usual 3.64 diff and speedo, original tires 165/70-13s would look like this;

5280 feet per mile x 12" = 63,360 inches per mile, divided by a wheel circumfrance of 69.4 divided by 3.64 means at 60MPH engine needs to spin at 3323 RPM with std 4 speed, 1:1 ending ratio in 4th gear.

So, the 3.90 speedo all being equal, is calibrated to expect 3560 RPM to equate to 60MPH. 63,360/69.4 x 3.90.

At that RPM the 3.64 car is doing 64.28, that is about 7%, but then add the fact I am running 185/60 -14's that have a circumfrance of 71.4, that would mean car is doing 66.13, very close to 10% over. Which is easy math while I am driving. 55 = 60, 60=66, 70=77. Actually may be good, so when that unsuspecting passenger looks over and says "How fast are we going? and slow down" They will only see 90% of reality!

I may attempt to correct all this by switching out the gearing on the odometer, and the speedo needle which I think is counterweighted differently that the 3.90 one, but for now just having a speedo that doesn't jump all over the place is great.

[Guest Anonymous]

URL: http://cgi.ebay.ca/ws/eBayISAPI.dll?ViewItem&item=2445359299

Not sure if that mucks up your calcs, but it's something to consider.

Al

[Guest Anonymous]

(nt)

[Guest Anonymous]

Nominal original diameter: 23.7 inch.

Nominal 195/60-14 diameter: 23.2 inch.

Difference is -1.9%. A difference that small is insignificant, because other errors of similar magnitude exist. For example, a typical tire changes 1.1% as it wears from full tread down to the wear bars. Speedo error can easily be 5% or more.