revs per mile - Page 3

Art161 · 06-26-2009, 12:50 PM

Quote:

Originally Posted by 28HopUp

But here's the question - If the correct number is 770, but you plug in 740 into your handheld tuner, will be speedo be reading faster or slower than you are actually traveling?

Probably.

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1morepart · 06-26-2009, 02:46 PM

the correct number, i looked it up again, for bf 235/50/18 97w is 759, but you can only put in 760.close enough
j

mustangtestuser · 06-27-2009, 10:00 AM

Use the manufacturers data! If they don't offer contact them or look at similiar tires from several companies. 770 revs sounds just about right.

Here's an interesting point when considering speedometer correction. The speedo needle on my Mustang naturally reads 3 mph LOW.

How do I know?

I have an Aeroforce II that reads the speed directly from the computer and a GPS. The speed registered by the computer is consistently 3 mph faster than the speed displayed on the speedometer of my 2006. Just an error in needle position from the factory!

The number from my OBDII scan gauge agrees very, very well with the data from my GPS at steady speeds. It takes a split second for the GPS to register the correct speed, so it's best to test on a flat road at a constant speed.

It took me a while to figure out the problem. I just assumed the needle on the speedometer was dead on from the factory. Apparently, the SAE allows for some variation in speedometer readings. Who knew?

On top of that the SCT tune I originally had in the car had the wrong rev per mile entered. I always correct the speedometer when I use a tuner, even if the "default" is supposedly correct.

Art161 · 06-28-2009, 09:13 PM

I think a lot of manufacturers set the speedo to read a little fast. They don't want customers to complain about a speeding ticket because the speedo reads a little slow. To compensate for various factors, they err on the side of being a bit conservative. Strange to have a speedo read slow.

157db · 07-02-2009, 11:08 AM

Quote:

Originally Posted by Art161

I think a lot of manufacturers set the speedo to read a little fast. They don't want customers to complain about a speeding ticket because the speedo reads a little slow. To compensate for various factors, they err on the side of being a bit conservative. Strange to have a speedo read slow.

I dont use the speedo.
The tach will tell you what you want to know.
Red line first, 30 MPH.
Red line 2nd, 60 MPH.
And so on.