On the Viper when you go down in overall tire size in the rear, I know it affects the speedo, but what about the tach? I would assume the tach is getting its signal from somewhere else. I ask as I went down from the stock tire dia. of 27.4 to 26.2 with the drag radials. So assuming this my speedo is slow by 4% from what I calculate. old tire size 27.4 divided by new tire size 26.2 =1.045 correction factor. So if my speedo is reading 180mph, I am actually going 188mph. Does this seem correct? If this is correct I am going alot faster than I think I am at high speed.
Yes it is around a 3-5% speed loss. At 4% you should be running at 187.2 since there is less rotation that the wheels have to travel. The tach will never change if you switch out for smaller wheels. At 180 if you are used to running at around 5200 rpm, you switch the tires and you see 5400 rpm at 180, it is your speedo that is off not the tach. The tach is tapped into the motor.
I thought SRTs used an ABS sensor so that there was no conflict when changing tire sizes.
This may be true, as I don't think I was going 198mph yesterday. It may not be tied into the ABS, but maybe a VSS somewhere? Anyone know?
***EDIT***
After thinking about it, my speedo has to be off as there is no way the car can know what tire size is on the car unless it is programmed into the system, which I have not. Since I have deviated, it must be 4% slow. So I guess I am seeing the higher speeds than what is being reported on the speedo. The only way it could know speed is via GPS, which this car does not have.
Last edited by NoRice : October 23rd, 2006 at 09:30 PM.
No Rice-I thought so to-only thing-since the gear change it seems like the speedo is off even more. I did change to 18 & 19 Pirelli tires awhile back and they are about 1/2" smaller in height than Michelin run flats ,and this will cause speedo to read off. But it does seem like the gear change has even made the speedo read off a little more ,but this is probably just in my mind. Ill have to get it checked.
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