Speedometer re-calibration

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Nov 18, 2004 | 12:26 PM

rganders

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From: Vidor Tx

Speedometer re-calibration

Can the speedometer on my '95 coupe be recalibrated if I change my rear wheels from 17" to 18". The tire size would increase from 20.25" heighth to 23" heighth. Calculates out to about 68 actual mph when speedometer reads 60 mph. Thanks rganders@netzero.com

Nov 18, 2004 | 01:44 PM

kwik_ta

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From: St. Peters MO Sometimes you have to prove yourself by doing alot of killing or alot of dying...

Yes the speedometer can be recalibrated... I'd like to add 'can it be done with a hypertech tuner?'

Nov 18, 2004 | 02:12 PM

CrvtBB

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From: North Carolina

From what I understand the hypertech programmer can adjust for rear end gears and tire size. You'll need to know the total height of the wheel and tire.

Nov 18, 2004 | 02:30 PM

xs650

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From: Northern CA

Quote:

Originally Posted by rganders

if I change my rear wheels from 17" to 18". The tire size would increase from 20.25" heighth to 23" heighth. Calculates out to about 68 actual mph when speedometer reads 60 mph. Thanks rganders@netzero.com

Your tire diameters don't look right. What sizes are they?

Nov 18, 2004 | 03:51 PM

rganders

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From: Vidor Tx

Got the tire height from some chart (can't remember where). The tires on the car presently are 275 40 17. Thanks

Nov 18, 2004 | 03:58 PM

Team Owner

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From: Phila., PA burbs

Quote:

Originally Posted by rganders

Got the tire height from some chart (can't remember where). The tires on the car presently are 275 40 17. Thanks

I believe that 275/40/17 tires are around 25.7" in diameter. You should be able to find an 18" tire thats pretty darn close.

Nov 18, 2004 | 04:50 PM

rganders

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From: Vidor Tx

I'm new to this part of corvette ownership. I found the chart for tire diameter and apparently I misread it. Looks like I could go to a 285 35 18 (diameter 25.85in) and my speedometer would be off less that .6 mph at 60 mph. Thanks Randy Anders

Nov 18, 2004 | 08:07 PM

xs650

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Quote:

Originally Posted by rganders

Looks like I could go to a 285 35 18 (diameter 25.85in) and my speedometer would be off less that .6 mph at 60 mph. Thanks Randy Anders

Find someone with a GPS with a speed readout and check your speedometer at highway cruising speed. Speedometers usually read a little high, so you could potentially make it more accurate than it is by using a slightly larger tire.

We are picking nits here, but why not?

If someday you don't have you chart handy, tire diameter in inches

= wheel dia + (nominal tire width x aspect ratio)/1270

in your case = 18+(285x35)/1270 = 25.854 inches.

Look like your chart calculates it the same way.

The 1270 is just a finagle factor to convert from mm to inches, account for two hunks of rubber and convert from percent to a decimal number.