How to correct speedo for change in tire size?

Can anyone tell me what the original size tires were from the factory
on the 1987 Civic Si?

If someone is good at math, can you tell me what the speed difference
would be, as indicated on the speedometer, between this stock size and
the currently installed P185-65-R15 at, say, 60mph?

Also, where can one obtain the driven gear (or whatever it is called)
that would correct the speedometer for the above current tire size?

 

speedo gear cannot be adjusted or replaced with a different size.

--
Charles Tague
93 Honda Civic DX HB
1.6L SOHC VTEC 14.85 @ 89 mph,1.98 60 ft.
With ZEX 85 hp ZEX 13.09 @ 103 mph, 1.81 60ft.
86 Pontiac Trans Am
225/50/15 GForce Drag Radials
305 peanut cammed 15.29 @ 88 mph
http://home.cinci.rr.com/mistab0ne/

 

The original size is 175/70 13 for a tire diameter of 22.65" and a
circumference of 71.14"
Your size is 185/65 15 for a diameter of 24.47" and a circumference of
76.87".

The difference is 8%, your speedometer will show 8% lower than it would
with stock wheels. If it showed 60 with stock wheels it would show 55.8
with the wheels you have. I don't know how accurate the speedometer is
in and of itself, but it is not unusual for speedometers to show a
percent or two too high a reading with stock tires.

Do a google search for "speedometer calibration" to find places that can
calibrate your speedometer.

 

I also like this one so you can set the gears:
http://gs.tolan-hoechst.com/tirecalc.htm

Although, personally, I crack out a spreadsheet and generate the formulas
myself (not hard at all if you think for a bit).
but you can't adjust the speedo, you can swap the speedo, with one from an
EX, for example, which might have stock tires close to that size.
Since most speedos under-read to begin with by about 5%, maybe more, (my 94
CX was closer to 10%) you may have increased the accuracy of your speedo,
but that also means, that if you used to cruise down a 35MPH road at 45, but
were really only going 41, a cop may not care, but now that you are actually
going 45, you could get pulled over.

 

When I compared my speedometer reading with a handheld GPS receiver, I
found that the speedometer on my '93 Accord read 3% high. Presumably,
this means that my odometer is 3% high too. The tires that came with the
car are 185/70-R14, which seems a little small compared to other cars
in its class which seem to come with 195's. If I changed to 195's, I
would cause my speedo to read 5.4% lower that it is now, and 2.4% lower
than true, so maybe Honda intentionally calibrated the speedometer to
fall between 185's and 195's so they could have the flexibility to use
either sized tire with reasobale accuracy.

Andrew

 

Don't presume! The speedo and odometer are separate instruments. You need
to check the odometer separately. My speedometer is 10% optomistic while my
odometer is almost dead on!

The tires that came with the

Few speedos are pessimistic, probably because the manufacturers don't want
to be blamed for speeding tickets. Most modern ones are very close to
correct with the stock tires and wheels.

George

 

Woops, yeah, I screwed that up...i mean to say over-read. Most speedometers
read a speed higher than you are actually travelling.
Actually, in the UK, it's illegal for the speedo to indicate a lower speed
than you actually go, but it can read higher than your actually speed by
about 10% plus 5km/h or something like that....I don't know what the
regulation is here in the US, or if there even is one.

 

Here is a great little program which actually calculates this for you.
btw, I scanned this for viruses when I DL this morning and it's clean.
as of VDAT 8/15/03 from Mcafee......
Hope this helps
PT

 

sorry but your file didn't post
can you try again?