Toy Civic

Regarding speedo and odometer accuracy……………………

I don’t know if this has ever been covered in previous posts. If you already are aware, or don’t give a ****, then disregard. But for those who are unaware, do care, or are new to Hondas and Acuras, then please read on.

When our Civics and Integras were stock, they came with 14 or 15 inch wheels with multiple tire sizes depending on model. I am here to tell you that the speedo and odometer MAY NOT BE ACCURATE with the stock tire sizes…………………between 3-11% depending on the stock tire size that came on the car. The only way to verify is by taking the car to a speedo calibration shop.

As many know, the whole system runs off the electronic vehicle speed sensor in the tranny and is driven by the supplemental ring gear on the edge of the differential and the gear attached to the speed sensor itself. Unless I am mistaken (95% sure that I am not), both those gears have the exact same ratio regardless of which Civic or Integra B or D series manual tranny you have. I cannot vouch for speedo gears in the automatic trannies. With the stock tire sizes, the only real variable is how the speed sensor electronic voltage signal itself is performing. Swapping speed sensors may or may not solve the problem. Chances are though that the vehicle speed sensor is functioning AS DESIGNED.

I bought my 1995 Civic EX coupe new and immediately noticed this flaw. When my gauges were reading 70mph, I was really doing 64mph. That 9% difference was also consistent at other speeds as well. The odometer was registering 1.1 miles per every mile driven. I had this verified by a speedo calibration shop. I had the stock 185/60/14 tire size. I took the car to the dealer and they replaced the speedo cluster, which didn’t solve anything. When I later went to 195/50/15 tires, the accuracy stayed the same. I drove the car like that for a few years and mentally compensated. My mileage accumulated a few extra thousand miles more than it should have.

In 2001, I switched to my current tire size of 205/40/17. With my original D series manual tranny, my speedo accuracy increased to 99-100% depending on my speed. My odometer has been perfect ever since.

In 2006, I swapped in a B series with an ITR tranny with factory LSD. I had the speedo and odometer checked again and it was exactly the same as my D series tranny.

So in summary, assuming that all B and D series manual tranny vehicle speed sensors are operating as designed, the only true fix is to use a tire size that has a total properly inflated external diameter of 23.5 inches to achieve 99-100% accuracy. Here is a rundown of some of the popular 92-00 Civic and 94-01 Integra tire sizes and their external diameters. These numbers were derived from Toyos website. There may be very small deviations in tire size dimensions from one tire manufacturer to the next. Smaller overall diameters mean more revolutions from point A to B, and vice versa for larger overall diameters. If all you do is race, then smaller is the way to go for most. But of you want accuracy for your street car, then follow this chart next time you decide to go with a new wheel and tire size:

205/40/16..........22.5 inches..........70mph=60.9mph.............87% accuracy
175/70/13..........22.7 inches..........70mph=62.3mph.............89% accuracy
185/60/14..........22.8 inches..........70mph=63.7mph.............91% accuracy
195/50/15..........22.8 inches..........70mph=63.7mph.............91% accuracy
205/50/15..........23.1 inches..........70mph=65.8mph.............94% accuracy
205/45/16..........23.3 inches..........70mph=66.9mph.............95.5% accuracy
195/55/15..........23.4 inches..........70mph=67.9mph.............97% accuracy
205/40/17..........23.5 inches..........70mph=69.3mph.............99% accuracy
225/50/15..........23.9 inches..........70mph=74.2mph.............94% accuracy

Dogginator

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Toy Civic &raquo;</TD></TR><TR><TD CLASS="quote">The only way to verify is by taking the car to a speedo calibration shop.</TD></TR></TABLE>

Or just use a handheld GPS to monitor speed and distance.

Toy Civic

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Dogginator &raquo;</TD></TR><TR><TD CLASS="quote">

Or just use a handheld GPS to monitor speed and distance.</TD></TR></TABLE>

Good point!!! I don't know how legal that is. The only legal way I know is the speedo shop.

WinningNinja

http://www.miata.net/garage/tirecalc.html

shiux

so with my GSR blades, on 45 wall tires, my speedo is going to be 10mph off?

Toy Civic

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by shiux &raquo;</TD></TR><TR><TD CLASS="quote">so with my GSR blades, on 45 wall tires, my speedo is going to be 10mph off? </TD></TR></TABLE>

Quite possibly. If so, the speedo will read 10 mph faster than actual speed.

nsxtasy

First of all, all speed sensor units do NOT function the same. So just because the speedometer on your '95 Civic is off by 9 percent with the stock size tires, does not mean that anyone else's speedometer is going to be off by 9 percent, even if it's the same year and model car.

As for tire sizes, it's easy to calculate differences in the sizes that are labeled on the side of the tire. For example, if your stock tire size is 185/60-14, and your speedometer reads 9 percent higher than your actual speed, if you switch to 205/40-17, that size is 3.2 percent larger. So all things being equal, your speedometer will then read 5.8 percent high instead of 9 percent.

But there are lots more causes of speedometer error. Some tires are larger than others, even when they are marked with the same size on the sidewall. Tire manufacturers have some latitude in marking their sizes, so they can only be considered approximate. This alone can account for differences of 3-5 percent. Even treadwear can result in speedometer error; for example, the difference in diameter between a tire that is brand new and one that has worn down to the treadwear indicator bars (2/32" tread depth) is about 2 percent, and that goes into speedometer error as well.

The bottom line is this: Anyone who is interested in verifying the accuracy of his speedometer can, and should, do so (such as by using a GPS to measure speed), but even then, it is only an approximation that can change over time.

We have a '94 Integra GS-R, an '01 Integra Type R, and a '91 NSX. With tires labeled as the stock size, the speedometers on all three are accurate to within 1-2 percent.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by shiux &raquo;</TD></TR><TR><TD CLASS="quote">so with my GSR blades, on 45 wall tires, my speedo is going to be 10mph off? </TD></TR></TABLE>

Well, you don't mention what size your tires are - 195/45-15? 205/45-16? etc - so it's impossible to answer your question. But if you provide your tire size, it's easy enough to calculate the difference that your speedometer will register due to the difference in the size of the tire labeled on its sidewall. The percentage difference in its calculated outer diameter will be added (multiplied, actually) to whatever percentage difference it may have been off with stock sized tires. This is aside from all those other factors that affect speedometer accuracy, such as tire tread depth, differences in actual tire sizes for specific makes/models/sizes of tire, and any inaccuracy inherent in your car's VSS/speedometer system.

lmike6453

good info guys..never thought about this

forumnoobie

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Toy Civic &raquo;</TD></TR><TR><TD CLASS="quote">

Quite possibly. If so, the speedo will read 10 mph faster than actual speed. </TD></TR></TABLE>

I have a stock gsr and my speedo is a good 5-7 mph faster than actual speed. Just to cheap and lazy to get it calibrated. Good read btw.

nsxtasy

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by forumnoobie &raquo;</TD></TR><TR><TD CLASS="quote">I have a stock gsr and my speedo is a good 5-7 mph faster than actual speed.</TD></TR></TABLE>

I have a stock GS-R, too, and mine is within 1-2 mph of actual. Which I guess just supports the need for calibration if yours is off.

94rs-turbo

iTrader: (6)

I took my needle off and put it back on so my 205 50 15's read perfect