So I've noticed this for a while now and I'm not sure how it "works".

I've got a stock 97 R and I think the speedo is off. I've done all the gear ratio calcs and read threads on engine RPM and they all say at 4500 RPM in 5th gear I should be at 80 MPH. However, my speedo tells me otherwise (reads at about 83). When I tested this with a "your speed is" radar thing, I drove past one at speedo 28 and the radar told me I was going 25. My RPM in 3rd gear was just under 2600 (where it should be for 25 MPH).

So here are my questions...

- I think my speedo is reading high, is there any way to fix it?
- If it's supposed to do that from factory to keep the driver from speeding by making them think they're going faster than they actually are, would the odometer reading be skewed as well? (I've read that in newer cars with electronic displays, the first part is the case but the manufacturer programs the odo to read correctly.)

If it helps, I'm on RE-11s mounted on stock 15"s.

Oh, and my speed reading at 0 MPH is also 0 on the speedo...

Curious note: would that fact that the speed has a gap between 0 and what looks to be 10 MPH have anything to do with this?

 

Might as well ask this easy question first, what size are your tires?

There is a gap between 0-10mph because when you drive 1-9 mph as well

 

everytime i go past those speed rador things my speed is usually 3 less than they say too

 

Use a GPS to verify the speed. Use a tachometer (OBDII reader) to verify the RPM. The gauge needles can be adjusted.

 

Tires are 205/50-15.

 

The spedometer vs. tachometer reading is independent of tire size as the VSS reads the differential speed.

 

Wasn't there a thread not too long ago stating that all caRs are built to say they're going faster than they actually are so that no-one could blame Honda for a speeding ticket or to keep people from going too fast?

 

I do remember when I got a set of 205/50/15's my speedo would read 79-80ish when my actual speed was 75. The 205's were just a bit shorter than the 195's allowing the the wheel to turn a bit faster at a given speed. Hope that helps.

 

Okay, and what about the odo? Is my current mileage of 70,5xx higher than it should be? I mean, if it is, It wouldn't be hard to calculate the real mileage since I have the exact date and mileage I put the RE-11s on so all I'd have to do is a simple math equation to correct for the higher reading (assuming the higher reading also affects the 'mileage').

 

The higher reading affects the odometer as well. Most cars have speedometers that display higher speeds than actual, probably for the same reason that Coke 2-liter bottles have more than 2.00L of Coke. Opening up your cluster and rolling it back is odometer fraud regardless of your tire size.

Gear ratio calculators spit out a vehicle speed assuming a specific tire rollout (among others). Not all 205/50-15 tires have identical dimensions. Not to mention that your tach is far from accurate as well, which makes any conclusions you wish to draw well, moot.

Also I'd say that your speed isn't mph+3, but probably something closer to mph*1.05 or so.

 

One thing I learned a few years back is that most people don't know that the 205 measurement isn't actually the tread-width on the tire, but it's the maximum width of the tire as it baloons out on the sidewall apparently.

This fact alone explains why tires have such variations from model to model and brand to brand.

Steve

 

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
195/55-15 4.2in 11.7in 23.4in 73.7in 860 0.0%
205/50-15 4.0in 11.5in 23.1in 72.5in 874 -1.6%

205/50-15 = 1.6% too fast. When your speedo reads 60mph, you're actually travelling 59.0mph.

 

I know, but it'd just be for reference to write down in a log book or when I ever sell the car and want to give the REAL mileage (not that this car is ever leaving my possession anyway).

Thanks for the help guys

 

When I took off my OEM wheels with dunlop direzza's and put on my 15 inch kosei's with re-11's I noticed there was a height difference with the tires. I'm not saying the direzza's were worn down because I had one that was practically brand new with less than 100 miles on it before I put on my kosei's. I think it would probably be the same way with an OEM wheel.

 

Car and Driver covered this. Speedometers typically read high, odometers are precise.

http://www.caranddriver.com/features...andal!-feature

There was another article there that covered the odometers as well. They had an Accord in it IIRC.