[speedlever]

This old beater Civic I bought back in December 2015 has a few oddities. It began life as an automatic and was converted to a manual when the auto tranny died. Ergo, it has a different FD ratio than the standard manual tranny would have.

Manual: 4.06
Auto: 4.36
http://www.csgnetwork.com/rgpspeedcalc.html

Ergo, I would think my speed would be lower than indicated. Do I have that right?

But the previous owner put on 15 inch wheels/P195/60R15tires instead of the OEM 14s with P185/65 R14 rubber.
https://www.tacomaworld.com/tirecalc?tires=185-65r14-195-60r15

I've noticed that my speedometer is right on the money with my GPS. But when I check my GPS speed vs OBD2 speed (Torque Pro), there is typically a 2-3 mph difference in speed (OBD2 speed being less than GPS speed). And the difference seems related to speed... ie, the higher the speed, the greater the difference.

I recently noted that when I made a 45-50 mile one way trip (GPS distance), my indicated mileage was substantially lower. IE, a RT that should have been 96 or 97 GPS miles was short about 8 miles on the odometer.

All this means is that I'm likely getting much better mpgs than I thought!

OTOH, I'm puzzled why my speedometer seems accurate (compared to GPS) and yet my odometer seems way off.

Maybe using GPS distances is not an accurate way to check. And a measured 1 mile is too short to draw any conclusions.

Any suggestions how I might (cheaply) check my odometer for accuracy?

 

[OhSix]

This old beater Civic I bought back in December 2015 has a few oddities. It began life as an automatic and was converted to a manual when the auto tranny died. Ergo, it has a different FD ratio than the standard manual tranny would have.

Manual: 4.06
Auto: 4.36
Actual Speed Change From Ring Gear And Pinion Change Calculator

Ergo, I would think my speed would be lower than indicated. Do I have that right?

But the previous owner put on 15 inch wheels/P195/60R15tires instead of the OEM 14s with P185/65 R14 rubber.
https://www.tacomaworld.com/tirecalc?tires=185-65r14-195-60r15

I've noticed that my speedometer is right on the money with my GPS. But when I check my GPS speed vs OBD2 speed (Torque Pro), there is typically a 2-3 mph difference in speed (OBD2 speed being less than GPS speed). And the difference seems related to speed... ie, the higher the speed, the greater the difference.

I recently noted that when I made a 45-50 mile one way trip (GPS distance), my indicated mileage was substantially lower. IE, a RT that should have been 96 or 97 GPS miles was short about 8 miles on the odometer.

All this means is that I'm likely getting much better mpgs than I thought!

OTOH, I'm puzzled why my speedometer seems accurate (compared to GPS) and yet my odometer seems way off.

Maybe using GPS distances is not an accurate way to check. And a measured 1 mile is too short to draw any conclusions.

Any suggestions how I might (cheaply) check my odometer for accuracy?

Speed - whats the source of your GPS info?

 

[AG87BlkLX]

I guess you could drive your Pilot on a short trip to the store, school, mall, etc. & back home. Note the miles you traveled & go the exact route in the civic.

 

[Carsmak]

Or you could use the airbag loaner for a typical trip to work, then repeat with the civic, Zero the odometer in the drive way each time?

 

[Dnick]

I don't know Speed.... it seems to me that your Speedo & Odo should be in synch, no matter what; unless they are both driven separately, which I would doubt. Personally however, I really don't have a clue as to how these instruments are driven in newer cars.

But having said that, I would doubt your GPS vs. ODO measurement reliability before I'd be thinking ODO & Speedo don't match. Now the OBD reading might be an entirely different story if the source input is not the same as is used for Speedo/ODO.

I know I've not answered any of your questions, except to say you're looking in the right place (validate ODO).
One simple check to verify Speedo/ODO match might be to get out on a stretch of road where you can hold at 60MPH for a couple of miles or more, and time the distance covered; should be exactly 60 seconds per mile @ 60 MPH. If you can do this for 3 or 4 minutes, you should definitely see if there is much variation.
I have a real hard time imagining these two won't be in synch.

 

[speedlever]

Thanks for the good suggestions guys. My gps comes from my phone, so I assume that to be accurate. I'll try to run a comparison trip using the loaner when I have a moment.

 

[OhSix]

Thanks for the good suggestions guys. My gps comes from my phone, so I assume that to be accurate. I'll try to run a comparison trip using the loaner when I have a moment.

Suggestion:

Download the app "GPS Test" from the play store. With your android mounted at the top of the dash on the air vent, your phone should {mostly be able to} resolve a 3D fix with 10 to 20 foot resolution. At that level, you can trust ground speed data it reports.

Couple of screen shots of info available from the app. These captured sitting still in the driveway w/phone mounted in the air duct between radio and instrument cluster.

BTW: this is what the altitude/speed screen looks like when airborne.