Base tire size: 175/65r14
Sport tire sze: 195/55r15

Does anyone know if the Sport and base model trims have corrections in their cluster (or a different VSS) for their different tire sizes? The difference isn't large; the Sport tires are only 1.7% larger diameter, but it could make a difference if picking a non-stock tire size. For instance, 185/60r15 is only 1.3% larger than the Sport's factory size (less than 1mph @ 70mph) but if the VSS/cluster are calibrated for the base model's 175/65r14, they would be a full 3% larger, resulting in a speedo and odometer error of a bit over 2mph.

 

Interesting. I've read that speedos and odometers from many brands are not that accurate anyway, varying 2% either way.

 

We run Base size snow tires every winter on our '07 Sport. Never had an issue (daughter never complained about, or noticed any inaccuracy).


I've never heard about any automatic correction in a car's speedometer. How would it know if different size tires had been fitted? They don't incorporate a GPS...

 

There wouldn't be an automatic correction, because it wouldn't know. Rather, the VSS might be different in the base and sport trims, or the cluster in the sport might have a correction factor for the larger tires.

I checked the part numbers for VSS and gauge cluster on G1parts and it looks like they're the same, so the only other possibility I can think of is a software difference between the two, to give a 1.7% correction on the odometer. At 200k miles, that's 3,400 miles the car didn't actually drive.

I'm starting to suspect there isn't a difference between the two, though. Most 90's Civics didn't account for tire size differences and so some trims had less accurate speedos and odos than others.

 

Don't confuse speedometer accuracy with odometer accuracy.


Honda learned that years ago after a class-action lawsuit (had to extend warranties on several models...speedos were correct but odos recorded extra distance).






These days, manufacturers tend to assure that odometers are correct but speedometers read optimistic. (So that you can't say: 'But Officer, I thought I was only going 60"...)

 

Both would be offset by the same factor if you change tire size with no change in cluster or VSS. Which is not to say that either is accurate to begin with.

 

How would that work though? The speedo and the odo have the same data source.

 

Its a calibration/software thing.


Yes they use the same input, but the displayed OUTPUT can vary between the two 'displays'.

 

Ah I see what you're saying. It'd be nice if cars started letting you adjust that stuff so you don't need to break out an Arduino to change the calibration if you decide to change wheel sizes, but I could see them not doing it because they designed the car for the OEM size.