That's pretty close though: 30mph with the 19515R15's would actually become 29.86mph.

 

I've got an '04 with 14" tires.
How do you find out what the speedometer/odometer is calibrated to? GPS, or some other source?

 

I've seen highway signs indicating start and finish of a measured distance. Not recently though, in our area. Hmm, I wonder if Google directions (with distances) would be accurate enough for this? The greater the distance the better, I suppose.

 

Yeah, we have mileage markers on the highways here too. I was just wondering if there was some other data source out there which is specific for the car model, or the calibration is specific to each individual car. I'm thinking it varies a little, depending on the individual car.

 

Great work .. really informative .. and thanks a lot for sharing ..




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I recently purchased the Fuelmax for my HCH II '06. Overall I am pleased with them. Most importantly to this group - my mileage did fall off a bit. I was getting 60 mpg or better (warm weather) with what was left of the original factory Dunlops at 65K. They had uneven ware all around because the rear control arm issue wasn't fixed until after 40k miles were on the tires. Anyway-I can squeeze about 55 mpg out of the new tires with my usual trips and less if I do steeper hill climbing. I am running with about 43 psi front and 41 psi rear. Same as with the original tires. I would give these tires a thumbs up for wet and dry traction. I've "tested" the grip in cornering and in torrential downpours and never felt loss of control. I am very pleased with the noise level drop off with the new tires. I couldn't easily talk using the bluetooth for the cell phone before. The typical noise now is like a wind sound not a roar and even rough roads and bridge joints are tamed.
I would give these tires a thumbs up over all and hope the mileage will improve with wear.
Dave

 

I have the GY Triple Treads on my HHR and have been very happy with them for 20K and 2 winters. Go good in the snow and look like brand new. Rated for 80K on wear so we'll see.

 

Thanks for that Dave. I think even if you replaced with OEM you'd see a drop, due to deeper tread, newer and more pliant rubber and slightly larger outer diameter.

The latter doesn't change your mileage, but changes your car's perception of it: less revolution to go a given distance means your car doesn't think you've gone as far.

In AutoCAD, I created a circle with a diameter of 25" (new stock tire OD), and then offset it by 6/32" (typical wear from new to "wear bars"). Listing statistics yielded:

New OD: 78.5398"
Worn OD: 77.3617"

With a spreadsheet I established the worn OD to be 98.5% of the new OD. I think that equates directly to the car's perceived mileage drop, ie: going the same distance as before the tire swap, the car thinks you've only travelled 98.5% of the distance. Ok, that factor alone would take you from 60mpg to 59.1mpg.

 

I have evidence of the cars computer being off more with the new tires. I calculate my mpg every tank and since the new tires the difference has increased by about 2 mpg.

 

Yes, there are various factors dropping your mileage, increase in tire diameter just one factor.