Yep. Even within one manufacturer's line, two tires of different types but of the same P-metric size may vary in actual dimensions.

 

Interesting. How do you judge operating temp, middle of the stock temp gauge? (~58C)

 

I'll try that method tomorrow when I travel to San Antonio (weather permitting). If I'm able to travel in the FD, I'll post the results tomorrow PM. San Antonio is approx 144 miles from Corpus.

Thanks.

 

Affirmative. At cold (a.m.) start-up the engine idles like it has a high lift cam . It takes about 3 minutes to reach smooth, steady idle (750 rpm).

 

I think that in order for the new Dunlops to have a positive enhancement in fuel consumption (additional 5.4 mpgs), they would have to be much taller than the Yokos that were replaced. Taller tires will effect final gear ratios by lowering rpm per mph ratios. Thus the engine doesn't work as hard to accomplish the same mph. Assuming driving habits remain unchanged.

I'm still cruising 80mph at 3K just as before when I had the Yokos.

 

Try testing with a GPS unit. Buy and return one after using it.

 

Why, may I ask, would I need a GPS unit to obtain MPG data??

 

it will accurately measure how far you've gone. you can also reference it with your speedo and odometer to see if they are off.

 

Agreed. The method Jim describes here is more accurate than the method you were using. And it is easier, you can fill the car up 100% every time, and it doesn't matter when you fill it up (1/4 tank, empty, half tank).

-Max

 

Unfortunately, you're overlooking one important aspect: your speedometer reads off the transmission, so it's always going to read 80 mph at 3000 rpm no matter what size tires you have. The speed you're actually traveling will be different, however.

 

^^good point, i forgot about that

 

Time to get a Valentine!

 

Two points:

1) Different tire types and brands can have significantly different rolling resistances, even at the same pressure. Obviously, lower rolling resistance means better mileage.

2) Different tires will have different effective radii, giving a different number of rotations per mile. A tire with more rev/mi will give better fuel economy on two counts - lower actual speed at a given speedo reading, and less actual miles traveled compared to the odometer reading.

I would suspect these tire factors have much more effect on your mileage than the type of cat, especially since the cat is really restrictive only at high flow rates, when you are producing more HP (say above 100), and you only need 75 or so to accomplish most non-spirited driving.

 

According to your explanation, [MPH to RPM] readings will not change even if rear-end gears are swapped to a lower/higher ratio?? EXAMPLE: If my car is equipped with 4.55 ring and pinion, it would still mark 3K at 80 mph??

 

Weather was a little foggy this a.m. so I decided to scratch the San Antonio trip. I did take her out for 107.2 mile round trip cruise. Temp was 52 degrees.

I fuel'd up as Jimlab recommended and zeroed out the trip odometer at the gas station. Got on the highway and maintained a speed of 72 mph (2600 rpm). Had a few passes at 90 and 115mph. Turned around and headed back to Corpus.

Stopped at the gas station and refueled in same manner before the trip was initiated. When the pump handle automatically stopped the pump read 4.425 gallons.

That comes out to 24.2 MPGs. A little dissapointed I didn't cross the 25mpg mark. It may be time to change the plugs and the fuel filter

 

Yes, because it reads off a gear in the transmission, not the differential.
The different sizing between each manufacturer despite the "same size designation" also affects the rolling resistance, including the actual rotational weight of the tire between brands.