How to compensate my MPG calculation for "taller" tires?

[jrlnc]

Ok guys, I'm appealing to those folks here with the technical knowledge . . .

I have the '07 Fit Sport. I recently replaced my tires (those crappy Dunlops) with some really nice Michelins (Primacy MXV4s) from Costco.

The new tires are: 195/60HR15 88H

The problem is, when I bought the tires they did not have an exact size match for the old tires. I agreed to get the "taller" tires and am pretty happy with them. I run mine at 38 psi and the others were definitely firm (i.e. harsh) riding and very noisy. The new ones at the higher pressure are dead quiet in comparison and ride very smoothly and comfortably.

Problem is, I noticed a mileage drop on the first 2 tanks on the new tires and thought it was an aberration because of higher %age city driving and constant a/c use.

One day I was using my GPS and noticed that it was reading a higher speed than my speedometer. Very consistently when the speedometer is locked at 70 mph the GPS reads 73 mph. That jibes with what I've been finding because I seem to be passing everyone now whereas before I being passed more.

I know that if your speedometer is reading low, then the tripmeter and odometer are also reading low. (btw - The Honda dealer said they definitely don't recalibrate speedometers for tires.)

In calculating MPG, I have now been multiplying miles traveled on the tripmeter by 73/70 and then dividing by gallons put in at the pump.

MPG = Miles traveled (tripmeter) / Gallons at pump X (73/70)

Does this seem correct? Does anyone know of a different way to adjust the MPG number?

[feddup]

On a sort of related topic how many RPM are you running at say 70MPH. In another thread I was contemplating running something like 195/60R15 in hopes of achieving slightly lower RPM on the Highway with an M/T Fit. I knew the speedo would be off but had forgotten that MPG calculations, of course, would be off as well.

[jrlnc]

Well the Fit Tachometer is not very precise, but I can tell you this.

With the original tires, at 69 mph my tachometer was just under 3400 (v-tech kick-in) at about 3380 rpms or so.

Now with the new tires when the GPS says 69 mph, the tachometer is between 3100-3200 rpms.

That is a nice drop that (I assume) reduces fuel consumption and it does quiet the engine a bit.

[feddup]

Excellent, that's about the decrease I was looking for. It'll be a while before I need tires but that drop in highway revs is about perfect for my taste. Sorry if I hijacked your thread!

[Sid 6.7]

There is a difference of 3.19% in the rolling diameter of the new tires from the old. So multiply your trip mileage by 1.0319 to get your actual mileage traveled.

[jrlnc]

Originally Posted by feddup

Excellent, that's about the decrease I was looking for. It'll be a while before I need tires but that drop in highway revs is about perfect for my taste. Sorry if I hijacked your thread!

No problem. It's all related and is a side benefit to the taller tires.

[jrlnc]

Originally Posted by Sid 6.7

There is a difference of 3.19% in the rolling diameter of the new tires from the old. So multiply your trip mileage by 1.0319 to get your actual mileage traveled.

Thanks!!

I knew there had to be some math/engineering wizards out there.


After plugging your adjustment into my spreadsheet, I show that my last tank was 37.947 MPG and my average since the new tires is 36.729 MPG.

I hope that it improves, and I think it will since I'm now adjusting my speed for the speedometer error and getting lower rpms on the highway.

[feddup]

Thanks again to jrlnc and sid 6.7 . I started a thread going for this exact sort of thing and it tanked. This is exactly the effect I hope to achieve for the sake of MPG, my ears and wear and tear on the car.

[SSM0423]

there's a website that calculates the difference in ration and speed. I think it was miata.net? tire calculator. I think the difference was 6.2%???
I too have 1956015 - running Goodyear Tripletreads as it rains alot where I work. ride is smoother too!

[Shaggs2Dope]

I started one in the 1st gen forum and people were hating. Thanks for the positive affirmation of the info I was trying to gather.

heres the other thread with lots of % ratios
Anyone running OVERSIZED tires