In-Dash MPG question..

[RioRed4v]

Since swapping over to my 20" Roush setup i've obviously noticed a drop in MPG's. On average, it seems to be about 1mpg.

Question though is how accurate that is. I don't have a tune so I haven't been able to adjust the speedo readout.

Using a tire size calc I get:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
255/40-19 4.0in 13.5in 27.0in 84.9in 746 0.0%
275/40-20 4.3in 14.3in 28.7in 90.0in 704 6.0%

Also, is this making a substantial difference in my odometer reading?

 

[03Cobra1979]

Since swapping over to my 20" Roush setup i've obviously noticed a drop in MPG's. On average, it seems to be about 1mpg.

Question though is how accurate that is. I don't have a tune so I haven't been able to adjust the speedo readout.

Using a tire size calc I get:

​

Also, is this making a substantial difference in my odometer reading?

Mine isn't all that accurate. It usually reads about 22-23mph on average and I actually get 25 - 26 hand calculated.

You can change your tire size in your tune so your speedo is reading correctly too.

 

[RioRed4v]

I dont have a tune.

But regarding the math, if I do it by hand it should be correct, right? It would only be the dash readout that is incorrect..?

 

[03Cobra1979]

I dont have a tune.

But regarding the math, if I do it by hand it should be correct, right? It would only be the dash readout that is incorrect..?

Yep, providing you have the math skills of a 2nd grader at a minimum you will always be more reliable than the dash is. Most people call the MPG read out a "Liar Gauge". It is off by like 4 - 5mpg in my diesel truck.

 

[Laztug]

my dash gauge is pretty accurate.

 

[faztcobra]

my dash gauge is pretty accurate.

My dash guage is usually .5 to 1.5 below actual. I think only once has it ever been dead even.

 

[dirtyd88]

actually, the miles of actual travel will be slightly higher than what your car is reading, since it still thinks you are turning a 746 revs/mile tire. It won't amount to much though, maybe a couple .

And FWIW, my dash MPG readout is spot on to my hand calcs in my 2013 GT. It also depends on if you stop pumping when the pumps clicks or keep filling up after it clicks, on how much you are off on the hand calcs.

 

[RioRed4v]

actually, the miles of actual travel will be slightly higher than what your car is reading, since it still thinks you are turning a 746 revs/mile tire. It won't amount to much though, maybe a couple .

And FWIW, my dash MPG readout is spot on to my hand calcs in my 2013 GT. It also depends on if you stop pumping when the pumps clicks or keep filling up after it clicks, on how much you are off on the hand calcs.

My current revs/mile would be 704 with the 20's and the stock setup would be 746.

*edit* nvm, i think i see what you're saying now.

I'll play around with hand calculations to see how it compares.

 

[dirtyd88]

try this:

Fill car up until pumps self stops and reset trip odometer. After burning through that tank of gas, fill the car up again until the pumps stops itself. Then compare your hand calc MPGs with the dash readout.

Then on the 3rd fillup, fill the car up past the point of self stop and see how far off from the computer your hand calcs are with the add't fuel. Shouldn't be much, but worth a look.

 

[Mike50]

I used to calculate it by hand each time I filled up, and it would always end up higher than the readout on the dash (which I reset after filling up) by 1-3 MPG.

 

[Red Turtle]

Isn't the Roush/Cooper tire a 275/35/20?

 

[RMoore92]

I'm about 1-2 mpg higher then it reads when hand calculated.

 

[RioRed4v]

Isn't the Roush/Cooper tire a 275/35/20?

Actually, i think you're right:lol1:

So even less of a difference:

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
255/40-19 4.0in 13.5in 27.0in 84.9in 746 0.0%
275/35-20 3.8in 13.8in 27.6in 86.6in 731 2.0%​