How do you figure MPG with oversized tires?

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FL-RAM

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Just curious how you figure your MPGs if you have bigger tires? These come with 31" tires, and I'm running 33" tires. Comparing the speedo with a gps unit, my actual speed is usually 1.5 - 3mph faster than the speedo display.

My simple mind tells me that if I put one gallon of gas in and drive for 20 miles on the odometer and it dies...the OBC will say I got 20mpg, but in reality I will have actually traveled something around 21-22miles. So I assume my MPG display will always be a couple MPGs low.

Just wodndering if there was a simple way of calculating it without getting too scientific or having the speedo recalibrated.
 

odinjunior

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Use the mileage from your GPS and hand calculate it. GPS must stay plugged into record all miles and don't reset the trip mileage.
 
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FL-RAM

FL-RAM

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Just did a long trip (Pensacola to Savannah). I filled up when I left then hit the interstate for the 7+ hour drive. The OBC said I was getting 20 mpg (averaged 72-73mph). Stopped and got some gas. OBC said I traveled 392 miles, and the GPS said 412 miles. That's what I figured on 33" tires.

Filled up the tank and put in 20.5 gallons. 412/20.5 = 20.1mpg...the same as the OBC. That threw me for a loop. The OBC should have read 19.1mpg (392/20.5 = 19.1mpg).
 

BlackGold6.4

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Don't use GPS's mph, use its miles driven and follow odinjunior's advice above. Or find a friend that has AlphaOBD software and change the tire size in the PCM
@FL-RAM as said in quote above, it's easy to get the dealer to change it ... that is what I did but you have to have a pre-programmed size in the PCM that the correct OBD reader can see. When I put on 35" tires the dealer could not make that adjustment so I bought a AEV Procal and with that you can change the tire size any time you need too.
 

John Jensen

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I'm not familiar with AEV Procal but isn't around 200.00? AlphaOBD is a lot cheaper and can do a lot more
 
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FL-RAM

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@FL-RAM as said in quote above, it's easy to get the dealer to change it ... that is what I did but you have to have a pre-programmed size in the PCM that the correct OBD reader can see. When I put on 35" tires the dealer could not make that adjustment so I bought a AEV Procal and with that you can change the tire size any time you need too.
I just thought it was curious that the OBC said mpg was 20, when mathematically it should have been 19. The odometer said I traveled 392 miles, but actual mileage was 412. So I know the odometer and speedometer are off by about 5%...but the displayed MPGs were right on the money.
 

BlackGold6.4

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I just thought it was curious that the OBC said mpg was 20, when mathematically it should have been 19. The odometer said I traveled 392 miles, but actual mileage was 412. So I know the odometer and speedometer are off by about 5%...but the displayed MPGs were right on the money.
I use the read out on my dash as a guideline sorta.. when I want accurate readings I use what @SniperDroid said but the issue is you need an accurate way to get mileage traveled divided by the receipt of fuel spent on said mileage.
 

MikoS

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Just curious how you figure your MPGs if you have bigger tires? These come with 31" tires, and I'm running 33" tires. Comparing the speedo with a gps unit, my actual speed is usually 1.5 - 3mph faster than the speedo display.

My simple mind tells me that if I put one gallon of gas in and drive for 20 miles on the odometer and it dies...the OBC will say I got 20mpg, but in reality I will have actually traveled something around 21-22miles. So I assume my MPG display will always be a couple MPGs low.

Just wodndering if there was a simple way of calculating it without getting too scientific or having the speedo recalibrated.
As John Jensen says above, If you have AlphaOBD or access to someone who has, it is really simple to change tire size. You can even pick the tire make and spec eg Goodyear Wrangler HP P265/70R17 or whatever tires you have. (not that the computer needs any more than the diameter of course... yet it can be done) This would correct your speedo as well as the MPG shown on the panel. My truck came with different tires to what the computer was figuring on, so my shown MPG actually showed worse after resetting it...:( but at least it is spot on accurate now.
I can give the steps through AlphaOBD if needed...
Edit. I just had a quick look at the steps on AlphaOBD and I see that you can enter the tire circumference too if that's what you prefer.
 
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Daw14

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I used to use the mile markers on the side of the interstate as a mileage gauge . Now gps on the phone handles it .
 

Dinky

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I just thought it was curious that the OBC said mpg was 20, when mathematically it should have been 19. The odometer said I traveled 392 miles, but actual mileage was 412. So I know the odometer and speedometer are off by about 5%...but the displayed MPGs were right on the money.

The vehicle displays is always off. I've never had one vehicle for it to be a 100% right even in stock form.
 
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FL-RAM

FL-RAM

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The vehicle displays is always off. I've never had one vehicle for it to be a 100% right even in stock form.
Yeah, I'm not going to obsess about it. I figure it's a ballpark number no matter what...close enough for jazz.
 

Dinky

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Yeah, I'm not going to obsess about it. I figure it's a ballpark number no matter what...close enough for jazz.

I really don't worry about fuel economy. I do if it's seems I sprung a leak in the tank bad. If the gauge say it's remotely decent I forget about it. With gas prices I prefer my wife not to be right a out my decision making skills lmao.
 
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