With $4/gal gas, I need more MPG's!!

OK, do the math dude. You said that tire size would not change the indicated mileage.

transmission in third gear, 1:1, with a 3:1 rear axle ratio and a 28" tire. At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire, or an indicated 58 mph. Change the tire size to 30". Now 2100 RPM transmission output speed (indicating the same speed and odometer rate to the driver) means 94.25" per rotation and an indicated 62.5 mph. Thus the odometer and speedometer will both be incorrect.

John Brown wrote: I have thought about it three times. A taller diameter tire will result in less rotations per mile, when coupled with an engine that has is more efficient at lower RPMS it may result in markedly greater MPGs even though you are adding a lot of unneeded rotational mass. From experience my Cobalt got better fuel mileage over 10K miles while running 205/65r15 tires when compared to the 195/60r15 OE tires in the range of 4mpg consistantly.

And he is right too, even if you corrected for it, you're changing the RPM range of the car.

It sounds like you need to run a GPS to get a perfect read on miles travelled. I use to run a vacuum gauge with best mileage on it to help with my driving habits. I run my tires at max pressure for best MPG.

has anyone else noticed the shift in car ads now saying that a car "has" so many MPGs. like its something they bolted on at the factory.

dunno, just really grinds my gears, like when clarkson says a car has x "torques"

Strizzo wrote: has anyone else noticed the shift in car ads now saying that a car "has" so many MPGs. like its something they bolted on at the factory. dunno, just really grinds my gears, like when clarkson says a car has x "torques"

Yes. yes. yes.

tuna55 wrote: OK, do the math dude. You said that tire size would not change the indicated mileage. transmission in third gear, 1:1, with a 3:1 rear axle ratio and a 28" tire. At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire, or an indicated 58 mph. Change the tire size to 30". Now 2100 RPM transmission output speed (indicating the same speed and odometer rate to the driver) means 94.25" per rotation and an indicated 62.5 mph. Thus the odometer and speedometer will both be incorrect.

In both cases, the indicated speed will be the same at the same 2100rpm. Real speed/distance travelled will vary.

njansenv wrote:

tuna55 wrote: indicating the same speed and odometer rate to the driver

In both cases, the indicated speed will be the same at the same 2100rpm. Real speed/distance travelled will vary.

Uhmm.. yeah. I wish someone had said that initially... oh wait

In reply to tuna55:

Re-read your 9:47 post. You said that tire size changes would change the RPM:indicated speed ratio(from 2100rpm:58mph, 2100:62.5mph). It doesn't.

njansenv wrote: In reply to tuna55: Re-read your 9:47 post. You said that tire size changes would change the RPM:indicated speed ratio(from 2100rpm:58mph, 2100:62.5mph). It doesn't.

Find where I said it and quote it back to me, please.

tuna55 wrote: OK, do the math dude. You said that tire size would not change the indicated mileage. transmission in third gear, 1:1, with a 3:1 rear axle ratio and a 28" tire. At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire, or an indicated 58 mph. Change the tire size to 30". Now 2100 RPM transmission output speed (indicating the same speed and odometer rate to the driver) means 94.25" per rotation and an indicated 62.5 mph. Thus the odometer and speedometer will both be incorrect.

leaner does not necessarily equal better mpg! Tune the mixture for best BSFC

You do realize that saying 'lean it out until it makes less power' is the same thing as saying BSFC except hundreds of dollars cheaper (monitoring), right?

You can buy a lot of gas with the money you'd spend getting the monitoring to determine BSFC.

They weigh a LOT. (Says the person using the front end off a 1990 Dakota)

No they dont (2g like op). My 96 ext cab 3.9/auto weighs 35XX lbs empty. Thats about the same as my Dodge Magnum, but ive only towed a front-loader on a 22-ft tandem axle trailer with one of those. The 3rd gen ones definitely put on some weight.

Tire size will not affect indicated mileage, since the speedometer is driven off the transmision.

That's just wrong.

pft...I'm only getting 40 with my BMW R1200 GS on this corn-syrup gas. My V-Rod is lucky to do 35

Most bikes get absolute crap for mileage.

So, roughly the same as my fullsize chevy that got 20 highway with the 350 and carb. Weird?

No, not really the same because a 2g dakota drives a LOT nicer than a fullsize chevy from the carb era, and if you compare a 20mpg 5.2 dakota to a 20mpg 5.7 chevy, stock for stock the 5.2 dakota will disembowel the chevy in acceleration. Ive driven a lot of 80s chevy trucks and if a stock one got 20 mpg i can say for a fact that was the best thing about it. They are worlds apart from a late 2g dakota.

I have thought about it three times. A taller diameter tire will result in less rotations per mile, when coupled with an engine that has is more efficient at lower RPMS it may result in markedly greater MPGs even though you are adding a lot of unneeded rotational mass. From experience my Cobalt got better fuel mileage over 10K miles while running 205/65r15 tires when compared to the 195/60r15 OE tires in the range of 4mpg consistantly.

This is probably true on a 3.9 dakota as well. Considering it can accelerate up a hill in 4th+lockup @ 1500 rpm, it could probably cruise 70 at light load at that rpm as well, but they are closer to 2100-2200 at that speed (with stock gearing).

At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire,

That is also just wrong considering the circumference of a 28" tire is closer to 90" than 8"

In reply to DeadSkunk:

I didn't mean the whole thing, man. I am trying to point out that I did not actually say that.

Vigo wrote:

At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire,

That is also just wrong considering the circumference of a 28" tire is closer to 90" than 8"

Sorry, typo, should be 88"

RE The lean thing, I promise that your car will get better mileage running 12:1 than at 9:1 A/F. I can get closer to the point on a dyno.

My truck weighs the same, that's what I was responding to. touchy?

tuna55 wrote: OK, do the math dude. You said that tire size would not change the indicated mileage. transmission in third gear, 1:1, with a 3:1 rear axle ratio and a 28" tire. At 2100 RPM transmission output speed, the car will be going 8" every rotation of the tire, or an indicated 58 mph. Change the tire size to 30". Now 2100 RPM transmission output speed (indicating the same speed and odometer rate to the driver) means 94.25" per rotation and an indicated 62.5 mph. Thus the odometer and speedometer will both be incorrect.

At the risk of being pedantic, see above. Typically:
Indicated = speedo readout
Actual = actual speed.

Iceracer is correct.

He said "indicated mileage".

100 rotations of the output shaft of the transmission = 100 rotations of the speedometer cable regardless of the tire size. Therefore, tire size does not effect "indicated mileage".

BTW- because of this, "indicated mileage" is virtually always incorrect, because of normal wear on the tires (tires change diameter, but the transmission is still linked to the speedo, with no consideration of the actual measured diameter of the tire).

Tire size effects "actual mileage". If you are using true measured distances instead of odometer indicated miles, tire size (of course) effects mpg.

However, it's irrelevant to the OP's question. He has already explained that he is mathematically correcting the 8% discrepancy he has in his indicated mileage.

So, he is (effectively) using measured mileage, not indicated mileage. Assuming his math is correct, he should be able to see the effect of tire sizes on his mpg.

disagree (obviously) but it's a moot point, and beyond that, completely irrelevant. I'll leave now.

Vigo wrote: This is probably true on a 3.9 dakota as well. Considering it can accelerate up a hill in 4th+lockup @ 1500 rpm, it could probably cruise 70 at light load at that rpm as well, but they are closer to 2100-2200 at that speed (with stock gearing).

I wish mine would instead of having to downshift and upshift 3 times to maintain decent speed and not lose, at least if not more, 25mph, bottom to top. Mine has absolutely no bottom end and by 4500 it's over up there. I am almost ready to dump the 3.9 lump for a 360. At least then I would have a reason for E36 M3ty mileage.

70mph cruise rpm is 2200, according to my craptastic factory tach, right now with my 27" tires and 3.55 gears. Even when I lost the speedo and there for OD and lockup, I had a 3000-3200 cruise rpm@65-ish and got 19mpg. Same exact setup as now.

Ranger50 wrote: I wish mine would instead of having to downshift and upshift 3 times to maintain decent speed and not lose, at least if not more, 25mph, bottom to top. Mine has absolutely no bottom end and by 4500 it's over up there. I am almost ready to dump the 3.9 lump for a 360. At least then I would have a reason for E36 M3ty mileage. 70mph cruise rpm is 2200, according to my craptastic factory tach, right now with my 27" tires and 3.55 gears. Even when I lost the speedo and there for OD and lockup, I had a 3000-3200 cruise rpm@65-ish and got 19mpg. Same exact setup as now.

I went from a 98 v6 Dak to an 01 Dak R/T and only lost 2MPG. The 3.9 is not real efficient. The extra fun via torque made up for the mileage.

But, I did dump the R/T for a Suzuki SX4 right before the gas shortage a few years ago. One of the very few times I timed something like that right.

tuna55 wrote: disagree (obviously) but it's a moot point, and beyond that, completely irrelevant. I'll leave now.

This isn't really an opinion point, it's factual simple math. There isn't any disagree, there is right and wrong.

I am willing to be incorrect, but I thought I had explained this pretty thoroughly. Have I made a mistake? I am perfectly willing to be corrected. Please do so if I have made a mistake.

Please explain otherwise if I have incorrectly summarized it.

If you run the car with the rear wheels off the ground, the speedo and odo will both work. If you set the cruise at 60 mph and run it for an hour that way, the indicated mileage will be 60 miles. Regardless of how much fuel you use or what tire sizes are on the car, the calculated mpg based on the indicated mileage will be grossly incorrect. The car will not have moved. The correct mpg based on the actual mileage will be 0 mpg.

I only have non constructive answers for you, as the only thing I can think of is the 3.9 kinda sucks.

So, MDS equipped 5.7 swap Cummins 4bt swap

Other than those two things I want to do to my slightly larger truck, I'm of no help.

Check out tips on ecomodder.com. It's an interesting place.

Grizz wrote: I only have non constructive answers for you, as the only thing I can think of is the 3.9 kinda sucks. So, MDS equipped 5.7 swap Cummins 4bt swap Other than those two things I want to do to my slightly larger truck, I'm of no help.

I have seen non-MDS 5.7 Hemi swaps into the older A/B/E bodies net 30 mpg with no real big hubbub.

And yes, the 3.9 sucks, much like the 3.7 which is the same design principle, chop two cylinders out of the V8 cousin.

I think the 3.9 is a superior motor to the 3.7 in the sense that they spent a bucket of money building the newer 3.7 which does nothing better than the 3.9 other than look good on paper.

IF your truck is having that much trouble you are probably getting bad mpg because you actually have a performance problem. Mine has 245k hard miles on it and its WAY down on power from when my dad got it (~30k) so now it doesnt spin all the way through 1st but it definitely doesnt have problems getting up hills. In fact, i keep meaning to loosen the kickdown cable so it will upshift earlier and not downshift so easily.