I'm leaving Eagan MN for Duluth MN to go visit my friends up at UMD for the weekend. It's about 168 miles one way, 159 of which are on I35 Northbound. The speed limit varies anywhere from 50-70, the most of it is 70 though. Last time I did this trip in October, I got 21.8mpg at about 75-80 mph. I'm gonna try and stay within 70-75 with the cruise control on, and I'm gonna shoot for 25mpg. I dont think thats unreasonable. I've heard of people getting 29mpg in these 91-93 deVilles (mines a '92). The oil and coolant are looking good, and the front tires are at 36 PSI, and the rears are at 32.

Do you think it's possible?

Very much so!
I got (as read on FDC) 29.7 from Beloit, WI to Grover, MO (360 miles or so) running 70 mph. 70 seemed to get best mileage. My 91 REALLY was affected by winds more than my 94 LT1 FWB. If you are into a headwind, you may want to slow some (as long as you aren't bottlenecking traffic) to see how it does.

Calc was around 27.9 or so. All time high? 31.x on FDC. All time low with good injectors? 16.x. With bad injectors? 12 highway....

Colder weather I got worse, warmer, better. A "trick" for better mpg is to allow the engine to suck up hot underhood air. Not always best for power, but helps mileage some.

may as well bring the rear tires up to 36 PSI also.

around 70mph is the best speed for these cars.

I got as much as 28mpg on an extended highway trip, AC on...

normally it is around 25

lately it is 22mpg... but I have a lead foot... if I could get my 25 back, I'd be happy.

Alright, so I got either 23.4 MPG or 24.3 MPG. I can't remember but I wrote it down on a piece of paper in my car. My speedometer read 72MPH, but the digital speedo in the diagnostic mode read 75-76 which is the accurate one. When I went up to duluth the first time in october, the speedo said I was doing 75-77, so in reality, I was running 81-83 MPH and I averaged 21.3 MPG.

On the way back, I'm gonna set the cruise control to 70 MPH on the digital speedo in the diagnostic mode. I'm gonna shoot for 25-26 MPG.

That will be astounding to get 26MPG in a car of this size with a 4.9 V8, a huge interior, and all that luggage space!

If your speedo is that far off, then there is mpg calc error. You should do some mile marker testing to get a relaively accurate speed based on time. I use several 1 mile, and some 10 mile times to get a good accurate average... Then if the speedo (as read, not the PCM reading) is off, you need to put in an offset multiplier on the mileage (as it is driven off the speedo). Like my Suburban, it came with 245 75 16's and I got it with 265 75 16's. I had to multiply by 1.05 my miles on the trip odo to get correct readings. As for it now, I had the PCM tweaked now and it is dead on correct now.

I can't see how the PCM is reading off than the speedo. I see some delays in readings, but not actual errors. Are you sure there isn't something else going on?

I'm confused :hmm:

Interesting to note...

Before I filled up, I calculated my mileage and found out I was getting 17MPG in the city :bonkers:
Last week I got 13.8!!

Interesting to note...

Before I filled up, I calculated my mileage and found out I was getting 17MPG in the city :bonkers:
Last week I got 13.8!!

That seems about right.

the FDC isn't accurate....

fill up, reset trip odoment, drive alot. Fill up again, note gallons used to fill, divide miles on trip odometer to gallons used. Thats your true mileage... of corse if the speedo is off due to tire seize etc.. then the mileage will be off as well.

I dont get why the speedo would be off due to the tire size. I have original rims and tires on the car, granted the tires are almost bald, could that make a difference?

How can I fix the speedo problem?

For the 17~ MPG I got, I filled it up one day, drove like 45 miles around town, and then filled it up the next day. I think i put in like 2.6 gallons or something. 45/2.6= ~17.

My MPG would decrease the further I drove it?

I remember I got 17MPG for a week last summer, I was thrilled :D

Like whenever I fill up, I count the mileage I drove that week, and divide it by the total number of gallons I used that week. I usually average 13-14 MPG in winter, and 15-16 in summer.

I'm not certain it's only the tire size that affects your digital readout of MPG. All sorts of variables come into play- are your tires over-inflated or under-inflated? How much weight is in the vehicle? Are you using a "Top Tier" brand of gas? Obviously, stop and go city traffic vs. long stretches of highway traffic affect MPG, but what about the terrain- do you have long stretches of uphill grades?

Personally, I tend to use the digital readout of my MPG as an advisor, not as a perfectly reliable indicator of the true miles per gallon I'm actually getting. The only way you're going to know exactly what you're getting is to reset your trip odometer at every fill-up, and divide the actual miles you've driven at the next fill up by the gallons of gas you filled up. And I tend to not do "top offs," but rather to go as far as I can on full tank.

Slightly off topic, but a bit of a curious thing I noted is two cars- even two of the same model car- can be off just slightly on how they are calibrated to read distance, and therefore speed. Last summer, I was in a caravan with some fellow Corvette owners going to Bowling Green, Kentucky. While traveling through Kansas, we all set our cruise controls at 7 MPH over the speed limit (Kansas State Patrol is merciless to anyone doing 10 or more over the limit). Well, the curious thing about this is that although our speedometers all said we were doing the same speed, some people were gaining on the car in front of them while others were drifting back- sort of a faction of a MPH increment effect.

And that fractional increment can also affect what you're computer readout of MPG is, I suspect.

45 miles is on the low side when calculating fuel MPG.... I personally say at least 100 miles... that is, if you are trying to find highway MPG, fill up right before you hit the highway, then set cruise and drive for atleast 100 miles then fill up as soon as you get off.... the more miles you drive the better (more accurate) the results.