FDNewbie

iTrader: (10)

5th gear at 55mph...what is that, 2200 rpm?

adam c

iTrader: (2)

Bullshit!!

DS2000

26mpg is about what I'm getting - plenty of redline action too, so I guess I'm just lucky

widebody2

Jim I've posted that number before and was called for BS that time too. Here's a picture of the page. Its actually 37.2 at 56. If you want to call BS on Mazda thats fine...but not on me. I actually believe the statement...it goes right along with some of the figures that others have claimed...DamonB's claim is one that comes to mind.

jimlab

That's fine, but you do realize that Mazda stating it and what people are actually getting are two different things, right?

My car never got anywhere NEAR that, and the window sticker sure as **** didn't mention 30+ mpg. The best I ever saw with highway driving was about 21-22 mpg, and it got far worse in town. As far as I've seen over the years, that's pretty average.

While it's possible that some get higher 20s with all highway driving and no boost, that's not the norm, and I doubt you'd find anyone on this forum who knew how to calculate mpg properly and didn't have a modified gear set who would honestly claim 30+ mpg.

I suspect there's a lot of rounding going on, or a lot people flunked basic math.

FDNewbie

iTrader: (10)

I thought it was basic math, too...nothing complex involved. Fill her up, press in the trip odometer buttom, zeroing it, and drive until you reach 1/2 tank, 1/4 tank, or very close to empty (not a good idea). I usually drive it until 1/4 tank anyways.

Example (I'm making up the mileage):
-Trip odometer reads mileage as 212 @ 1/4 tank
- Gas tank capacity = 20.1, and using 3/4 of the tank gives you 15.075 gallons
- Divide mileage driven (212) by gallons used (15.075) = 14.06 miles per gallon.

Some ppl may forget to factor in the unused portion of gas, and instead of dividing by 15.075 gallons, they divide by 20.1 gallons (a complete tank). That would, of course, yield significantly lower gas economy (in this case bringing it down to 10.55 mpg). Am I missing something?

4rotors

I don't think anyone can get 30 mpg. I get good gas mileage on my 95 VR. I only have 28000 miles on it. I notice after I use the Racing Beat inlet duct and K@N airfilter with some royal purple I started getting better gas mileage. I was getting 21-23 now I get about 23-25. That is with driving under 4000 rpms.





4 Rotors

ROTARYFDTT

iTrader: (4)

To add yet another bit of information. I averaged a little over 20 MPG on my last trip of the year. However, this was traveling 80-90MPH nearly the entire way, including a few pulls. One flat out through 3rd, 4th and 5th. That’s also with 850 primaries.

jimlab

As usual.

First, your gas gauge is not an accurate indication of how much gas you've used or how much is left in the tank. Fill the tank some time, drive until you hit 1/2 tank, and then refill. Let me know if it's exactly 10 gallons.

Second, are your tires stock diameter? If they're taller (P275/40-17, for example), you're effectively reducing your gearing and you'll travel farther per each rotation than you would with stock tires. If you change your differential gearing (as I mentioned earlier), you're also going to get an inaccurate reading.

The easiest way to determine gas mileage is to fill the tank until the pump shuts off automatically (no topping off the tank), drive a known distance (i.e. 120 miles from point A to point B), and the refill the tank until the pump shuts off automatically. Divide the actual mileage by the actual amount of gas you pumped.

Use a calculator instead of rounding off and trying to do it in your head. Rounding can make a big difference.

120 miles / 4.19 gallons = 28.64 mpg
120 miles / 4 gallons = 30.0 mpg

If your tires are stock or near stock diameter, you can probably trust your odometer to be close enough. Reset the odometer at the beginning of your trip and divide the reading when you fill the tank again by the amount of gas pumped.

FDNewbie

iTrader: (10)

Jim, Jim, Jim...always got something smart to say, eh?

Yea I figured that, but just how inaccurate do you think it is? Your method is definitely very accurate however (using the amount necessary to refill).

Stock is 225/50/16, right? 50s are pretty darn tall... But I run 245/35/18s in the front, and 285/30/18s in the rear, so my tires are pretty thin...

Like I said, that's a real accurate way of doing it. But just how big of a difference do you think there is w/ my method vs. your method? More than 1 mpg? I'm gonna try both and see, just outta curiosity's sake. If anything, it'll tell me HOW inaccurate the fuel gauge is.

Talk about down to the tee...using exact distance vs. odometer. You're not leaving any room for error, huh?

jimlab

Quite a bit, and not even necessarily the same amount from car to car.

The sensor uses a plastic float on the end of a rod which floats on top of the fuel in the tank. It can tell you the relative (to its calibrated maximum and minimum range) level of the fuel in the tank reasonably accurately, but the position of the float does not correspond to any specific number of gallons of fuel.

The rear tire is all that matters, and you're pretty close to stock. P225/50-16s are a nominal 24.86" tall (78.09" circumference). P285/30-18s are a nominal 24.73" tall (77.69" circumference).

Why would you want to use inaccurate figures if you want to accurately calculate your mileage?

FDNewbie

iTrader: (10)

So Jim, would you happen to know why my tank never reads completely full? It's always just short of the full mark, vs. in other cars, I can get it to be directly on or even higher than the full mark... is it just not calibrated?

Also, is this the same system used on older cars? Cuz I know w/ my FB, the gas gauge fluctuates as you drive lol. You can start out at half tank, and in a few min it'll be up to full, or visa versa.

What forumla are you using to calculate tire height? Cuz I was thinking of going w/ 285/35s instead of the 30s.

I think it was just a matter of guesstimating just how bad the FD's fuel consumption is lol. "Poor," "Very Poor," or "Already Empty"

jimlab

Bingo.

Many car's gas gauges will read differently if the car is on an incline instead of level ground (example: if the car is pointed uphill, the gauge will read high). Some older gauges will even bounce as fuel sloshes in tank. Most newer gauges are now electronically "damped" to prevent this, but they may change rapidly to reflect actual fuel level after you start driving.

Tire width (mm) * (Aspect ratio / 100) * 2 / 25.4 mm/in. + Wheel diameter (in.)

285 mm * 0.35 * 2 / 25.4 mm/in. + 18 in. = 25.85" tall (nominal)
25.85" * 3.14159 = 81.21" circumference

widebody2

Jim haha yeah of course I'm not claiming anyone can actually get these types of gas mileage...I'm claiming that with a brand new car, new plugs, new everything, that while traveling at a constant rate of 56 miles/hour in 5th gear, no accelleration, no deceleration it is very believable that an rx7 can get 37.2 miles per gallon. I am talking about only the section that the car is going at a steady rate...for exaple something like what many of the newer cars have on them where they are giving a miles per gallon rate for that exact second. Mazda never claimed that you can get 30+ miles per gallon for high way driving. They claimed that while traveling at 75 and 56 miles an hour with a constant rate an rx7 gets 29.XX and 37.2 mpg respectively. So in a nut shell, if you want to get great gas mileage accellerate as slowly as possible and do 56 on the high way fun fun fun

jimlab

Well, I'm one of the few people on this forum who has owned a brand new FD, and I say it isn't even remotely possible. Maybe if you put the clutch in and just let the engine idle, but not when it's turning ~2,100 rpm maintaining 56 mph in 5th.

Highway speed limits are typically 60 mph in western Washington except for some stretches of I-5 and I-90. I did plenty of highway driving while the car was stock and even on a trip averaging mostly highway miles at or near the speed limit, I never once came close to averaging 30 mpg, let alone 30+. Even after the car was lightened substantially and my power steering and A/C were eliminated, I never once came anywhere near to averaging 30 mpg on a trip. Not even high 20s.

If you could truly get 30+ mpg just by cruising around 56 mph in 5th, you'd see a lot of people averaging better gas milage just by traveling at about 60 mph. But you don't.

oneflytrini

iTrader: (1)

30mpg is possible if you swap an Echo motor into it

sferrett

Note also that the article may be from a UK magazine (based on the comment in italics under the article) in which case you should bear in mind that the UK gallon is bigger than the US gallon, so the MPG numbers would be bigger.

1 UK Gallon = 4.55 Litres
1 US Gallon = 3.8 Litres

So 37MPG (UK) is around 30MPG (US) - which makes it seem much less miraculous. I could imagine you might get 30MPG for those periods of time when you were cruising under light throttle in top gear. To average 30+MPG over a whole tank of normal driving is a bit of a pipe dream, IMO.

Simon.
(trying not to be the 0.1%)

widebody2

Its not a european article...it said 17.xx mpg for urban...thats right around what mazda claimed for US cars. Jim unless you have installed some aftermarket digital gas gauge that tells you what gas mileage you are getting in that specific instant (which wouldn't surprise me with some of the stuff you do) you would have no way of measuring your instantaneous gas mileage. This isn't something you can figure out by driving your car to a highway then cruising at 56 mpg...how did your car get to 56 from 0...you had to accellerate. Is your house on a highway? how'd you get there? You drove there at an unconstant rate. I also highly doubt that you've driven anywhere near 56 mph for an extensively long trip in a brand new rx7. Yeah the speed limits here on long isalnd are 55 on all the high ways. If someone is actually cruising at 55 on the highway they are a hazard.

jimlab

"Instantaneous" gas mileage is not only irrelevant, it's ridiculous. Average gas mileage is what matters, and that's fuel consumption over a given distance. Not a snapshot of one moment in time during the trip.

Doesn't matter. If you could obtain 30+ mpg at any point simply by traveling at 56 mph in 5th gear without accelerating or decelerating, travelling at or near that speed for extended periods of time would raise your average gas mileage significantly. Travel in that manner long enough and the average would be near whatever the "instantaneous" gas mileage was at 56 mph, or whatever speed you were traveling at the longest.

Doesn't matter, and being a smart *** won't help. See above.

Big deal. You just claimed 5 posts ago that the article claimed 29.xx mpg at 75 mph, and I've obviously logged a lot of miles at less than that speed, yet never approached an average in the high 20s.

Anyone on this forum can verify this. Follow my instructions for filling your tank without topping off, and take a highway trip anywhere without boosting, cruising at 56-75 mph as legal. Refill the tank in the same manner, then divide the mileage you covered by the amount of fuel you replaced. I guaran-*******-tee that you won't have anyone with a 30+ mpg average if they've done their math right, and I doubt you'll have anyone average in the high 20s. More people will be closer to 20-22 mpg, I'll bet.

FDNewbie

iTrader: (10)

Can we also factor in bad gas quality? I know for a FACT that gas in my area (Northern VA) sux...regardless of the gas station. Don't ask me why. But when I go fill up in Manassas (a good 30 miles or so south, in the actual "South", I get MUCH better gas mileage.

widebody2

So those numbers from mazda are completely made up? Pulled out of thin air? They claimed 17 for urban...would you say thats pretty close? Driving in a regular fashion I'd say that was pretty close. Most of the people who posted pretty much confirmed that number. I personally get about 10 mpg. My car only sees boost and only sees action once or twice a week. I also saw quite a few posts with people claiming 24-26 for highway. Are their cars brand new? Brand new plugs, fresh oil, etc? Were they driving in the absolute perfect conditions as I'm sure mazda did their testing in? Were there hills during their driving? Were they driving at a constant 75 or 56, never once moving from cruise control. Jim please answer this one question and I'll stop arguing with you: why would mazda claim such rediculous numbers for constant speeds but tell the truth about city mpg? Those 29.xx and 37.2 mpg claims by mazda weren't their highway mileage claims...they were their constant speed mileage claims. Maybe your car just was bad on gas? haha I personally believe that they did the tests and these were the numbers they came up with. Could they have done the tests with ten cars and this was the best one: ABSOLUTELY. Did they do the test on 10 different days with 10 different cars? Why not? You're a smart fella, I'm sure you've taken a statistics class or two. Do you remember the part about advertising? The people advertising are always going to give the best results, not the worst and not right in the middle.

widebody2

Jim, you also think that YOU of all people should be telling me that being a smart *** isn't helping?

jimlab

Have you ever heard of the EPA? Read up...

http://www.fueleconomy.gov/feg/how_tested.shtml

There's a difference between being a smart *** who's right and just being a smart ***... see if you can figure it out.

rpm_pwr

We're talking about a company that rounded off 1.146 to 1.2 litres. I'd say their maths is suspect. This is the same company that claimed 27mpg for the 20b cosmo. In the real world you can get as low as 7mpg in city driving.

Seriously who cares? If you can't afford the fuel you can't afford the car.

FDNewbie

iTrader: (10)

From the EPA website Jim put a link to:

"In the 1980s, an EPA study found that drivers were typically achieving lower fuel economy than predicted by EPA laboratory tests. As a result, EPA required the laboratory-derived city and highway MPG estimates posted on the labels of new vehicles to be adjusted downward by 10 percent for city estimates and by 22 percent for highway estimates to better reflect the MPG real-world drivers can expect"

They're saying that Mazda originally achieved 20/27-28 mpg, and this was marked down to 18/24????