Regards to all, I'm new here and have been catching up on my reading. My stable consists of an 88 CRX DX, 91 DX Hatch, 88 RT and 91 RT. I drive over 600 miles per week at freeway speeds (70-80 mph) in the CRX (for obvious reasons). All this time behind the wheel gives me way too much time to ponder the vagaries of the mpg question. The question that keeps coming back to me is accuracy of the speedometer (and therefore the odometer), which can affect the mpg calcs. As many of us do not run stock size tires, the calculated mpg can become skewed by the difference in rolling circumference of our tires. I am seeing some crazy high numbers from people with the RT (numbers that rival what I get in my CRX). I haven't yet checked the 88RT, but the 91RT gets 27 mpg (best ever, 30) at the above stated speeds (according to my speedo), which is slightly higher than the figures posted on fueleconomy.gov. This was running on 185/70R-14 tires. The circumference of that tire is 76.017 in. (http://www.csgnetwork.com/tiresizescalc.html). Simply changing to a 195 from 185 changes the circumference to 77.748, a 2.3 percent increase and an increase of 4.5% over the stock (I believe) 175/70R-14. As tire circumference increases, the number of miles recorded by the odometer over a given distance decreases, decreasing the calculated mpg. Similarly, smaller sizes result in higher numbers. Anyway, you get the picture, tire size can affect mpg calculations, so the numbers are questionable unless corrected for tire size. I suggest that when you post your mpgs, tire size should be included. That way the numbers can be adjusted for the variation between stock size (for which the speedo is calibrated) and actual size.
I apologize if this is all too anal, but enquiring minds want to know
I have often considered this argument too. Another thing to consider even on top of that is that car speedometers don't always seem to be very accurate. I have done plenty of comparisons between my 2000 civic speedometer and gps speeds and found that my speedometer is actually something like 6% slow (this is with the oem 185/65R14 tire size). So if the speedometer directly converts to the odometer we would be looking at mpg readings that are off by 6% right away let alone variance in tire sizes.
Now to add further confusion though I did a 123 mile drive the other night in 103 minutes. Do the math and that gives you an average speed based on the odometer of 71.65mph. According to my speedometer in my car I drove between 75 and 85mph the whole way never dropping bellow 75. SO how could my calculated speed actually be 72? Maybe the odometer is actually correct and the speedometer is actually set a little high on purpose to keep people from speeding. You can drive 70 in a 65 and think you are going fast when you are actually only going the speed limit of 65. Just some food for thought.
On the subject of actual speedometer accuracy, I am running stock (175/70R-13) tires on my CRX. When I pass one of those radar speed signs, it consistently indicates that I am going 5 mph slower than my speedo indicates. I pass no less than 2 of these signs during my weekly odyssey and the results are consistent. I noticed the same thing in my '91 HB when I was using it for the long hauls. I just asked a CHP about those signs and he said that they are regularly calibrated, though rough handling at the construction site may throw them off a little. The odometer is driven by the speedometer cable so they should be in synch, but if the speedo is just set a little high from the factory then we don't know if the odo is correct or not. In the old days, California used to have "Speedometer" checks on the freeways consisting of a measured mile with the tenths posted, but not anymore. They do, however put marks on the side of some freeways, consisting of a short section of double stripe on the left side of the fast lane, which I believe are used to clock cars from the air. They are spaced 1/10 mile apart for a distance of several miles. When I checked my odometer against these, I found there was an accumulating error. When I passed the first mile marker (tenth mark), my odometer was about halfway between 9 and 0, indicating that the odometer showed less mileage that actual. This is inconsistent with the speedometer reading higher than actual speed (as indicated by the radar sign). Maybe we expect too much.
So on my way to work today I checked my wagon speedo vs the gps. I am running 175/65R15 tires on my 4wd wagon. Which according to this nifty little tire calculator means my speedometer should read 1.3% too slow. So pretty close to factory tires. I checked my speedo against my gps speedo and they were dead on upto 60mph (didn't go any faster). Apparently my wagon is calibrated correctly....
Mr Whooppe, I think about that too and factor it in when I want to figure out my mileage or my actual speed. The Miata tire size calculator webpage is visited frequently by me. My tires are 4.1% shorter than the stock size. I don't sweat MPG too much since I am driving a wagon, and it is RT4WD. But I get about 29-31 MPG on the highway. I just know it costs about 10-12 cents per mile when I drive around. That's really what we are concerned about, right?
Just drove a full tank on my RT4WD and got 400 miles (60 miles a day hwy mostly), which translate to 33mpg. All stock with d16z6 head.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
Just drove a full tank on my RT4WD and got 400 miles (60 miles a day hwy mostly), which translate to 33mpg. All stock with d16z6 head.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
Just drove a full tank on my RT4WD and got 400 miles (60 miles a day hwy mostly), which translate to 33mpg. All stock with d16z6 head.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
'90 Rt4wd, 231k miles, 15 inch snowflakes with 205-50's set at 35 psi. I get 27-30 in town, 32-35 on the highway, and around 29 when I'm really trying to get somewhere. This car really hums down the road when you are doing 80-85.
I do alot of freeway driving, just driving to and from work. At lowest I was getting 27mpg, and most at 30mpg.. I understand the tank holds about 11 gallons, and when I fill up, I usually add about 8 gallons.. I didn't see a low fuel light so I don't want to run dry one day. So I usually refuel around the 200-230 mile make on the trip.. I got some gsr blades with 195/50/15 tires.. I haven't changed the air filter or even looked at it since I bought the car 3 months ago. I'm gonna buy me a ram or cold air intake one of these days off eBay. I wanna see if that does anything drastic to my mpg..
Just drove a full tank on my RT4WD and got 400 miles (60 miles a day hwy mostly), which translate to 33mpg. All stock with d16z6 head.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
Assuming you put 11 gallons in upon fill-up, that would equate to 36 mpg. Stock tank is just shy of 12 gallons empty.
I do about between 28-31, city and highway on a daily basis.
Geeze ive barely managed to get 29 out of my fwd 5speed d15b2 with a fresh tune up.... Even with the rpms mostly staying below 3k except when i do 65 on the freeway.
Fail....
My RT4WD that burns oil at the rate of about a quart a tank has been toying with trying to break the 30 mark in my usual "country" driving. Past few tanks have been 29.5. Thats with a DOHC ZC and the 4wd disengaged. Engine rebuild coming soon...we're hoping a fresh engine might get us that last 0.5mpg to break into the 30's.
90 with a 99 civic head. Rebuild with Honda parts. With the A/c on, 70 mph hwy is 37/38. In town not over 55 with the a/c on easy driving, I get a firm 34. Now- driving it hard with a/c on its more like 31/34.
The factory speedometers tend to read 5 to 10% high. On my CRX the speedo is dead on when I run 185 70 14 tires, it overegisters with the stock 1856014. I've never fully checked the odomoter reading either wiht mile markers on the interstate or my Garmin.
im getting 38 around town with a d15b2 w/ z6 intake manifold and converted to obd1 running a p06 and some heavy ass 16in wheels. might get better with some hx or lighter wheels.
1990, RT4wd, stock, d16a6, SOHC, the speedometer registers low, by about 5mph (when over 40mph), but odometer seems accurate. Best tank was 43mpg, cruising around 60mph between CO and UT in the summer. Regular MPG, around town is 32mpg, even with the studded snow tires on. I usually have >1000 pounds of gear in the car, and a full roof box. If I take that off, and empty the car, and put on the summer tires, it only goes up to about 33mpg.
Comments
I apologize if this is all too anal, but enquiring minds want to know
Thanks for listening
Now to add further confusion though I did a 123 mile drive the other night in 103 minutes. Do the math and that gives you an average speed based on the odometer of 71.65mph. According to my speedometer in my car I drove between 75 and 85mph the whole way never dropping bellow 75. SO how could my calculated speed actually be 72? Maybe the odometer is actually correct and the speedometer is actually set a little high on purpose to keep people from speeding. You can drive 70 in a 65 and think you are going fast when you are actually only going the speed limit of 65. Just some food for thought.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
With 14" black steelies, I was at 230 miles and gas gauge was at half way. Switched over to 15" at that point and so I think I would of had better results.
Assuming you put 11 gallons in upon fill-up, that would equate to 36 mpg. Stock tank is just shy of 12 gallons empty.
I do about between 28-31, city and highway on a daily basis.
Fail....