Just curious what you guys would expect for gas mileage under the following conditions:

'93 240 Wagon
VX Cam
75mph speeds
Thule roof carrier (box)
Tires are 215/55-16

I am seeing about 21mpg highway under the above conditions. Just switching from 195/70-14 tires to 215/55-16 tires cost me 4% due to more accurate odometer readings with the new tire size.

Still, 21mpg seems a little low, but maybe I am expecting too much. (I was more expecting 23mpg, but maybe the speed with the roof carrier is a killer)

What do you all think?

Thanks

'89 740T wagon; same diameter tires; roof top carrier; easy driving (mostly hyway); loaded down 25MPG for a Canadian gallon (20.8MPG US Gallon)
--
Norm Cook Vancouver BC; 1989 745T 200,000KM

In theory, the difference in size of those tires is only a bit over 2%, but whatever.

With the roofbox and the speed your 21MPG is not surprising. It would be interesting - if you have the opportunity - to see how much improvement you get with no roofbox and 65mph speed. I'd bet you'd see about 27MPG. If you try it, let us know.

I did an 1100mi highway trip in the 83(M46/OD) in late August. With mixed Interstate and 2-lane mountain driving at 60-70MPH it averaged 29.4MPG(US). I'm a data freak, and have an Excel spreadsheet for each of the cars and account for every gallon/litre that they burn.

The best mileage occurred at the Oregon fillup, as usual. State law prohibits self-serve fuelling in Oregon, and the pump attendants never fill the tank right up, so it always looks like you got great fuel economy. Next fillup, however...
--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Thanks everyone for the feedback. BTW, I forgot to mention that the AC=0n too.

Speedlimits in Texas and on through Florida are 70mph. When you only do 75mph people want to run you off the road. I tried 85+ at one point and it still wasn't fast enough to keep the traffic flow happy....I got passed by someone towing a trailer. Totally nuts!

When I first got the car 5 years ago I would get 27mpg driving 60mph. But that was with the wrong size tire - 185/70-14...these were good for a big bonus, since my speedo read 8% high.

I'll let you know what I get next time I'm in a 65mph area and I drive 65 :-)

btw...with the VX Cam and Sport exhaust I actually have passing power at 75mph with toating a load, the AC, and the roof carrier! Of course it will suck the gas though.

I monitor my mileage informally, using the trip odometer. I never write it down so I don't have a great record, but under my normal driving cycle, a bunch of around town 0-30-0 stuff, and then my drive to work: 1 mile of 0-30, 5 miles of 0-55-0, a flat out blast up one exit on the highway to 80-85 for 2 miles, then 2 miles of 40mph back roads.... got all that?... I get 20 mpg in the winter and 21 in the summer.
When I do long trips, 50+ miles of highway, I get 25-26mpg. I do drive pretty hard, averaging 75 on the interstates wherever possible.

This is my 87 240DL sedan, with 195/60-15 tires, and a 5-speed.
My wifes 88 740GLE automatic got almost identical readings, which isn't too surprising, given 185/70-14 tires and the fact that the 740 is probably just a bit better aerodynamically than the 240. Weights of both the cars were within 100 lbs- 3100 or so with our usual junk in the car.

I think you're probably not far off. A tuneup will never hurt however, including a new air filter.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 234K, '82 245T/M46-182K, '89 244DL/AW70- 212K Not too distant past: 86 244DL 215K, 87 244DL 239K, 88 744GLE 233K, 88 244GL 147K, 91 244 183K

Well, the roof 'box' doesn't help nor does the 75mph. Driving at 65 would get at 10% more, if not 20%, more economy. But who can drive at 65? Oh, and the wider tires add a lot more rolling resistance and aerodynamic resistance. 21mpg probably is 'not bad'.

I recently installed a VX cam, and put on 205/55/16's (Toyo Proxes TPT). I have an 89 245 DLA with 135k miles. After 5 tanks, I'm averaging 23mpg (2/3 city, 1/3 highway). This is a drop from 24.5 mpg--but darn well worth it, in my opinion.

Just for reference, the expected gas mileage for an automatic 240, according to the original invoice in my glovebox, is 19-24 mpg.

I don't know enough to comment on how your changes might affect the results.
--
'89 244 GL -- 106,520 miles (see profile for info on car)