Redonkulous (yet preliminary) MPG figures for a 1981 Caddy

[Quantum TD]

So, I bought a Caddy a few months ago. Owner claimed it was a runaway. I brought it home and it sat for about 3 months before I looked at it. Someone put in the wrong dipstick, so there was about 8 qts of oil in the case (wonder why it ran away?!)

But, at that point, the damage had been done. The thing was burning oil BAD (about 2 qts every 500 miles), and stunk like a ***. Then the head gasket went. I kept driving it though, and was getting about 37-39 MPG, even with low compression, oil consumption, a leaking IP, a blown HG, and a poorly geared 4-speed.

So, about a month ago, I installed a used 1.6 NA hydro engine out of a 1987 Golf. The motor has about 157k on it, but you'd never know given how great the compression is. Anyhoo, I installed it, with all new seals, glow plugs, new clutch, resealed IP, etc. I also put in a FN code trans to drop the RPMs. It shifts great and really helped with RPMs. A 1998 TDI cam was installed (broke the old one trying to remove the sprocket), and it really give the truck more pep at higher RPMS (to the detriment of low end torque).

I put it all together, and then let it sit for 3 weeks while I was on vacation. I came back, made sure everything was good, and started driving it again. I fllled the tank before I did all the work, and had about 120-150 miles on the old motor (getting 37-39 MPG). I just ran the truck down to a half a tank on the new motor, and went 406 miles. I filled up the tank as I normally do (all the way up the neck), and I had used 7.5 gallons.

That's 54 MPG in a pickup! I drive as I normally do (with a lead foot), but do alot of coasting when I can and don't use the AC. The truck is bone stock. I did take the carpet out (gross), removed the front bumper (to straighten it out), and don't carry anything in the back really. I'm pretty hefty myself (about 240), so that probably makes up for the missing carpet and bumper. Here's the best part: about 1/3 of those miles were on the old motor, so, if these numbers are right (i.e. I didn't fiill up, but forgot to reset the trip odometer), then I'm actually getting closer to about 60-63 MPG!

I don't want to jinx things here, and I'll post an update after another 1/2 to full tank. But I figure this truck has got to be getting at least 55 MPG on the current motor, and maybe more. If that's the case, I'm keeping it (I was gonna sell it on ebay once I fixed everything).

[jtanguay]

keeping the rpm's low with a good tranny is what you want  :wink:

[Darth Garry]

Pretty cool, question, are you positive you have the correct speedo gear on the transmission?  If you have the wrong one, fuel mileage can be up to 20% off.

Garry

[theman53]

let me know if you still want that cam
It would be great for you just to have a look at it. I never got a response back that is why I am asking. cya

[Quantum TD]

Pretty cool, question, are you positive you have the correct speedo gear on the transmission?  If you have the wrong one, fuel mileage can be up to 20% off.

Garry

Yeah. MPH looks right. The old trans had a 3.94 final drive, and the new FN has a 3.89. Both use the white speedo gear. If anything, I think my MPH is lower than actual by about 1-3 MPH. So, true figures might be even higher.

I'm taking this all with a grain of salt. I've owned several diesels (about a dozen and a half), and none have done this well. Best I got was 47-49. Most of those cars had compression near the wear limit, and leaking injection pumps: which may account for the disparity.

We'll see in the coming weeks if that number is accurate, high, or low. I think it may be a bit low, but that may be wishful thinking.

[jtanguay]

well if you're getting good mileage, be sure to run a good synthetic oil to make it last.

[zukgod1]

well if you're getting good mileage, be sure to run a good synthetic oil to make it last.

I tried that, went from a Dello 400 15w 40 to a full syn 0w-40 (maybe it was 30) anyway, no difference at all in mileage.
I usually change my oil every 3k regardless but I stretched it out to 5k on the syn if for no other reason then the price I paid for it.
Did a mileage check on several tanks and it stayed right @ 40mpg
Switched back to good old Dello and did another mileage check right at 40 mpg. I like to burn my waste motor oil after a change and I'm not going to burn the syn like I will the dino based oil so that raises the cost even more on the syn.

I'm rambling now..

[Quantum TD]

I think he was referring to maintaing the compression, and hence MPG. Not necesarily trying to get even better MPG.