no problem
get some 50/50 ASAP and check again - ill bet your temps go down

Perfect timing on this thread for me! I came on here to look for some info on the cooling issues of D1's. I've just driven from Boulder to Steamboat Springs and return this past weekend, and was monitoring my coolant temp via the Ultra-Gauge. I have a 190 thermostat installed, new water pump and hoses, big time flush when I got this truck last winter. Anyway temps were 192-199 on the cruising portions, and heading up the passes, (I-70 to the summit and coming back up Rabbit Ears, both which are big grades), the temps got up to the high teens low twenties. But it's only May and not even close to hot out! So I'm concerned for the summer driving season. I'm running oversized tires which will make the engine work a bit harder, but I shifted down to third and kept the RPM at around 3200 up the big hills. I will definitely be installing a 180 thermostat, new res cap, and I guess I should consider a new good radiator. It is a bit nerve wracking to essentially be staring at the temps! Great highway mileage though IMHO...high 19's. That's with the 8% conversion for the increased tire circumference.

my 97 d1 has an ome lift [springs,shocks,bilstein steering damper] and i'm running 255-70 michelins, no question 180 stat has reduced temps but really haven't sat in traffic to test! if mine holds below 220 i'll be happy. btw,19mpg are you serious! if i coasted down a mountain maybe!!! if at highway speeds i run 199-201-204 i'll be happy even with 180 stat these trucks [buick v8] seem to operate best at 190-210 +/- a few

Holy crap are you kidding me you're getting nearly 20mpg highway? That's unbelievable. I calculated my mpg for the first time the other day (ultragauge mpg doesn't work as I'm missing a cat) and I'm getting just over 11 city :(

On another note, you make it sound like shifting down into 3rd to keep RPM up would help the engine run cooler. Am I misunderstanding? That seems counter-intuitive, but maybe it is! Just curious in case I'm ever in that position.

As far as shifting down is concerned, what I do is drop it into third and back off the throttle position so I'm climbing at a steady speed, 3200 is what I tried yesterday. This puts less power to the engine and circulates the coolant . The fuel mileage is accurate, I've checked it several times, but you need to know that also somewhat counter-intuitively, usually a vehicle will get better fuel mileage driving in the mountains, as the downhills seem to do more to help than the uphills seem to hurt. I have seen this in many cars over the years. In the old days with carbureted engines this was not the case as the mixtures would get too rich the higher in altitude you went, but with the newer FI stuff with computerized fuel controls, a car's mixture will get leaner as it gains altitude, hence using less fuel. This is the same as a AvGas powered airplanes (I'm an A&P mechanic), you lean back as you climb and use less fuel. Driving on the interstate on the plains it gets around 15 best, and around town the absolute best I get is around 14, usually less because I tend to have a heavy throttle foot!

this is just for fun but i never leaned out a lycoming or continental [4 or 6 banger] when climbing to altitude,only a little after leveling off and cruising! i'd be surprised if our ecu even knows the difference [maybe basic ambient temps] anyway any tips on getting better mpg i'm listening,i do use a k&n air filter but i don't think that makes any diff [might even hurt!! maf] but rpms are much quicker

Since we are already starting at 5260 ft here in BDU, most guys do lean as they are climbing, especially heading west as we rapidly have to get to around 14K or so to clear the divide. At this altitude unless the engine is turbo'd you could run at peak EGT and you wouldn't do any harm. On the Rover, I do think the ECU does lean, as it senses the O2 sensors and adjusts accordingly to keep the correct F/A mixture. As far as tips are concerned, I run the tires at around 38-40 on the highway, put new plugs, and wires,but what I think has really helped is to readjust the kickdown cable to keep the engine in the HP band as much as possible. This truck had the bigger (8%) circumference tires, and was extremely doggy. I played with the kickdown so now it is pretty aggressive and drops a gear going up almost anything. That's why I manually shift down when climbing a big grade, as the tranny will shift up and down too much. I have just discovered that it seems my fan clutch is inop, so I'm going to look for that GM clutch the forum is talking about. Also ordered a 180 max flow thermostat. I do have a K&N, but a word of caution...most people when they clean the filter spray WAAY too much of the oil on the filter when reapplying, and either impede the airflow or worse, damage the MAF sensor.