I’m in the “it is as quick as I remember” camp.
When I had my original ‘78 DT175MX back in 1986 I weighed 135lbs and most of my riding was farm tracks or single lane tarmac farm roads where 60mph was fast.
I’m now 182lbs and my new to me ‘78 DT175MX still feels quick, definitely faster than both of my CT1s. It revs freely and just wants to go.
When I get it reassembled I’ll take another video, but here’s one it took after moving the needle.
I was hanging back as there were cars ahead and I wanted to make space so it’s not the best video.
Funny thing is I’m happy to bob along at 45-50 on my bikes and just enjoy the scenery, but feel obliged to stick up for my DT’s performance 
DT175MX

I can agree about the gasoline, as a friend of mine came down from Oregon and brought me two 5 gallon jugs of 92 octane, ethanol free. Wow, what a difference it makes in my motorcycles, especially me 1981 DT-175.

As far as top speed, I can push mine up to 70 mph, but that is the limit and it takes a while to get there. I don't know why anyone would want to do 60, or 70 on this little bike anyway.

A classic mistake on the last few years of the DT-175 is when people rebuild the carb and forget or do not notice the small pin on the bottom of the black plastic spacer that goes under the clip on the jet needle. This small pin locates in a hole in the slide; it is easy to not gt it aligned and then it does not drop in the hole which has the same effect as moving the needle clip from position 3 to position 4, which makes it run rich in the midrange. My 1981 DT-175 has only one clip position, and a cap blocking off the pilot airscrew, presumably for smog reasons. I found the black spacer was not aligned with the slide such that it's pin was not in the hole, and mine ran really rich below 7K. The guy I bought the bike from told me he just cleaned the carb....so that is where I started and found the afore mentioned problem.

All the DT Yamaha's were designed to run on proper fuel. Power will be reduced when ethanol type fuels are used, but can be partially restored with changes to jetting.

Leaded fuel, means more power, better fuel economy, and cooler running. Used to be available in the UK, but not sure now?

Yamfan wrote: "Leaded fuel, means more power, better fuel economy, and cooler running. Used to be available in the UK, but not sure now?"

Yamaha recommended unleaded fuel, not leaded.  Power, economy, and cooler running were not achieved due to lead in the fuel. Leaded fuel means far more frequent spark plug fouling and more carbon fouling of the engine and exhaust which meant more frequent disassembly to decarbonize. 

Lead in the fuel was a necessary evil in days gone by...  it was a cheap, albeit dirty, octane booster.  Modern fuels are more highly refined and much better quality.

The use of ethanol (alcohol) as and additive is a whole different topic.  By far the biggest concern with ethanol in modern fuels is the "corrosive" and "solvent" properties...  since ethanol was not in use when these bikes were built, the metals and rubber in the fuel system may be susceptible to damage from the ethanol.  It's usually not too big an issue, especially if a fuel additive like StarTron or Stabil is used during longer term storage.

In terms of dyno testing, what sort of test results would you expect, when comparing different types of fuel?

Do you remember Amoco white gas?

Hi Wes, never heard of it, maybe they didn't sell it out west where I grew-up. 

Good morning Tony the gas was clear high octane. Hope you have been enjoying your new bike

Yamfan asked: "In terms of dyno testing, what sort of test results would you expect, when comparing different types of fuel?"

Your question is extremely vague and therefore impossible to answer without writing a book of all the possibilities and factors. 

I've actually done this type of testing and since you weren't specific at all, I will be.  You mentioned "dyno" so I'll choose "best to tune for maximum horsepower" and I will limit the comparison to street-legal fuel available at service stations in the USA

The clear "pump gas" winner for maximum horsepower is E85 which contains up to 85% ethanol. 

I personally would not recommend it, but it would give you the most dyno HP from "pump gas". 

My DT-175G will wheelie on command in 1st and 2nd gear, and if you are not careful and have your toe on the rear brake, it can easily loop when the powerband hits at 5K.   By contrast, my CT1-C  will not wheelie unless I sit back on the seat, rev it to about 5 K, let off, and then whack it WFO.

It is mainly piston port versus reed valves and the lower torque band, as both engines have the same BHP.

BTW, ethanol fuel will eventually rot out the jets, and if it attacks the Needle jet, the midrange will be too rich.