Current setup is 1975 DT250B bottom end, 1974 MX250A top end and carb, and Jemco pipe. Bike is setup for pure street riding. I am really happy with the setup overall. The MX top end really opened it up and the heavy points flywheel still makes it easy to putt along through slower traffic. I'm running 15x47 final drive gearing if I recall correctly. 4.00 rear tire but not a knobby so the OD may be a little smaller than stock.

That being said I am looking to do a bottom end rebuild before this summer and am wondering if I can dial it in a little closer to perfect while I'm tearing things apart. As it sits 1st gear is pretty much useless due to it being so short. With the higher power band of the MX setup and the kind of riding I do, I never really have the revs to get it into 5th except for the rare occasions that I jump on the highway for a few exits.

I have been looking at specs and comparing gear ratios between the DT250B and the MX250A (those trans bits should swap right?) but its hard to judge just looking at ratios without being able to experience it first hand, so Im looking for the community's opinions.

The main goal is more useable gearing off the line but I wouldn't mind 5th gear being a little shorter as well. The tall overdrive gear is nice but the bike is still pulling at 80+ and i really don't need to be going that fast on this thing. Pretty happy with 2-3-4 currently.

So long story short, would the more evenly spaced mx gearing make more sense for me or would I regret losing the wide ratio DT gearing?

Sorry for the bit of rambling, any insight appreciated.

Awesome pipe!
Sorry, no comments on your questions.
But I wanted to be the first about the pipe.

Sorry I can't answer your gearing question, but that pipe looks pretty awesome! It's looks like you have a nice road bike project.

Looks like the question about the pipe is covered, what front end is that.

Do you have a list of the MX ratios you're considering?

Also, looks like MX had a lower primary gear ratio... are you keeping the DT ratio?

Glad you all like the pipe. Probably wasn't worth the cost or headache to get my hands on it but I do love it.

I should not have posted a pic haha. Complete yamaha virago 250 front end. If I did it again I think I would have gone with something a little beefier but it works well for my purposes.


Hi Mark,

These are the specs I was comparing as I know those engines are most similar. I was not sure if any of the mono shock stuff would be compatible

MX250A - www.yamaha-enduros.com/index.php/250cc-s...mx250-specifications
DT250B - yamaha-enduros.com/index.php/250cc-specs/44-dt250-b-c-75-76

Good question about primary drive, I was unsure how much mixing and matching I could do in there. Im comfortable enough to do a rebuild but messing with internal gearing isn't something I have any experience with yet. I think Id like to keep the DT250 primary drive gearing if possible, but unsure.

Let me see what I can come up with...

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This is what I really appreciate about MarkT . . . . Besides having a well rounded understanding of most things motorcycle, what really sets him apart from "almost" everyone else, is his willingness to sacrifice abnormal amounts of his own personal time to help others.

It wouldn't surprise me if he ends up spending literally hours doing research, calculations, reasoning, charts, composing understandable text .... you name it . . . the guy is amazing, and you will come away with a thorough and correct answer to you question every time.

I've seen him do it many times before . . .
Thank you Mark .... for your willingness to help others to the extent that you do!
It's one reason why this site is head and shoulders above most of the other motorcycle sites out there today,



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You're welcome Enduronut!

Thank you for the kind words... you are wrong about one thing though... I'm not right "every time"... I just do my best. I also learn something new here almost every day from the smart and experienced (and sometimes inexperienced!) members on here that contribute..

Anyway, on to the gearing...

Why do we need/want "close ratio" gears? Basically it has to do with rpm drop when shifts are made. If the rpm drop is too big, the engine will bog down when the shift is made. For racing, you want to stay as close to maximum HP as you can... the compromise is it takes time to shift and there is a practical limit to how many gears you can have...

It's the nature of gearing that the "steps" between gears are a percentage of the RPM...

Here's an example... First chart is shifting a bike at 5000 rpm...



Notice that the RPM drop from first to second is 1467 and the max speed in first is 17 mph.

Now here is the same bike, same trans, same gears, same tire size... only change is the engine has been heavily modified so now the shift point is 10,000 rpm.



It makes sense that the max speed in first is now exactly twice what it was... 34 mph. But notice the RPM drop between first and second is also twice as much 2933 rpm versus 1467.... which means when you make the shift from first into second, the RPM will drop from 10,000 rpm to just over 7000 rpm. Depending on the torque curve of the modified engine, it might bog down after the shift until the RPM gets up closer to 10,000 rpm...

As speeds go up, load (wind resistance) goes up so most gear boxes are designed to have less RPM drop in the higher gears.

Next I'll post up a comparison of the DT250b, MX250a, and MX250a with DT250b primary ratio.

P.S. Just a side note, if you changed the final drive ratio (drive chain and sprockets) on the 10,000 rpm engine so your max speed in first was back to 17 mph, you'd still have the same RPM drop between first and second. You can't fix RPM drop between gears with final drive sprockets.

Okay... more specifically... your final 15-47 gearing is very close to the 14-44 stock gearing... 3.13 vs. 3.14

Also, when I was looking at the specs on the primary ratio I looked at a DT250a which has a taller primary ratio (2.66)... DT250b has the same 2.83 primary ratio as the MX250a.

So here's a chart showing the speeds in gears and RPM drops between gears for the DT250b. I guessed at a rolling circumference of 6.5 feet for the rear tire and an 8000 rpm shift point.



And here's the data with the MX250a gears:



So you gain a few mph in first gear with the MX trans... but the DT250b gearbox actually has a lower 5th gear than the MX so you'd have a higher top speed with the MX gears.

All the above is based on the Yamaha spec numbers which could have errors....

Here is the DT250b Road Speed Graph:



And here is the MX250a Road Speed Graph:





I don't know if this helps your decision... I strongly suspect the gearboxes are interchangeable... I'm not sure it's worth it.

From what you said about the gearing now, what I would consider is keeping the stock gears, lowering the final drive ratio a little (higher numerically) and simply starting off in second gear instead of first.

P.S. I have spreadsheets with the data I can email you if you PM me your email address... you can make changes and adjustments to see what happens.