T250 Gear Ratios 30 MPH in first gear at 7,000 RPM

[Tripleking]

My 71 T250 is a "rescue." I bought it back in 2013. It had a 1973 GT250 engine, which I rebuilt from the crank up and it runs very well. However, It seems to be geared (or sprocketed) very high. It will do 30 MPH at about 7,000 in first gear and will cruise at 55 MPH at about 5500 RPM in 4th gear. The speedo is pretty accurate as it reads the speed limit when I am keeping up with traffic. If anything, it reads a bit low by an MPH or two. The rear sprocket checks out as stock for a T250. I have to pull the cover to check the front sprocket. How many teeth should my front sprocket have? Were the gearbox ratios different in the GT250 engine versus the T250? Launching the bike in first feels like launching it in second. It pulls like a small train once you get past 5500. Pulls strong to about 7500.

[Tripleking]

Figured out my problem. I pulled the sprocket cover and the front sprocket had 15 teeth. As far as I know, the front sprocket should have 14 teeth. That would explain it, eh?

[yeadon_m]

If it was my bike I'd investigate what the standard sided sprockets were, front and rear, on the T250 and GT250 of the relevant years. I wonder if you've too small a rear sprocket as well as too large a front sprocket? I prefer to consider ratios, eg on my GT750A I've 15T/43T or 0.349. I would have similar gearing if I used 16T/46T (if there is a sprocket of that size) but of course its much easier to swap the front!
Good luck,
Mike

[Tripleking]

From what I have read, the OEM setup was 14/41. My bike's setup is 15/41. The front sprocket has less wear than the rear. It looks as if a previous owner changed out the front sprocket. As I live in the mountains, with lots of hills to climb and I weigh about 235lbs, I was thinking of running a 13 tooth front sprocket, just one tooth down from stock.

Additional:

I think I found an additional problem. The T250 had a sprocket set-up of 14/41 (I have 15/41), but the GT250 had a 14/43 setup. Thus, the GT had a lower final drive ration than did the T250 (3.07 vs. 2.93).

The T250 gearbox is geared slightly lower in gears two and three, but is identical to the GT250 in first gear and over the final three. I will probably change both sprockets to get the OEM 2.93 or the GT250's 3.07. If I dropped the front to 13, I would have a final drive ratio of 3.15. That would be nice for my kind of riding. I.E. lots of hills and speeds rarely above 60 MPH.

[Tripleking]

One last question. What is the size of the front sprocket nut? I need to get the appropriate socket.

[yeadon_m]

Is it actually a nut? the 380/550 sprockets are held on by a triangular plate and lock washer and secured by 3 screws through those into the sprocket. The 750 uses an f off big nut and I don't recall the size. If you can get to it easily, I'd do that and get some calipers on it to be sure.
Cheers,
Mike

[Tripleking]

My GT250 engine has a hex nut.

[Tripleking]

Swapped my 15T front sprocket for a 13T. The difference with the stock 41T rear is wonderful. 30 MPH is at 5000 RPM IN third, instead of 4000 and the bike cruises nicely at 55 MPH in faith instead of fourth. Now I will replace the chain and rear sprocket so it all wears together.