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post #1 of 50 (permalink) Old 01-02-2015, 04:08 PM Thread Starter
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Gas milage?

What I have is a T140V engine going in a 1970 T120R frame so the rear sprocket will be 46 teeth and the front sprocket will be 20. So what kind of milage do you think I'll get at around 70 mph? Also, what do you think, would it be necessary to increase the front sprocket to 21 teeth for the engine to cruise at 70 comfortably for an hour or an hour and a half?
I was figuring that my tank is 2.5 gallons capacity and if it only gets 40 miles to the gallon @ 70 mph then it looks like 100 miles on a tank.
However, if it's only going to get 20 mpg then it's only 50 miles. Do you think it's possible to get 50 mpg out of this set up with no windshield on flat pavement?
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post #2 of 50 (permalink) Old 01-02-2015, 06:07 PM
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I'd get a bigger tank.

I expect you'll get about 45 mp(US)g and I don't think messing around with gearing will change that very much.

Last edited by Triton Thrasher; 01-02-2015 at 06:09 PM.
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post #3 of 50 (permalink) Old 01-02-2015, 06:24 PM
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These days with the vibration and the cold I need to stop,for a pee anyway
Might as well do it at a petrol station
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post #4 of 50 (permalink) Old 01-02-2015, 06:35 PM
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Mine would do about 60 mpg, UK gallons, if i sat at 70 mph.Not that i ever do constant speeds.One annual run with passenger over the moors yields 60 mpg easily.I dont think i could hold onto the bars for 1 hour at 70 mph.
I use a 47/20 combination and it is a good set up.
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post #5 of 50 (permalink) Old 01-02-2015, 06:37 PM
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While appropriate higher gearing could increase your mpg running on the flat at speed, the amount you are going to be talking about is very small in real terms. Lets say to keep the maths simple you were getting 50 mpg originally and you could achieve a 10% increase in fuel mileage on 2 gallons you will get 110 miles instead of 100 miles- not really a margin of safety and one that could quickly disappear with a head wind or even changes in temperature, inconsistent fuel etc.

If I were going to cruise regularly at 70 mph for the duration of time you mention, I'd consider raising the stock gearing, to be a little easier on motor by reducing the rpm, but with a small tank I don't think that will help your overall range that much. Based on my 750 Tiger, I would definitely go higher on the gearing while I had the opportunity, I'm at altitude of 4500-5000 feet most of the time and mine still feels under geared in 5th.

I agree with Triton that about 45 mpg on a US gallon is about the best you will regularly do with a T140 motor running in the best of tune. Rambo's 60 mpg on a UK gallon is 48 mpg on a US gallon. I think 50 mp(US)g is achievable, but I think that would be your fair weather favorable conditions mileage and wouldn't want to rely on getting it, if a long walk was going to be the result of running out.

Last edited by redhawk4; 01-02-2015 at 07:46 PM.
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post #6 of 50 (permalink) Old 01-02-2015, 06:46 PM
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I don't agree that reducing rpm is easier on the motor.
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post #7 of 50 (permalink) Old 01-02-2015, 07:13 PM
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I don't know the US/UK gallon conversion factor, all I can tell you is that my old 1976 T140 had the standard 20T/47T sprockets and when set up properly to run without silencers and air filters (I'm not recommending the latter but I would certainly recommend the former, and this makes the biggest difference in my experience), 5000rpm was 80mph, and that was an easy cruising speed to maintain all day, when I was 20 and didn't have bad wrists. Fuel consumption never went below 50mpg (British gallons) and was generally around 55mpg. After fitting higher compression pistons and a T140D 2-1 exhaust (no silencer), fuel consumption improved to nearer 60mpg. My current TR7 has never dropped below 60mpg and will easily do 70mpg if ridden at a steady 70mph over a long distance. Efficiency is the key, ie: less throttle opening for the same power, and the easiest route to that is to free the breathing up by ditching the silencers.

Last edited by JohnA; 01-02-2015 at 07:44 PM. Reason: Removed haggis related remark in the interests of amity!
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post #8 of 50 (permalink) Old 01-02-2015, 07:37 PM Thread Starter
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Quote:
Originally Posted by DAVE M View Post
These days with the vibration and the cold I need to stop,for a pee anyway
Might as well do it at a petrol station
Ha, Ha, You're right. I'd probably need to take a piss too! Mostly I was just wondering what type of mileage I could expect from my set up and whether or not you guys would think I'd be over revving the engine at 70 mph for long distances. I was reading John A's thread on his T140 rebuild and saw he was putting a 21 tooth front sprocket on his engine and was starting to ask questions that I could see was leading off the subject of his thread so decided to start this separate thread.
So far it looks like I can expect around 40 to maybe 45 mpg according to Redhawk4 and Triton Thrasher. I really don't care about the mileage I just wanted to know what to expect and don't want to ruin the engine by cruising to long of a time at high rpm's.

Thanks for everybody's responses,
Gary
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post #9 of 50 (permalink) Old 01-03-2015, 04:38 AM
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Just be aware that a 21 tooth needs a fiddle to fit as it will not fit through the cover.
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post #10 of 50 (permalink) Old 01-03-2015, 12:48 PM
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Quote:
Originally Posted by Triton Thrasher View Post
I don't agree that reducing rpm is easier on the motor.
Yeah,like the discussion we had on Britbike about gasoline combustion speeds, sometimes yes and sometimes no...

My bikes are for sport so fuel mileage isn't too important so long as it's better than my Jeep. And I don't have speedos on my bikes.

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