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Discussion Starter · #1 ·
I've experienced a major loss in gas mileage recently. I'd been routinely getting 85+ or so miles out of a tank before needing to switch to reserve. Now I'm getting about 60 miles.

The only thing changed is a new rear tire. I think it's tad wider than what was on there before (perhaps 110 - 120??). The bike runs great, acceleration fine, not bogging down or anything.

Can a new tire have this much effect on mileage? It's the same tire that was on there before, just a little wider.

Thanks,
BaileyMan
78 CB400TII
 

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That's a strange one. I can see a different tire affecting it somewhat just due to a weight difference, but I doubt it would be that much. Has your riding style changed at all with the new tire? I know my mileage is greatly affected by how I'm riding on that tank and whether the rides are short commutes in stop-and-go traffic or longer rides out in the country.
 

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Discussion Starter · #3 ·
It is kinda strange, I agree. My riding style hasn't changed, and this decrease has been consistent across two tanks.

I don't get it. The bike runs like champ, maybe not like it did 30 years ago, but no smoking, no leaking gas from the carbs, maybe the exhaust smells a little stronger than it has (but maybe not), no backfiring. I suppose I could check the plugs, but it start's right up, first push of the starter or first kick, idles nice, no changes in acceleration. Just a 25% decrease in gas mileage...

bike has 15K miles.

:?
 

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Are you sure you reinstalled the rear wheel properly? It's not crooked at all? No rubbing? All the parts went back in the proper place? Brakes not dragging? Did you air up to the proper amount?

That's a severe drop in mpg. Last time I had something like that, it was the e-brake on my truck dragging.
 

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I think the dragging brake issue would be my bet.
 

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feel the hub after riding for heat, you'll know if the brakes dragging, you may have gotten a smaller circumference tire as well, tire sizing between brands and sometimes between their own lines is somewhat sketchy IE Avons run tight in the bead, try mounting one, its a struggle!!! Michilins seem to mount themselves...

but here is the run down:

different tire size
(under inflated) check the front too.

dragging brake

misalignment (measure from the swing arm pivot to the axle center on both sides should be basically equal and sight down the chain, if its not obviously wrong its
probably OK)
 

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Keep an eye on your gas mileage.
It should actually improve as the tire gets exercised more. Although you'll also have to take into account changing weather/seasons as this will have a large effect on you mileage (depending on where you live).

<-Tire engineer

Alex
 

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Discussion Starter · #8 ·
Thanks all for the comments. I had the tire done at a shop that I trust. I assume they did everything properly, but then again I like doing my own work because 1) I know what's been done, and 2) if something's not right, I know who's fault it is and can't get angry for spending $90 / hr for something that's worse than when I brought it in.

I'll check the brake dragging and alignment. Tire pressure is ok. I'm on my 4th tank of gas now since the new tire. I think that even by the 3rd tank the mileage was better. I say this because at 62 miles I was expecting it to be empty soon, so I filled her up with less than 1.8 gallons. There was probably at least another half gallon or more in the tank.
 

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4 comments:

lots of good suggestions have already been made but there are 4 that come to mind that haven't been mentioned
- did you try lifting the bike and spinning each wheel to make sure it spins freely?
- have you looked at your speedo drive and cable to make sure it is fully engaged...is the speedo reading consistantly?
- chain alignment and adjustment
- is either of your axel bolts overtourqued. bike shops are notorious for using an impact gun on axels which overtightens them and could cause binding on the wheel bearings
 

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Discussion Starter · #10 ·
Just to update...

After about 250 miles, my mileage came back to where it should be.

I didn't check all the other stuff suggested, but feel pretty confident the decrease in mileage was due to the new tire.

Thanks for the suggestions all!
 

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I would bet a $1 the rear brake finally worn down a little and is spinning free now. Just a guess!
 
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