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I put some larger tires on my Insight recently and it is registering lower fuel economy. Mathematically is it possible that the mileage is not actually lower but only seems that way because the computer is calibrated to calculate mileage according to revolution of a tire with a smaller diameter?

If this is the case, is it possible to recalibrate the computer?
 

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Yes, tires of a different size will affect the MPG reading.

No, it's not possible to recalibrate the computer.

However, as we all know, any tire other than the Bridgestones will lower fuel economy.

You'll be able to tell when you do some fillups. The FCD in the Insight overall is very accurate, but I still go by actual fillup numbers and not the FCD.
 

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The problem is that even if you fill up and put lets say 10 gallons in the car in order to do the math correctly you need to know the correct number of miles driven. With the bigger tires your reading will be off because of the tire size. So unless you are on an interstate and can go by mile markers it will get you in the ball park but will still leave you wondering if it is right.
 

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I put some larger tires on my Insight recently and it is registering lower fuel economy. Mathematically is it possible that the mileage is not actually lower but only seems that way because the computer is calibrated to calculate mileage according to revolution of a tire with a smaller diameter?

If this is the case, is it possible to recalibrate the computer?
Hi .. what size larger fit your car? I have an insight 2001 that needs new tires. I'm looking to maximize my mileage.
 

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Hi .. what size larger fit your car? I have an insight 2001 that needs new tires. I'm looking to maximize my mileage.
If you're looking to maximize your mileage, you have one choice; the Bridgestone RE92 165/65R14.
 

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so putting 185/65/14 won't increase mileage?
This might give better milage - I've got to figure out how to fit them under my skirts... Eli, you know this would help your milage...

http://www.clublexus.com/forums/attachments/photoshop-forum/13670d1024126068-big-***-rims-on-lexus-gs400-plz-140inch_wheels-or.jpg
 

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You can imagine, this is a very old discussion around here and sometimes people get tired (so to speak) of answering this same question repeatedly. The fuel economy winner is clearly the original tires Bridgestone Potenza RE92s in the correct size. It has been demonstrated numerous times around here. Spend a little time searching and you'll find lots of posts. Aftermarket wheels, too have proven to hurt mileage as most are heavier and generate turbulence. No one around here would encourage you to buy anything but the RE92s.
 

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so putting 185/65/14 won't increase mileage?
No, doing such a thing will decrease your mileage, probably considerably.
 

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Many people belive that rasing the gearing or putting bigger tires on a car will improve fuel economy.

For very low speed operation, this may in fact be partially true.

Oncce the speed though gets above the rolling resiistance and built in mechanical drag of the vehicle and starts to push air aside (highway speeds), raising gearing or using a bigger tire really won't improve fuel economy. This is because once the drag takes over as the limiting factor, it basically just becomes a function of horesepower, and for a given true speed, you need x amount of gas going into the injectors to maintain that speed. A bigger tire will mean the engine is going slower, but you will have to use more throttle to keep the same speed, so the injector is still putting in the same amount of gas.

I did a lot of research on this (raising the gearing) about 5 years ago and that was the gist of what the research turned up.

For a standard car, this happens at about 45 to 50 MPH (drag becomes the limiting factor in fuel economy). The Insight is super slippery, so maybe a bit more than this.

I know my car will do huge MPG at 50 though, and when I hit about 57 or 58, I really can see the toll of wind resistance starting to influence the MPG.
 

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MT Insights have the added issue of Lean Burn. Anything that adds drag, whether turning a heavier tire, pushing more air, etc makes LB more difficult to maintain. Lean Burn is a rather fragile state.
 

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Yes, this would be my guess. The larger throttle opening needed to maintain the same speed would likley make lean burn hard or impossible in the manual transmission cars.

I did a lot of research on this. I had a Mustang convertible at the time, and was really looking to improve fuel economy. I did the underdrive pulley for lowering the AC drag (maybe a tiny tiny improvement) and played with ride height (again, maybe a TINY tiny improvement).

Then I started to consider re-gearing. This is a relatively expensive and non-novice kind of mod, so I didn't want to do it without researching it carefully.

As I studied it, it quickly became apparent that for improving highway speed MPG, gearing really would not improve anything. About the only thing gearing will do is improve/impact acceleration and make towing easier, and reduce some mileage based wear to improve engine longivity, but basically, I found that moslty it is the weight, rolling resistance, and DRAG that influences the mileage a car can get.

And yes, I would think that the larger tire would be a bad idea on the MT insight, and really not a good idea on the CVT car either. It just doesn't by anything.

Even the argument for ride quality that a bigger tire offers is I think negated by the fact that you can simply keep the stock tires at recommend pressure (the ride is not terrible at factory recommended pressure) or even LOWER the recommend pressure a couple of pounds, and the rolling resistance of the factory tire is probably STILL less than with most other tires.
 

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Thanks Guys

You can imagine, this is a very old discussion around here and sometimes people get tired (so to speak) of answering this same question repeatedly. The fuel economy winner is clearly the original tires Bridgestone Potenza RE92s in the correct size. It has been demonstrated numerous times around here. Spend a little time searching and you'll find lots of posts. Aftermarket wheels, too have proven to hurt mileage as most are heavier and generate turbulence. No one around here would encourage you to buy anything but the RE92s.
I appreciate your clear reiteration of the basic facts. I did spend quite a bit of time searching and reading and got more confused. It seemed like folks were putting larger tires and getting better mileage. I'll stick to the stock tires then. Thanks!!
 

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Yes, this would be my guess. The larger throttle opening needed to maintain the same speed would likley make lean burn hard or impossible in the manual transmission cars.

I did a lot of research on this. I had a Mustang convertible at the time, and was really looking to improve fuel economy. I did the underdrive pulley for lowering the AC drag (maybe a tiny tiny improvement) and played with ride height (again, maybe a TINY tiny improvement).

Then I started to consider re-gearing. This is a relatively expensive and non-novice kind of mod, so I didn't want to do it without researching it carefully.

As I studied it, it quickly became apparent that for improving highway speed MPG, gearing really would not improve anything. About the only thing gearing will do is improve/impact acceleration and make towing easier, and reduce some mileage based wear to improve engine longivity, but basically, I found that moslty it is the weight, rolling resistance, and DRAG that influences the mileage a car can get.
And yes, I would think that the larger tire would be a bad idea on the MT insight, and really not a good idea on the CVT car either. It just doesn't by anything.

Even the argument for ride quality that a bigger tire offers is I think negated by the fact that you can simply keep the stock tires at recommend pressure (the ride is not terrible at factory recommended pressure) or even LOWER the recommend pressure a couple of pounds, and the rolling resistance of the factory tire is probably STILL less than with most other tires.
If you are speaking about the insight, then I agree. The problem with the insight is the lack of power (torque) to "pull" a taller gear. It simply does not have any extra power. So taller gearing, whether through larger tires (and ignore added weight for a moment) or taller diff gearing will not help.

But i have done that in two other cars and noticed the expected increase in highway MPG consistent with the gearing change (10% gearing change equals about 10% highway fuel mileage increase). That is because both cars, V8's, had enough power to pull the taller gears ( I did it by changing ring and pinion).

The increased mpg comes from turning less revs at a given highway speed. One car is an auto, the other a close ratio 6 speed manual. Both cars seem to give the best combination of speed and mileage in the 60-65 mph range. Above 65-70 will show a noticeable drop. If I can run at 55, I can increase mileage, but the boredom factor goes up exponentially! The best speeds vary slightly by car as a function of gearing. The 6 speed actually gets better mileage at a higher speed in top gear as too low a speed lugs the engine.

Again, this will not work on an insight because of it relatively low power output. So stock gearing and stock wheels and tires (LRR RE92's) have proven to be the best to maximize mileage.

Regards,
Jerry
 

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Bottom line answer...........Its the excess or heavy load on the engine, no matter what size that kills the mileage. Trying to find the sweet spot is the challenge That's why I run a vacumn with boost gauge (For the turbo) on the LRR..
(My opinion)
Willie
 

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I think he's saying he has both a vacuum gauge and a boost gauge. Vacuum gauge would give an indication of the engine load, and the boost gauge is for the turbo.
 

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I think he's saying he has both a vacuum gauge and a boost gauge. Vacuum gauge would give an indication of the engine load, and the boost gauge is for the turbo.

Just need one guage to show vacuum or boost. one side shows boost and other side shows vacuum.
 

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Just need one guage to show vacuum or boost. one side shows boost and other side shows vacuum.
Oh... Duh. ;) LOL.

<-- has never owned a turbo automobile.. :(
 
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