Hi All,
Just put on 4 new Kumho Solus from tirerack on my 125K 88 245 and have noticed my mileage went from 28mpg to roughly 23mpg. Are the new tires to blame or should I be worried about something else like an O2 sensor.

are your tires the same size? have you checked if they are loosing pressure? leaking valve stem, or the such?

keith

Differences in rolling resistance can effect fuel economy, but I highly doubt tires alone would cause a ~20% change. Any chance one of your fill-ups shut off a bit short of actually filling up?

The O2 sensor is a good thing to check. The fuel pressure regulator is also a good thing to check. In fact, if your '88 is still running its original fuel pressure regulator and/or O2 sensor I would replace both as a preventative maintenance item.

What is the pressure in the tires compared to before? If it's low it can kill your mileage.
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David Hunter

Both around 40psi. I keep them at the max that the tire recommends found on its sidewall

I have the same tires in size 195-75-14. Taller than stock and I average 24 mpg on the odometer. I notice they are quieter than my previous treds ... Yoko avid TRZ.

I have a Chey S10 pickup that the summer tires I am running (installed by PO)cost me a speeding ticket .... after I got the ticket I checked to see how much larger they were and the website said 6.04% larger... sooo I installed my TomTom and sure enough when the speedo said 100 kph the TomTom informed me that I was actually doing 106 kph. When I put my Blizzaks on for the winter ... which were the correct size for the truck the readings of the TomTom and the speedo agreed. So when I figured out mileage I increase my distance travelled by 6.04% and got an accurate figure... go figure eh?

Brett

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Brett Sutherland & the 1.5 million mile 122 CANADIAN --- and has been Joined by a 1990 780 BERTONE TURBO Coupe -- WINDSOR, Nova Scotia the birthplace of HOCKEY

195-75-14 is exactly the right size to use on a 245 (wagon). I'm running these on our '93 245 and have checked the speedometer against our portable Garmin's speed reading. The car's speedometer is right on the money compared to the Garmin.

Wagons originally came with 185/80-14 tires which are almost impossible to find now. 195/75-14 tires calculate out to almost exactly the same circumference.

Find a site that makes the math of comparing tire sizes to determine speedo/ODO error when running rear tires of a non-stock size.

Sorry, I don't have a site for you, I'm not on my own computer.

The speedo/odo measures rear wheel revolutions, and how far the car moves per rev is built-in according to the tire diameter. With taller than stock tires, your car moves a bit farther than the speedo "knows" so you have a error in both speed and miles driven.

Probably not a large error, 2 percent or so (SWAG). Be aware that the car is going faster than the speedo says. Important in 'Aggressive Enforcement' zones.

Also, the actual miles driven is a slightly larger number than your odometer says, so your gas mileage could be better than you think.

Regards,

Bob

:>)

The website says that I am going 0.5% slower than with the stock tires.

Here's the site for calculating. Thanks!

http://www.miata.net/garage/tirecalc.html

Different compound tires will have different rolling resistances as well. Could be a combination of both. I also know that when I get a new set of tires on any of my rigs, I'm tempted to "excersize" the car a little more until I get used to the feel of it.