New tires! - Fuel Economy, Hypermiling, EcoModding News and Forum

Fat Charlie · 12-31-2011, 09:53 AM

It was time for new tires. LRR was a given, but I wanted to do more and go oversize. I've been too busy lately to do real research on LRR, so I bowed to the dark gods of marketing and just went with Goodyear Assurance Fuel Maxes. Then I went over to the tire size calculator over at Miata.net. My car takes 215/45/17s, a size I've never been happy with. It's a pretty rare size and way too little sidewall for dealing with crappy roads. I spent a lot of time dithering between 215/50/17 and 215/55/17, the issue being that I had a stock radius of 12.3". The 50s would take me up to 12.7" and the 55s to 13.2". I figured I could fit another half inch of tire in there, but didn't know if I dared try for a whole inch. I went with the 50s and it turns out I guessed right- at full lock the inside edge of the tire still clears the wheel well. Barely.

Because of all the time I spend gliding the main benefit to me isn't the size, it's the LRR. I'm happy with it- my standard glides are going farther now. My last glide on the way home didn't work, though. I started my second to last glide earlier than usual and didn't have to goose it to get over the last little hill. I made it over the hill, but without enough speed to glide all the way to the stop sign. That's huge: I couldn't glide all the way to my stop because I was still on the previous glide. Viva La Low Résistance!

Where is the size benefit? The 45s got 819 revs per mile, the 50s get 792, and the 55s would have been 767. Now for every turn of the wheel I'm moving 3.4% farther than before (and dreaming of the 6.9% farther that I could have been going on the tires that wouldn't have fit). I'm not going to be measuring this in real time. I couldn't find anything on the net about flashing the ECM to change the tire size, and I called a Subaru dealer to ask and got told no. You can't take that as Truth because dealerships are just as half-assed as any other place, but it's not looking good. The difference is small enough that I'm not worried about getting stopped. Sure, if I've got the cruise at 70 then I'd get tagged for 73 but I've been careful about traffic cops for long enough that I think I can be careful about traffic cops for a little longer. Since I can't change the car's instrumentation I'm not going to change the UltraGauge either, I'm just going to use the 3.4% when I calculate from the pump. That'll be fun to do again- I haven't logged a full tank in quite some time.

Ecky · 12-31-2011, 05:16 PM

Nice!

I'd up my tire size if it weren't for throwing my gauges off. My tach is spot on right now - only 0.3% off. I've got tons of room to lower my car further though, even with a larger wheel size.

mcrews · 01-01-2012, 07:26 AM

The whole Idea behind runing the gauge is that you can calibrate it exactly.....
Why would you want ALL your gauges off?
I runn a 245/50/18 where factory was 245/45/18. It's almost 6% off. I use the scanguage for my exact mph (and my gps). cant remember the last time I looked at the speedometer.

mcrews · 01-01-2012, 07:30 AM

Quote:

Originally Posted by Ecky

Nice!

I'd up my tire size if it weren't for throwing my gauges off. My tach is spot on right now - only 0.3% off. I've got tons of room to lower my car further though, even with a larger wheel size.

this is almost funny if it wasnt so off base.....
Fact: lowering the height effects the ride. period.
So you'd rather have a crappy ride quality, instead of know that the aftermarket gauge is correct?
PS.....tire size doesnt effect the tach.......the tach measures the ENGINE.

like when your sittting at park......not moving......the tach is still correct.....right?
take the tires off, but the car on jack stands.....the tach is still correct.....right......

Ecky · 01-01-2012, 11:25 AM

Quote:

Originally Posted by mcrews

this is almost funny if it wasnt so off base.....
Fact: lowering the height effects the ride. period.
So you'd rather have a crappy ride quality, instead of know that the aftermarket gauge is correct?
PS.....tire size doesnt effect the tach.......the tach measures the ENGINE.

like when your sittting at park......not moving......the tach is still correct.....right?
take the tires off, but the car on jack stands.....the tach is still correct.....right......

My apologies, I meant odometer. (had taken something to fall asleep before I typed that post)

What I meant to say was, even with larger tires I could still lower my car significantly, possibly to the ground, so it's not a factor stopping me. It's not a shame that larger tires would throw off my odometer and speedometer and that they can't be corrected for.

mcrews · 01-01-2012, 11:42 AM

sleep deprivation will do that!!!!!

weddingcoo · 01-04-2012, 01:21 AM

nice~~~~~

SentraSE-R · 01-04-2012, 11:10 AM

Change the distance & speedometer calibrations on your Ultra-gauge. Then you can rely on the UG for your speedometer and trip odometer. There's no need to be flying blind unless you want to.

Fat Charlie · 01-05-2012, 11:48 AM

I'd rather have them match the car, and "maybe a couple over, depending on how fast you're going" isn't really blind. Instead of keeping the cruise a hair under 70, 80 or whatever to avoid alarms, I'll just leave a little more space. Besides, if I'm not in a hurry I don't need to worry about radar any more and if I am in a hurry it's not going to be the extra 2 or 3 mph that gets me in trouble.

Fat Charlie · 01-09-2012, 09:58 AM

I finished my first full tank on the new tires, and it's a success. Mileage has been down lately- it's cold and dark and I've got no grille block or kill switch.

I don't have any proper tank logging for some time, but I've been struggling to keep it in the 26 mpg range. This tank read at 27.4 before I factored in the taller size, which rought me to 28.37. I'm happy.