North American Turbocoupe Organization



Wider tires and gas mileage
Sophosis Offline
Member
#1
Anyone notice a correlation in a drop in gas mileage and a change to wider tires? I've got 255s on it now after doing a 5-lug swap.

I may be just over thinking and over observing it. I'll run a full tank through and see what the actuals show me over the tripminder. If the block and IHI are just too tired (260K), then hopefully the gas mileage will be solved in the next few weeks.
1988 Turbo Coupe
- 0.020 Diamonds w/stock rods, T3 w/stock IC, BoPort 1.5 cam, LA3 w/QH on MAF, water/meth
- 5-lug swap, 31-spline, 4.10s, vacuum brakes, PBR front brakes, 2000 Mustang GT 17" rims
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Not B Anymore Offline
Administrator
#2
I didn't notice any difference going from 225's to 245's. :dunno:
I log all of my fill-ups and keep track of my mileage.
Brian Leavitt
'86 TC 5-Speed -- MS2x w/COP | 83 lb. injectors | T3/T4 50 Trim Stage 3 .63AR | Full 3" Exhaust - No Cat | Motorsport FMIC | Ranger Roller | Ported E6 | Walbro 255HP | Kirban | 20psi | 120-amp 3G | 8.8" 3.55 rear | '03 Cobra Wheels
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Jeff K Offline
Administrator
#3
Wider tires, it theory, have higher air resistance as they present more surface area to the front, and wider tires generally have higher rolling resistance, so in theory, MPG will drop slightly. With that said, I noticed zero difference when I went from 225s to 245s. I keep MPG records in a spreadsheet for all my cars just like Brian does.
Jeff Korn

88 Turbo Coupe: Intake and exhaust mods, T3 turbo at 24 psi, forced air IC, water injection, BPV, Ranger cam, subframes, etc., etc.
86 Tbird 5.0 (original owner): intake, exhaust, valvetrain mods, 100 HP N2O, ignition, gears, suspension, etc., etc.
05 Taurus SEL Duratec daily driver
04 Taurus Duratec (wifes car)
02 Pontiac Grand Prix GT
95 Taurus GL Vulcan winter beater
67 Honda 450 Super Sport - completely customized
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5.0TurboCoupe1988 Offline
Posting Freak
#4
you guys are anal.

Soph, you probably lost some MPG "testing" the new increase in traction of the wider tires...
1988 TC 2.3/5-Speed, 148K
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Sophosis Offline
Member
#5
5.0TurboCoupe1988 Wrote:you guys are anal.

Soph, you probably lost some MPG "testing" the new increase in traction of the wider tires...

lol I wish.. the only traction testing I did was to see if I could lock them up with the PBRs... I can. I'm in the camp with Jeff.. I didn't expect an appreciable diff.. but the tripminder is showing me a 5mpg diff highway since before I started this round of upgrades. That's after driving 60 miles highway.

I track in fuelly.com too.. I'll just run some miles through her and see what I get I guess.
1988 Turbo Coupe
- 0.020 Diamonds w/stock rods, T3 w/stock IC, BoPort 1.5 cam, LA3 w/QH on MAF, water/meth
- 5-lug swap, 31-spline, 4.10s, vacuum brakes, PBR front brakes, 2000 Mustang GT 17" rims
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natmac3 Offline
Administrator
#6
how does the overall diameter compare to previous setup?

are these rims/tires heavier too?

any chance the brakes are dragging a little?

alignment issues can contribute a little bit as well, is it straight & true?

probably a combination of many variables going on here... Smile

1987tc
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Sophosis Offline
Member
#7
Funny thing about the brakes.. When I first put the reman PBRs on, you could not free-spin either wheel - at all. It took a lot of torque to rotate the wheel.. new calipers - junk out of the box. So instead of tracking down another set of calipers, I just took these apart and fixed them. The bleed screws had been overtightened at some point and were locking the pistons into place. I had to clearance the aluminum on the inside of the bore. They seem to spin normally now (1/2 - 1 spin freewheel with a push by hand).

They're 02 Mustang GT rims. I think they're lighter overall than the stock rims, at least it seems so the last time I put them on. I'll have to actually check, one day.

They're 255/45R17 .. so taller than what I had on it, but shorter than stock. I've still got the Auto TC speedo gear in it from the small tires i had on it, so it's even more accurate than it was with the I had on it before the swap (reads 1-2mph low at 70+).

Drove it today, about 60 miles.. 45 of them highway. Tripminder is sitting at 22.3mpg. Almost to half tank and barely at 200 miles.. not encouraging.
1988 Turbo Coupe
- 0.020 Diamonds w/stock rods, T3 w/stock IC, BoPort 1.5 cam, LA3 w/QH on MAF, water/meth
- 5-lug swap, 31-spline, 4.10s, vacuum brakes, PBR front brakes, 2000 Mustang GT 17" rims
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Dan S Offline
Administrator
#8
The change from stock size(225/60R16) to 255/45R17 reduces the tire diameter by 2.4". The 255/45R17 turns 775 revolutions per mile, while the stock size only turns 710.5 revs per mile. The width has very little resistance compared to stock, provided the tire is properly inflated.
What you have done is lowered your gear ratio, thus using more fuel per mile.

Now that's bunch of anal calculating!

I have 235/50R17 on mine, which are very close to stock diameter.
Dan S
Custom 88 TC, Mandarin Copper Pearl Metallic
http://natomessageboard.com/ubbthreads.p...5#comments
1972 Ford F-100 SWB Styleside
2015 Lincoln MKC 2.3 EcoBoost AWD
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Sophosis Offline
Member
#9
Dan, how are you getting 2.4"? I was only getting a diff of about a half inch in overall diameter.. 2+" is a lot, more than one speedo gear step. I was running 225/50R16s before the swap (old tires I had, might as well not let them waste away) and with the auto speedo gear, the speedo was fairly close.. read a bit higher than actual for speed.

So did some anal calculating earlier today. With these tires and the auto speedo gear in, compared to GPS, my odometer was reading 6.5% less mileage than reported by GPS. Using old fuel data, 450 miles was about 16.75 gallons, 27mpg. So if I'm now reading 6.5% less, 420.75 miles, I would get just over 25mpg.. tack on maybe 1mpg loss for the difference in tire weight, profile, and contact patch.. and that's a 3mpg loss. All that being said, I might not actually be having a problem with fuel mileage loss.. it might just be closer to right :<

I wanted to go with 255/50/R17 (should've been about 0.4in taller) to keep the tire diameters near the same but the guy I ordered the tires from talked me into 255/45. Oddly enough, my speedo is a lot closer to correct than my odometer.. which means I didn't put the needle back on in the 100% correct spot last time I had it apart...

Btw, I got really lazy this time and used this to do my tire calcs for me:
https://www.1010tires.com/Tools/Tire-Siz.../255-50R17
1988 Turbo Coupe
- 0.020 Diamonds w/stock rods, T3 w/stock IC, BoPort 1.5 cam, LA3 w/QH on MAF, water/meth
- 5-lug swap, 31-spline, 4.10s, vacuum brakes, PBR front brakes, 2000 Mustang GT 17" rims
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Dan S Offline
Administrator
#10
According to the calculator I used the stock 225/60R16 is 28.4" dia.; 89.2" circum.; 710.5 Rev/mile.

The 255/45R17 is 26" dia; 81.8" circum; 775 rev/mile.

Engine RPM's have to be higher, thus using more fuel to go the same distance. Not really a problem for casual drives but could make a big difference as a daily driver when driving a distance back and forth to work. I used to get 29-30 mpg before I rebuilt the entire vehicle and started making mods. Great fun when I take it for a spin on 93 octane, but the extra for gas is worth it.

Have you compared the speedo to the speed on the GPS? Speed over ground (actual speed) is very acurate on a GPS which is cross tracked by up to 5 satellites at once. How are you comparing odometer readings using a GPS?

Anyway, the cars are fun to drive and the MPG is caculated by the spirit of the drive multiplied by the pressure of the foot.

Enjoy!

Dan S
Custom 88 TC, Mandarin Copper Pearl Metallic
http://natomessageboard.com/ubbthreads.p...5#comments
1972 Ford F-100 SWB Styleside
2015 Lincoln MKC 2.3 EcoBoost AWD
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