Now I know i shouldn’t be complaining with this just wanted to get opinions. I have a 3800sc fiero with a 4 speed Muncie trans. How is everyone getting with a full tank from their car. I have a fiero sending unit and it is actually really accurate. Before I did the autolite 104, and Taylor 8.5mm wires I was running ac delco iridium plugs with stock ac delco wires I was seeing 200-210 miles a full tank at 65-70 miles an hour. This was on 205/60R15 stock cross lace tires. I installed 17 inch Scion tC wheels 205/50R17 my Miles to full tank have dropped now seeing 170-180 a full tank. Now would this be because of the taller tires or going with copper plugs I’m seeing less of burn from fuel? The car runs like a bat out of hell and pulls hard to 80 like nothings so what could it be? 🧐

All other things being equal, the slightly larger circumference of the 205/50R17 tyres over the stock 205/60R15 tyres would actually have you travelling further in real terms for the same fuel consumption though only by about 1.6%. The catch is the odometer will still think you are travelling on 205/60R15 tyres and will be measuring distance on that basis so 200 miles on the odometer will actually be ~203.1 miles travelled. This doesn't account for the big drop in fuel economy. Tread pattern and stickiness of the rubber makes a difference but it's hardly measurable. Tyre pressure could be a contributing factor but you're talking about a 14.6% drop in fuel economy so unless you are running with very little air pressure, I'm betting on the autolite plugs being the main contributing factor. Are they colder than the delco plugs?

[This message has been edited by Mike in Sydney (edited 07-13-2021).]

My gas mileage first dropped from 28-29 Hwy MPG to 25-26 hwy mpg. Later a miss developed. It was all due to corrosion on the ignition module base and mounting plate where the ignition module gets its ground.. Corrosion is an insulator and the coil packs were not getting a steady voltage. Took off the ignition module and coil packs, cleaned the bottom and the mounting plate with 400 grit W/D paper, applied a film of conductive heat sink grease (not dielectric grease as it is an insulator) , bolted it back in place and engine efficiency was 100% restored. This must be a common problem as a friend also had the same issue . Might be worth looking into.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

quote Originally posted by Mike in Sydney: All other things being equal, the slightly larger circumference of the 205/50R17 tyres over the stock 205/60R15 tyres would actually have you travelling further in real terms for the same fuel consumption though only by about 1.6%. The catch is the odometer will still think you are travelling on 205/60R15 tyres and will be measuring distance on that basis so 200 miles on the odometer will actually be ~203.1 miles travelled. This doesn't account for the big drop in fuel economy. Tread pattern and stickiness of the rubber makes a difference but it's hardly measurable. Tyre pressure could be a contributing factor but you're talking about a 14.6% drop in fuel economy so unless you are running with very little air pressure, I'm betting on the autolite plugs being the main contributing factor. Are they colder than the delco plugs?

Yes the 104 plugs are 1 degree heat range colder I mean I could careless I was just wondering on what actually caused the drop in mpg. It goes like a raped ape so I’m happy with it

quote Originally posted by pontiacfierokid1985: Yes the 104 plugs are 1 degree heat range colder I mean I could careless I was just wondering on what actually caused the drop in mpg. It goes like a raped ape so I’m happy with it

I've heard that colder plugs can affect gas mileage but if you're happy with the performance and can live with the lower fuel economy, go for it.

Getting back to the question. Larger diameter tires slow the speedometer calibration and will show less miles traveled than there actually are. The solution is to edit the program and correct for tire size. Larger diameter tires also slow your shift points. If everything is up to par a 3800SC engine in a Fiero in good tune should give 27-30 hwy MPG.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, P-log Manifold, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, Champion Radiator, S10 Brake Booster, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

If you switched from pure gas to a 10-15% ethanol blend, you could see a MPG drop.
With the larger diameter tires, the speedo will read lower than actual distance travelled AND you will be driving the car faster than before for the same speedo reading - both will result in lower MPG measurements if you don't take them into account by either fixing the speedo or making adjustments in the testing and calculations.
The new parts you installed might also be loose or have a poor connection, which could drop MPG as well.

I mean it honestly doesn’t bother me I’m just wondering the factors of why mpg dropped I would think it would be better with the better parts installed but guess that was wrong lol. I mean the car runs amazing, love having c5, c6 vette pull up along side and I stay right along side them when when getting on the car with them. 3800sc is literally the best thing to do to these little mid engine go karts