New tires are on!
I got my new tires mounted and balanced on my F250 truck. I noticed a huge difference in coasting and on my vac. gauge. My MPG jumped into the 14's :D even in these freezing winter temps. I bought Bridgestone Dueler A/T Revo's. They are the stock E rated 235/85R16's, my old ones were D rated 285/75R16's. Here's some pics:
https://www.fuelly.com/attachments/fo...1d8461e8cd.jpg https://www.fuelly.com/attachments/fo...95432d953a.jpg https://www.fuelly.com/attachments/fo...d573e7a829.jpg EDIT: I almost forgot, I have them at 80 PSI - sidewall max. |
Did you check the manufacturers' actual listed tire size to compare your actual circumfirences? GPS data for actual speed?
I'm potentially looking to do a swap of my 285/75R16's to an E rated 235/85 or a 265/75 myself, so I'll be watching. :thumbup: Looks like the Revo 285/75 does 631 turns per mile and the Revo 235/85 does 648. |
I didn't check out any of that info, I figured there would be some difference though. I was going to measure the circumference of the old ones compared to the new ones, but I ran out of time. My old tires were basically bald and looked about the same height when they stood next to each other. Another factor that didn't help with my old tires, was that one of them was the spare on the rear axle. Its a stock size, but looks smaller than these new tires. The rear axle has clutches in it for positrac, which are mostly burned out. I do have a gps that I can compare speed with, I'll have to try that out.
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I know my PSD '96 E350 used a VSS to calculate data for the speedometer.
I'm not sure if yours would have it, but you could try this. Hold down the odometer reset button with the key off, then turn on the vehicle without starting. There might be a number there that you scroll to using the reset button. If you have that, it can be reset to work with the actual tire size that you have. Quote:
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Holycow has a great link in his signature for calculating the speedometer correction: https://miata.net/garage/tirecalc.html
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My speedo does have the calibration feature. I tried it one time but never changed any settings. The guy who owned the truck before me, i think had 33"s on the truck judging from the sawsall hack on the front bumper for clearance. So it might be calibrated for taller tires. I just did a run with the GPS along, and with the cruise set at 60 mph the GPS fluctuates between 58.4 to 58.7.
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I have a few comments about the chin spoiler you've added. They all pertain to the added frontal area and nearly vertical position (max drag) versus the intention of reducing air drag under the vehicle to offset that added front area.
The height stance is more than enough that air will curl around under the vehicle and still expose, probably from the transfer case back, the underside to the same air as without the front dam. Have you considered side skirts to block this air ingress? Or just a smooth belly pan to reduce drag and be much less frontal area than the present dam? On a lower vehicle a dam makes sense, but on raised ones (think solar racers and such) a dam adds much more drag than it eliminates elsewhere. |
Off topic, but I don't think a belly pan would help clean up the flow over two large axles and the large leaf springs of a large vehicle. Isn't a solar racer smooth underneath and driving the wheels via a different configuration where there is no axle exposure?
Tractors pulling trailers have continued to use front air dams rather than underbellies. Would be easier to do on a tractor as they are built more for service. |
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I'm on the lookout for discarded corrugated plastic bedliners to use. I'm not sure if the corrugations will help by directing the air straight back, or if they'll be worse than a smooth pan because the air gets turbulent while escaping sideways, but they are large and stiff and it should be easy to do the job with them. |
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