Reader Question: Speedo Accuracy?

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Here’s the latest reader question, along with my reply!

Paul asks: I once saw, but lost track of a formula (maybe an app now?) to apply to figure out the impact of the increase of tire sizes on speedometer readings. Do you have anything like that? I once figured it out longhand but misplaced my notes. Thanks. Also, great article on rim size and mpg.

My reply:  Here’s a link to a calculator which will give you the correction. Keep in mind, though, that factory speedometers can be off even with the factory wheels/tires and everything else. This goes for both newer and older cars.

I have a GPS device I use when I test drive new cars and among other things, it gives me a speed readout. This often varies by as much as 2-3 MPG indicated. And this is why armed government workers have “calibrated” speedometers in their vehicles – the better to ticket you with!

On the upside, it’s often still a viable defense (depending on the judge) to present evidence that your speedometer was reading slower than you were actually driving and thus you had no idea, your honor, that you were speeeeeeeeding!

. . .

Got a question about cars – or anything else? Click on the “ask Eric” link and send ’em in!

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  1. I’m surprised that it wasn’t the other way around as far as traction (narrower better).

    But maybe there is a “sweet spot” in aspect ratio?

    Here is what I used to find wheels/tires with same OD:

    It’s worthwhile to pick a tire size that actually exists and is fairly common. In my case I had to keep the same OD because there was no extra fender/strut clearance.

    • Anon, the sweet spot is always what you desire for the job at hand. Those narrow 235’s sucked for handling and needed more air pressure to not have a saggy sidewall(Michelin’s). The much wider and same overall height Toyo’s were heaven on earth getting really great traction, no noise and never needed re-balancing. Sure, a 2″ wider wheel and tires several inches wider cost me 2mpg but it was well worth it in handling and esp. load hauling. I’d put 4,000 lbs in the bed and 10,000 lbs GVW on a 7500GVW trailer and never miss a beat. Of course the trailer was biased for load to the front since that tail wagging the dog sucks.

      Everyone who had ridden in it was amazed at the ride quality(of course, Edelbrock IAS Performer shocks were instrumental in the handling) after factory size wheel and tire. I didn’t have to run near as much air pressure either although when you’re well over GVW you’d better be 80psi on the rears at the least.

  2. Good morning eric, thanks for the calculator. It showed my changed from 235/85/16’s to 265/75/16’s to be 64.84 as a perceived 65. I’d go along with that. It always seemed I was right with everyone else. The change in my pickup from the first to the second was like night and day. It rode better, handled a few times better(it felt), stopped better and got the townies come back to the country to retire unstuck in the sandy barditch they couldn’t seem to understand is a trap for the unwary 2 WD pickup owners.

    It was funny one day I came across a retired couple in their Ford 2WD diesel(now we know who buys 3/4 T diesels and no 4WD). I had thrown my chain out for some reason so only had my 6′ chain I used for other things. I got us close and hooked us up(No, I’ll get it I told the man driving….last thing I need is help with something like that since so many people are completely incompetent). I reached in and turned his steering wheel so the front tires were pointing straight and told him to hold the wheel right there. After a few seconds of spinning tires I realized we had a malfunction so I got back and see his tires are turned to right lock, the exact worse way he could have turned. I turned the wheel back and told him DO NOT turn the wheel and repeated first step. I get out and go back and here he is with the front tires cranked as hard right as they’d go. I told him if they wanted out in the daylight(it was nearly dark….naturally)to leave the wheel as is. I had help this time. His wife jumped down his throat and said Do what the man says. Next time it was easy to get them out.

    I could understand Ca. yankees doing this…..but they seem drawn to not so rural parts…..thankfully.


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