Does upsizing tire affect acceleration in all gears?

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Hi Friends.
Please correct if i am wrong,
If we upsize tire diameter by 10%, we would feel the sluggish acceleration all the time?
Because depending on the gear and our cruising speed, most of the time, engines are running at lower than optimal RPM range where it can deliver decent amount of torque to the wheels. And if engine reaches to a RPM where it delivers a good amount of torque, and we still continue to accelerate, it will shifts upper gear, resulting in reduced torque again. So in this case, closer (or infinite) gear ratios would affect acceleration more than just having optimal final drive ratio (which is affected by tyre size also) ?

Can we safely assume upsized tires would affect maximum speed and acceleration only when car starts moving, but not make car sluggish during most of the cruising time ?
Regards.
 
Tires that are 10% bigger have *almost* the same effect as changing your differential ratio by 10%. Why *almost*? Because a 10% bigger tire is probably heavier, and wheels are unsprung mass, and that extra mass impairs acceleration and braking. If you changed the differential ratio by 10% it would have the same effect on the gear ratios but it wouldn't change the wheel mass.

In short, expect slightly better fuel economy and worse performance/acceleration. But the difference should be small.

PS: now that your car is going 10% faster than it "thinks" it's going, this may may make your speedometer and odometer read 10% lower, and may impact the function of other systems like antilock brakes.
 
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When I had my 09 Ranger (Duratec) I had undersized tires by choice and it felt like it had a little more torque for acceleration in the city where I did 90% of my driving. Could have placebo/confirmation bias. Didn't matter to me because I didn't like that vehicle on highway.

I wasn't anywhere near 10% undersizing though.
 
I put 31" tires on a 1995 Explorer that originally had 235/75/15s on it and it was fine. 3.27 gear ratio. Really barely noticed it, and it had the OHV 4.0.
 
My Ford's 1st gear is so low, I start in second almost all the time. So It would likely IMPROVE my acceleration if I could use 1st :)

In general increasing the tire diameter will slow acceleration due to torque mutiplication loss along with a the mass being at more distal radius from the axle centerline - Think "flywheel" effect.

Most moderm cars with autoboxes have a torque converter AND gearing and can accomodate modest changes.

Note: your speedo will read low vs actual ground speed

- Ken
 
If you've got a recent manual trans car I think bumping up a size can be a good thing, top gear in all of them is too short for good mileage. Probably not a good idea with a modern automatic car as they are all geared for maximum mileage from the factory.

I've been very happy with going from 195/60R15 to 195/65R15 on the Focus. It's about a 3% "lower" gearing difference and put the speedometer dead on, tires are the same weight, and the ride is a touch quieter. I used to run 195/55R15 summer tires which was ~3% "higher" gearing ratio which was fine too. With a 6% difference the only thing I really notice is that I have to downshift to 4th for more passes, instead of lugging it out in 5th.
 
PS: now that your car is going 10% faster than it "thinks" it's going, this may may make your speedometer and odometer read 10% lower, and may impact the function of other systems like antilock brakes.
This is a good point. I bought a 2001 Silverado new. It always seemed to hold gears a little longer than necessary, which was especially noticeable with the 4L60E's low first gear. One day I drove past a mobile radar sign and realized the speedometer reading was slightly off. The stock tire size was set at 30 inches, but when I measured tire height and figured it by circumference, my numbers came out closer to 29 or 29.25.

When I reset the tire size with my Hypertech programmer, the speedometer was dead on and the trans shifted a little earlier.
 
If you've got a recent manual trans car I think bumping up a size can be a good thing, top gear in all of them is too short for good mileage. ...
That depends on the car. My 2014 Mazda 3 has a 6 speed manual and 6th gear is so tall it's virtually unusable below 60 mph. Our old 2004 Subaru is the opposite, 5 speed manual and 5th gear is too short, it really needs a taller gear for freeway driving.
 
I went from 185/65R15 to 205/60R15 on my Saturn…

20 millimeters wider, just a little bigger.

This car is used primarily on the interstate, so a slower 0-60 speed is no problem…

Use the “Tire Comparison“ on the left side to see the difference between tire sizes…

 
Most answers above are only half-right, at best. Assuming a manual transmission or automatic with fixed ratios, yes it definitely would reduce acceleration in a specific gear ratio at a specific engine speed. However, that's largely counterbalanced by the fact that you could stay in a lower gear up to a higher actual speed before upshifting at the same engine speed (and same indicated road speed). Obviously that effect wouldn't apply at launch, and you'd still have the added weight to accelerate.
 
That depends on the car. My 2014 Mazda 3 has a 6 speed manual and 6th gear is so tall it's virtually unusable below 60 mph. Our old 2004 Subaru is the opposite, 5 speed manual and 5th gear is too short, it really needs a taller gear for freeway driving.
I am surprised and happy that Mazda figured out a way to have a proper top gear! What rpm do your run at 60mph in 6th?
My Focus is ~2550rpm @60mph. The Outback runs about 1550rpm at 60mph and at that speed my wagons get very similar mileage, inspite of the extra 1000lb, tire size, AWD, and physical size of the Outback. I guess the smooth underbody and "silly" grill shutters on the Outback must help a little, but if you just look at the fuel consumption difference of spinning the Focus engine at 1500rpm vs 2500rpm sitting in neutral, you can see that engine friction ramps up quickly with rpms.
 
The Mazda runs about 2000 RPM @ 60 in top gear. Anything taller would be too much for its 2.0 liter engine; going up more than a 5% grade already requires downshifting. The old Subaru runs 2500 RPM @ 60 in top gear.
 
The Mazda runs about 2000 RPM @ 60 in top gear. Anything taller would be too much for its 2.0 liter engine; going up more than a 5% grade already requires downshifting. The old Subaru runs 2500 RPM @ 60 in top gear.
I seem to be going either 45 or 80 in top 60mph on a interstate is so 1960.
 
Well, I didn't say I actually drive 60, but just provided the engine RPM at that speed ;)
 
The Mazda runs about 2000 RPM @ 60 in top gear. Anything taller would be too much for its 2.0 liter engine; going up more than a 5% grade already requires downshifting. The old Subaru runs 2500 RPM @ 60 in top gear.
That sounds like a good top gear in mazda. That's the first compact car I've heard of with a decent gear for mileage in here.
 
Hi Friends.
Please correct if i am wrong,
If we upsize tire diameter by 10%, we would feel the sluggish acceleration all the time?
Because depending on the gear and our cruising speed, most of the time, engines are running at lower than optimal RPM range where it can deliver decent amount of torque to the wheels. And if engine reaches to a RPM where it delivers a good amount of torque, and we still continue to accelerate, it will shifts upper gear, resulting in reduced torque again. So in this case, closer (or infinite) gear ratios would affect acceleration more than just having optimal final drive ratio (which is affected by tyre size also) ?

Can we safely assume upsized tires would affect maximum speed and acceleration only when car starts moving, but not make car sluggish during most of the cruising time ?
Regards.
Purely academic answer I can think of. No width change, no wheel change, only tire of the same kind and just 10% diameter change of the tire (not wheel).

Your "gear ratio" will change as you said. You will in theory be closer to a taller gear overall and will impact acceleration, but this alone would not give you worse mpg. It may actually give you better mpg for taller gear change alone.

Your tire will be thicker and therefore have more rolling resistance than a thinner tire of the same kind.

Your vehicle is now taller off the ground so more air goes under your car and you will have more air drag, making the final speed lower and your fuel economy worse.

Your tire is heavier so you have more inertia to turn when accelerating and decelerating, this will slow you down.

Your engine is likely oversized for most day-to-day driving so you will get used to it and just step on the gas pedal further, but it will use more fuel.

Done that once but not the same tire size change only, and the Escort becomes quite sluggish as a result (185 60 14 S to 185 65 14 H).
 
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